Everything You’ve Always Wanted to Know About Shellfish Allergy (Except How to Cure It)

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Shellfish cause allergic reactions in millions of people around the world, especially among those who live in the Asia-Pacific region, where shellfish is a popular staple. Shellfish allergy is also relatively common among people who work in the shellfish industry and people who are allergic to insects. Crustaceans are more likely to cause allergic reactions than molluscs, but not all reactions to shellfish are caused by allergy; sometimes it’s just a case of food poisoning.

The sheer variety of edible shellfish and the great potential for cross-reactions makes shellfish allergy difficult to diagnose, but once a diagnosis is made, the advice is often to avoid eating all shellfish. However, the shellfish-allergic rarely react to all types of shellfish; people who are allergic to crustaceans can often eat molluscs, and an allergy to just one type of shellfish (or even just one part of a shellfish) is more common than you may think. So, if you’re allergic to shrimp, a plateful of salt & pepper squid may still be on the menu, but you should undergo a supervised oral food challenge first, just to be sure.

And now for the details, which include:

What is an allergy to shellfish?

The culinary term ‘shellfish’ is made up of two major groups of invertebrate animals: crustaceans and molluscs.

Crustaceans belong to the Arthropoda (‘jointed foot’) phylum, the largest phylum in the animal kingdom, coming in at around 11,340,000 species. There are over 52,000 species of Crustaceans (subphylum Crustacea, meaning ‘hard-shelled’) living in salt water and fresh water.

Most edible crustaceans belong to the order Decapoda (‘ten-footed’). Decapods have five pairs of legs on their main body which they use for walking (thus, the 10 ‘feet’) and five more pairs of swimming legs on their abdomens. Decapods include shrimp, prawn, crab, lobster and crayfish.

Other edible crustaceans include barnacles (order Cirripedia, which roughly translates as ‘slender, wispy feet’, like the slender, wispy cirrus clouds), krill (order Euphausiacea, which roughly means ‘true shining light’ and refers to the fact that many types of krill are bioluminescent) and mantis shrimp (order Stomatopoda, literally ‘mouth-feet’ which alludes to the first five pairs of thoracic limbs that are used for feeding).

Crustaceans are closely related to other arthropods, especially the (mostly eight-legged) arachnids such as spiders and house dust mites and the (Hexapoda, ‘six-legged’) insects including house pests like cockroaches and edible delights like mealworm (beetle larvae) and crickets.

Crustaceans can be eaten raw or cooked, and products like dried shrimp and shrimp paste are widely used to add flavour to packaged and processed foods like instant noodles, soup and surimi. In countries like Russia, Japan and Korea, krill is used to jazz up various types of processed food. In Europe, krill oil is gaining popularity as a supplement because of its health benefits and the fact that it may be easier for the body to absorb than fish oil. Crustacean skeletons and shells are also used in a range of health and beauty products including calcium supplements and skin care products.

Molluscs belong to the Mollusca phylum, the second largest phylum of the animal kingdom, made up of over 100,000 species that live in salt water, fresh water and on land. They are divided into 8 classes and the ones we commonly eat can be found in three of them:

• Bivalves:(from the Latin bis = ‘two’ and valvae = ‘leaves of a door’) which have a shell made of two systems called valves and are generally symmetrical. They include clams, mussels, oysters, scallops and razor shells
• Gastropods: gastro = stomach, pod = foot) which have a shell but no bilateral symmetry. They include snails, abalones, limpets and periwinkles
• Cephalopods: (cephalo = head, pod = foot) which have an internal shell or no shell and a large head. They include octopus, squid and cuttlefish

Molluscs are a common food in many European countries. Although they are not often present as ‘hidden’ ingredients in processed foods, they can be sneaked into sauces and soups.

Shellfish make up a large part of the Asian-Pacific diet, with around half of the global shellfish production being consumed in the region. However, increased awareness of its high nutritional value—lean protein, healthy fats, beneficial micronutrients—and potential health benefits—for the brain, heart, immune system, skin, bones, joints, waistline—has led to a rise in shellfish consumption in the West which has, in turn, led to an increase in reported cases of allergic reactions.

These allergic reactions happen when a person’s immune system mistakes one or more harmless shellfish proteins for toxic invaders and creates IgE antibodies against them. The next time that person eats some shellfish, their IgE antibodies recognise the shellfish proteins and prompt a response from immune system cells. These, in turn, release a variety of chemicals into the bloodstream, including histamine, the chemical primarily responsible for the symptoms of allergy.

Shellfish allergies are more commonly reported to crustaceans than to molluscs, and over three-quarters of reported reactions are to shrimps or prawns.

In fact, although Hippocrates (c. 460 BCE to c. 375 BCE) is often credited with being the first person to recognise the existence of food allergy, centuries before the ancient Greek physician was pointing out the potential hazards of cheese, the Chinese emperors Shen Nong (c. 2735 BCE) and Huang Di (2698-2598 BCE) were advising people with skin lesions and pregnant women to avoid certain foods including shrimp in their ‘Shi Jin-Jing’ (‘Interdictions concerning food’).

That said, the fact that shrimp and prawn are reportedly more problematic than other species of shellfish probably has a lot to do with the fact that they are the most fished and eaten types of shellfish. They are also often the first type of shellfish to be introduced into the diet so, if they cause a reaction in a person, that person tends to avoid all other varieties of shellfish which are therefore not given the chance to be problematic.

Different patterns of shellfish consumption around the world also influence which types of shellfish are reported as problematic. In Portugal, barnacles are on the menu often enough to cause problems, and in Spain, squid figures highly among the list of usual allergenic suspects. In Italy, people in the north of the country (where eating habits are more similar to those of central and northern Europeans) are more likely to be allergic to crustaceans like shrimp, whereas people in the south (where eating habits are similar to those in other Mediterranean countries) are more likely to be allergic to bivalve molluscs like mussels and clams. In America, the most likely types of shellfish to cause problems after shrimp are crabs, lobsters, clams, oysters and mussels. In Korea and Singapore, whelk and limpets figure on the list of usual suspects and, in South Africa, abalone is in the top 5.

Shellfish are also responsible for provoking delayed, non-IgE-mediated allergic reactions, notably Food Protein-Induced Enterocolitis Syndrome (FPIES) eosinophilic oesophagitis (EoE) and delayed-onset atopic dermatitis (AD), aka allergic eczema, which I shall now just call ‘eczema’ (although, strictly-speaking, AD is the most common subtype of eczema).

Shellfish can also cause reactions that are not related to the immune system—namely shellfish poisoning due to toxins. The results of both shellfish allergy and toxicity are very similar and can easily be confused with one another.

Identified allergens

The proteins (and occasionally carbohydrates) in a food that are capable of provoking allergic reactions are called allergens. Allergensare named using the first three letters of the genus, the first letter of the species and a number reflecting the order in which they were identified. For example, one of the most important allergens used for diagnostic purposes is a tropomyosin from the brown shrimp— Penaeusaztecus—named Pen a 1.

There are thousands of different types of shellfish, all containing several different types of allergens. Dozens of allergens from several crustacean (including shrimp, prawn, crab, lobster, crayfish) and mollusc (including oyster, clam, whelk, abalone, squid, snail) species have been described and are officially recognised by the World Health Organization/International Union of Immunological Societies.

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How common is shellfish allergy?

A definitive diagnosis of allergy can currently only be established with a food challenge, but challenges are costly in terms of both time and resources, as well as potentially risky. So many prevalence studies measure sensitisation—using skin or blood test data—but sensitisation is not allergy. Being sensitised to something simply means that your immune system recognises it, but you may not actually react to it; in fact, many people don’t react to whatever it is they are sensitised to. Studies that use sensitisation data therefore tend to produce allergy prevalence numbers that are larger than they should be.

In a similar vein, studies that estimate allergy prevalence using questionnaires usually produce somewhat inflated numbers as people can self-report allergies that they do not actually have, although robust studies will use certain criteria to evaluate respondents’ answers and determine whether their symptoms suggest an allergy or not.

IgE-mediated allergy

A 2016 analysis of 61 studies put the worldwide prevalence of shellfish allergy between 0% and 10.3%, depending on the method of diagnosis; when researchers used self-reported questionnaire-based methods, the percentage was relatively high and where food challenges were used, the prevalence of shellfish allergy was between 0% to 0.9%.

Prevalence numbers tend to be higher in countries where shellfish makes up a larger part of the diet; namely Southeast Asia, possibly South America (which is short on data) and coastal countries everywhere.

A review of 27 shellfish-related studies reported that around 0.01% of Europeans had been diagnosed with an allergy to shellfish during their lifetimes (the lowest prevalence among the ‘Big 8’ allergens), with previous research finding that shellfish allergy in adults and children is highest in Iceland and Mediterranean countries.

A study published in 2020 investigating food allergy in children aged 6 to 10 in eight European countries revealed a physician diagnosed allergy to crustaceans in schoolchildren that was highest in Iceland (0.6%), followed by the UK (0.4%), Spain (0.4%), Lithuania (0.3%), the Netherlands (0.2%) and lastly Germany, Poland and, somewhat strangely (because of all the coastline and alleged healthy seafood eating), Greece (0%).

A British study that has followed a group of children on the Isle of Wight since birth, reported that shellfish allergy seems to show up around the age of 10 and put the prevalence at around 2 in 1000 children. A Danish study carried out in young adults found shrimp to be the second most common food allergen among 22-year-olds, also affecting around 2 in 1000. A German study reported that 1 in 1000 adults were allergic to mussels and 12 in 1000 to crabs.

In Portugal, a questionnaire-based study determined that shellfish was the leading food allergen among adults, affecting over a third (34.6%) of all food-allergic patients (and 2.1% of the entire study population of 840 adults), and a Spanish study reported that, among the food-allergic, shellfish was the third most common allergen, after fruits and nuts.

In the US, a survey of 40,443 adults carried out between 2015- and 2016 which applied certain criteria (at least 1 symptom on a list developed by an expert panel) to identify convincing self-reports of allergy reported a 2.9% prevalence of shellfish allergy, which was the most common allergy reported, affecting an estimated 7.2 million adults.

The same approach was used in a survey of parents reporting allergies in 38 408 children that found a 1.3% prevalence of shellfish allergy. Shellfish allergy was the third most common allergy reported, affecting an estimated 1 million children. The onset of shellfish allergy was, on average, around the age of 6, with allergy to crustaceans appearing a few years earlier than allergy to molluscs.

In Canada, a 2020 review of children’s electronic medical records reported a doctor-diagnosed prevalence of shellfish allergy of 0.2%, making it the seventh most common food allergen among Canadian children. A telephone survey based on the same kind of methods as the ones used in the US reported a prevalence of probable (either with a convincing history of symptoms or physician-diagnosed) shellfish allergy in children of 0.5%, with initial reactions occurring, on average, around the age of 6 and a half years old.

The prevalence of probable allergy in adults was 1.7%, with adult-onset allergy appearing, on average, around the age of 25.

An analysis of 2006 Canadian Census data focusing on vulnerable populations reported a 1.6% prevalence of probable shellfish allergy among people with a high income and 1.3% among those with a low income, and 2% among people with a high education and 1.3% among those with a low education. The difference between the vulnerable and less vulnerable may have been partly due to underdiagnosis because of less access to healthcare sservices but, regardless of education or income level, shellfish was the number one food allergy.

In Mexico, a study of Mexican schoolchildren reported that shrimp (1.3%) and other shellfish (0.7%) triggered most of the allergic reactions and in Brazil, parents asked about food allergies in their preschooler children reported shrimp and molluscs as the most frequent offenders. However, quite a few of these reported allergies were not confirmed in subsequent testing.

In Iran, a meta-analysis of 23 studies revealed shrimp as the top food allergen in the population, with the number of people sensitised to the crustacean increasing with age.

In Kuwait, a parent survey put the estimated prevalence of shellfish allergy in schoolchildren aged between 11 and 14 at 1.3%, making it the third most common allergen in that demographic.

In Russia and India, shellfish is not commonly eaten. The 2020 EuroPrevall-INCO survey which included Russian and Indian schoolchildren aged between 6 and 10 reported that 1.8% of Russian children and 10.3% of Indian children were sensitised to shrimp but that the rate of probable food allergy in Russian children was 0.02% and 0% in Indian children.

The 2016 EuroPrevall-INCO survey of Indian adults and children had similar results; although the sensitisation to shrimp was estimated at around 15.5% of the population, the prevalence of shellfish allergy was put at 0%. A possible cross-reaction with house dust mite was put forwards to explain the discrepancies.

The prevalence of shellfish allergy in the Asia-Pacific region tends to be higher than in other parts of the world, in part because saltwater and freshwater shellfish figures more often in people’s diets and it’s introduced early to infants, at around 12 months. In Singapore, for example, the rate of sensitisation to shellfish is as high as 10.6% among atopic children younger than 3 years old. Additionally, shellfish can be eaten raw and is available in a more extensive variety of foods, such as fermented fish sauce, shrimp head and roe, exposing people to allergens that may not be problematic in other parts of the world.

The idea that eating habits may be to blame for the higher prevalence of shellfish allergy in this part of the world is supported by a study of children living in Singapore that found that shellfish allergy was more common in native children than in expatriates.

High exposure to cross-reacting arthropods like dust mites and cockroaches in tropical and subtropical areas of the region is also thought to increase the prevalence of shellfish allergy.

Shellfish is the leading cause of food allergy in Australia, where 0.4% of 1-year-old infants are already sensitised to shellfish and the prevalence of confirmed allergy among adolescents has been put at 0.3%.

It’s also the leading cause of food allergy among children and adults in Taiwan, preschoolers in Thailand, adults in Singapore, children and adults in Vietnam and in preschoolers and older children in Hong Kong, accounting for over a third of all reactions to food. In China, shrimp and crab have been reported as the most common food allergens in preschoolers, but China is a vast country, and shellfish is not always the top allergen in this group, although it can generally be found in the top 3.

Shellfish allergy is less common in East Asia; In Japan, crustaceans are only the 7^th most important allergen, accounting for 3.4% of all reported food allergy cases. In South Korea, it accounts for 0.84% of parent-reported shellfish allergy in schoolchildren, which makes it the second most commonly allergenic food group, behind fruit.

Most of these studies have tested for and reported allergy to crustaceans (with shrimp being the designated representative crustacean). Data on mollusc allergies are rare and data based on actual testing rather than self-report is ever rarer. Very broadly speaking, confirmed mollusc allergy varies from a low of around 0.05% in Northern European adults to a high of around 1.5% in Asian adults.

In fact, because fewer people seem to have an allergy to molluscs than to crustaceans, for a long time mollusc allergy was suspected to exist only as a secondary allergy—a cross-reaction resulting from a crustacean allergy. But a recent study has convincingly demonstrated that molluscs are perfectly capable of provoking a strong allergic reaction without any prior sensitisation to crustaceans.

Roughly-speaking, it seems that, if you’re allergic to shellfish, you’re about twice as likely to react to crustaceans than you are to react to molluscs.

Among American adults, 1.6% have a probable allergy to molluscs (compared with 2.4% to crustaceans) and among American children 0.5% have a probable allergy to molluscs (compared with 1.2% to crustaceans). Children tend to be younger when they become allergic to crustaceans than to molluscs (perhaps reflecting when the food appears in their diet); while the prevalence of crustacean allergy peaks in children between the age of 6 and 17 years old, that of mollusc allergy peaks a little later, between the age of 14 and 17. Children are also more likely to be allergic to just crustaceans than to both types of shellfish.

An allergy to crustaceans among Australian children is also more common than an allergy to molluscs, with a 2011 study reporting that only 5% were allergic to molluscs in general, compared to 27% who were allergic to prawn.

In Vietnam, young children in the southern province of Tien Gang have a doctor-diagnosed prevalence of 1.36% for molluscs versus 2.8% for crustaceans. Among university students, the doctor diagnosed rate of mollusc allergy is 1.27% compared to 2.95% for crustaceans.

As for which types of mollusc, that depends on where you live; in northern Spain, cephalopods are the most allergenic category, with squid being the biggest trigger of allergic symptoms, although limpets deserve a special mention for being the third most important trigger of symptoms, even though they’re not that popular on the menu.

In Texas, oyster, clam and scallops are the top three triggers of symptoms among the mollusc-allergic, with squid, octopus and mussels tying for fourth place.

Finally, an American survey reported that, although people living near the coast were more likely to report an allergy to crustaceans, people living inland were more likely to be allergic to molluscs, possibly because they have a greater tendency to cook their seafood, which can increase the allergenicity of certain types of shellfish like scallops.

Non-IgE-mediated and mixed allergies

Specific prevalence information for non-IgE-mediated conditions is more hard to come by.

About 2.6% of the global population is estimated to be affected by eczema, which is just over 204 million people. It’s a condition that’s more likely to affect young children and females, and food is thought to be a trigger in 20% to 30% of the cases, with the most common allergens being milk, egg, soy, wheat, peanut and fish.

Although food-triggered eczema affects children more than adults, quite a few adults still have the condition. The prevalence of food allergy in children with eczema is estimated to be somewhere in the range of 15% to 30% and the prevalence of food allergy in adults with eczema is thought by most experts to be between 1% and 3%, with between 9% and up to 24.5% of that number estimated to be new, adult-onset cases.

Adults with eczema tend to be allergic to different foods than young children; namely those which are cross-reactive with airborne allergens—like celery, carrots, hazelnuts and shellfish—and those which people tend to start eating when they’re older (such as fish, in Western countries).

Shellfish (shrimp) is a common food sensitisation among Asian children and adults with eczema, but research in this area is quite scarce.

Not everyone with eczema and shellfish allergy suffers from a worsening of their skin condition after eating shellfish.

A 2020 study of 100 Czech adolescents and adults with eczema reported that 6 were allergic to shrimp, and only 2 of them experienced a worsening of their skin condition after eating shrimp. The other 4 suffered from typical immediate symptoms of shellfish allergy, namely stomach pain, diarrhoea, vomiting and cramps.

Experts do not know the exact prevalence of food protein–induced enterocolitis syndrome (FPIES) but it’s estimated to occur in the general population at a prevalence ranging from 0.015% in Australia to 0.7% in Spain and reports of cases have been on the increase in recent years, either because of an increase in new cases or because of an increased awareness of the condition among doctors.

FPIES is the most well-studied of the shellfish-related non-IgE-mediated conditions. Unlike FPIES to other foods like milk and wheat, FPIES to shellfish is a condition that tends to affect adolescents and adults—women, in particular—more than young children. In fact, when it comes to adolescents and adults, shellfish—notably crustaceans—is by far the most common trigger food reported.

In a study of all cases of FPIES seen at two American hospitals between November 1991 and December 2018 among the adult-onset FPIES cases, 82.4% were triggered by fish or shellfish and all of those people were able to eat shellfish without problem until they developed the allergy, which is a puzzling characteristic among most adult patients with FPIES and makes it difficult to identify what (if anything) provokes adult-onset FPIES.

Although research on FPIES is gathering pace, prevalence data is almost non-existent. A study using data from a large survey of American households carried out between October 2015 and September 2016 reported an adult prevalence of 0.2% so, if we assume that seafood is the trigger food in most American adults, we can very loosely take that number as representing the prevalence of FPIES to seafood in American adults.

Cases of eosinophilic oesophagitis (EoE) have been reported to be on the increase since the turn of the century, probably because the condition is better recognised. EoE is now thought to affect 1 or 2 people in 2000 but, in people who have food allergies, the number is more like 1 in 20. EoE to a food often develops in someone who already has a standard, IgE-mediated allergy to that food.

Eosinophilic oesophagitis is more common in males and can occur at any age, but it becomes more common as people get older, peaking in adults aged between 30 and 50.

Shellfish is associated with EoE, but it is a much less frequent trigger of this type of disease than it is of IgE-mediated allergy, especially when it comes to children, and especially in areas of the world in which it is less frequently on the menu.

So, for example, it is an uncommon trigger of EoE in American children, and one study reported that it was the trigger for just 1 in 10 American adults with EoE. However, in Spain, where seafood is more commonly on the menu, one study has reported that fish and shellfish were responsible for 4 in 64 (6%) of the cases of EoE among children and adults and another study that focussed on adults reported it as the cause of around 1 in 5 (19%) of the cases.

Finally, a recent study examining the associations between IgE-mediated allergy and eosinophilic oesophagitis also reported that the prevalence of EoE in people with IgE-mediated food allergy is higher than the rate of EoE in the general population, and that children who have an IgE-mediate allergy to milk, eggs and/or shellfish are most at risk of being diagnosed with EoE.

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Will it go away?

IgE-mediated allergy

Because of our eating habits, shellfish allergy tends to affect adults and adolescents more than younger children in Western countries,

According to the reports of American parents, the average age at which their shellfish-allergic children have their first reaction is around 6 years old. However, shellfish allergy is generally more of an adult problem. The rate of shellfish sensitisation peaks among adults aged 20– 59 and shellfish allergy is the most common type of adult onset allergy, responsible for about half (54%) of all new allergies detected in adults. The age of onset can vary widely—anywhere between 18 and 86 years old—with most adults having their first reaction in their late 20s or early 30s.

Shellfish allergy rarely goes away; just 4% of people allergic to shellfish reported outgrowing their allergies in an American phone survey carried out in 2004, and a 2015-2016 survey of American adults found shellfish allergy to be a particularly enduring type of food allergy, reporting an estimated prevalence of shellfish allergy of 2.8% among 18- to 29-year-olds and an estimated prevalence of 2.6% among people 60 years or older. This is a lower rate of decrease across the life span than observed for other food allergies. Whether this is indeed because of an attribute of shellfish allergy, a greater awareness of shellfish allergy and/or other factors remains to be determined.

The prevalence of shellfish allergy in Western children also seems to be increasing; two decades ago, a survey reported a prevalence of 0.5% in American children, then one carried out a decade later reported a prevalence of 0.87%. More recently, a survey carried out between October 2015 and September 2016 revealed a prevalence of shellfish allergy in 1.3% of American children. It also noted that the average age of the first reaction to shellfish was younger than previously acknowledged, possibly because more children are being exposed to shellfish at an earlier age.

Research carried out in Western and Middle Eastern countries suggests that children with a shellfish allergy have less chance of developing tolerance than children with other most of food allergies.

A Canadian study that followed a group of 26 people with shellfish allergy for anywhere between 2 to 8 years reported only 1 case of resolution. The team of researchers then calculated that, having followed the shellfish-allergic for a total of 124.7 person-years, the resolution rate was 0.8% per person year for shellfish. I don’t know what that actually means in any concrete sense, but I do understand what they mean when they say that their ‘findings indicate a low resolution rate for seafood allergy.’

By contrast, Thai research seems to suggest that up to half of the shellfish-allergic may grow out of it, with a study on 14 young adults finding that, 10 years after a positive oral challenge, 8 (57%) of them were no longer allergic. Avoiding eating shrimp in the meantime did not seem to help in the resolution of their allergy—in fact, those who had reintroduced shrimp into their diet were more likely to acquire tolerance.

A follow-up study which included 3 more people from the original group determined that the tolerance level was 46% 10 years after initial diagnosis. Another predictor of the development of tolerance was a smaller skin test reaction.

Ultimately, the news for shellfish-allergic Westerners may not be quite as bad as the previous studies suggest; one study that used the blood of 53 Spanish and American shellfish-allergic children and adults to examine the reaction of the IgE antibodies to different shellfish allergens found that children with shrimp allergy had higher specific IgE antibody levels and recognised a greater number of shellfish allergens and epitopes (the surface part of the allergen that the antibody attaches itself to) more intensely than the adults, suggesting that, over time, reactivity to shellfish may decrease and the allergy could be lost.

Non-IgE-mediated and mixed allergies

The outlook is not as good for non-IgE-mediated conditions.

People with eczema caused by shellfish are less likely to outgrow their condition than people with eczema to other foods like egg, milk, soy and wheat.

The story is the same for FPIES. An Italian study of 70 children with acute FPIES to seafood reported that the age onset was later than that of FPIES to other foods, and it also took longer to outgrow. If, indeed, it was outgrown at all.

As for adults, age of onset is also quite late; around the mid-30s, A Spanish study of adolescents and adults reported that symptoms were ‘commonly persistent’ and an American study didn’t report any resolution for the subjects in their study with FPIES to shellfish. A recent study from Spain is slightly more reassuring, however, stating that ‘Some patients may outgrow adult-onset FPIES.’

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Risk factors for shellfish allergy

As with all food allergies, a risk factor for developing shellfish allergy is atopy; the predisposition to develop allergies. A French study looking for cases of allergy among people working with seafood found that half of the affected adults were atopic, and an Australian study investigating seafood allergy among children noted that 94% had another atopic disease.

American research investigating adults and children with food allergy has found that the shrimp-allergic are more likely to have doctor diagnosed asthma, perhaps due to an allergy to house dust mites, and investigations of children with shellfish allergy have found that they are also more likely to have hay fever as well.

Working in the seafood industry puts people at particularly high risk of developing an allergy to shellfish—especially crustaceans—as workers are constantly exposed to allergens in the air while the shellfish are processed.

These include not only seafood (primarily muscle) allergens, but also allergens from the exoskeletons, gills and internal organs as the shellfish are cleaned, steamed, washed, sawed, cracked and crushed, releasing multiple types of allergen in the form of dust, steam and vapour. IgE antibodies in the blood of workers at snow crab processing plants have been shown to react to allergens in both crab meat and crab cooking water.

Around 1 in 5 Norwegian employees working in 5 plants processing king and edible crab were recently found to be sensitised to the crustaceans that they were handling.

A relatively old study of British workers in a prawn-processing factory reported that nearly 2 in 5 of those exposed to the airborne allergens had developed wheezing and other respiratory symptoms.

A recent study of 382 Greenlandic seafood processing workers found that 18.1% were sensitised to to snow crab and 13.6% to shrimp. 5.5% of the workers had occupational asthma and 4.6% had occupational rhino conjunctivitis (hay fever symptoms). The study also noted that the longer a person had spent processing snow crab, the more likely they were to be sensitised to it.

These numbers are similar to those reported in an older study of Canadians seafood processors, which also demonstrated that molluscs—in this case, clams—are less allergenic than crustaceans; although both clam and shrimp were handled just as much as each other, the prevalence of sensitisation was lower to clam than to shrimp (7% to 14%, respectively).

Smoking while working in the shellfish processing industry has also been shown to increase your chances of developing an allergy to shellfish, especially snow crab (although that could just be where all the studies took place)

Being a woman in the seafood processing industry has been linked to a higher risk of developing symptoms to shellfish, as well, although some researchers have pointed out that this may be partly due to the kind of jobs women tend to have as well as the fact that they tend to be in them for longer than men.

Job sorting aside, however, being a woman has also been linked to a higher risk of having adult onset shellfish allergy in general, reversing the sex trend which sees more boys developing allergies during childhood.

Supporting the data on IgE-mediated allergies, being a woman is also a risk factor for developing the non-IgE-mediated conditions of atopic dermatitis and FPIES to shellfish; case reviews in Spain, Canada and the US have all found that a large majority of new cases (around 3 in 4) of FPIES among adults are diagnosed in women.

The reasons behind women being more prone more to developing allergies than men during adulthood are unknown, but the influence of sex hormones is strongly suspected.

And a study carried out among children younger than 14 in Hong Kong reported that girls were more likely than boys to have allergic reactions to shellfish (and fruit) so, in that part of the world at least, being female seems to be linked to a higher risk of shellfish allergy at any age.

Multiple American studies suggest that being non-white is a risk factor for shellfish allergy. The first study to point this out was a 2004 telephone survey on allergy among American children and adults, which found that the highest rates of seafood allergy were reported by black subjects, although the rate of physician-diagnosed shellfish allergy wasn’t much higher than white subjects (40% vs 38%, respectively).

However, around the same time, the National Health and Nutrition Examination Survey (NHANES) reported a link between shellfish allergy and ‘non-Hispanic black race/ethnicity’ and, a few years later, a study looking into food allergy in ‘urban minority children’ also concluded that black children had significantly higher rates of shellfish allergy than their counterparts,

This was corroborated by a study examining all cases of children who had reported to Texas Children’s Hospital Allergy and Immunology Clinic over 11 years and another one examining cases of food-allergic children who went to the allergy clinics of Rush University Medical Center (RUMC) and Cincinnati Children’s Hospital Medical Center (CCHMC) over a 6-year period.

A review of intensive care admissions between 2010 and 2015 involving American and Canadian children also reported that shellfish was the most common trigger of anaphylaxis among black and Hispanic children.

A 2015-2016 survey that included 38,408 American children added Hispanic and Latino children to the list of young people who were more likely to be allergic to shellfish than white children. The study also found no difference in the odds of having an allergy to shellfish among households with different incomes, suggesting that race was a stronger predictor of the development of food allergy than socioeconomic status.

It’s a similar picture among adults. An analysis of the medical records of all patients entered into the system of ‘a large health care organization’ in Boston between 2000 and 2013 found that blacks and Asians were more affected by shellfish allergy than whites and Hispanics.

A 2015-2016 survey of 40,443 American adults reported that ‘adults reporting non-White races/ethnicity’ were more likely to have shellfish and crustacean allergy than their white counterparts. It also found that black crustacean-allergic adults were also likely to have more severe reactions.

This finding was corroborated by the Study of Asthma Phenotypes and Pharmacogenomic Interactions by Race-Ethnicity (SAPPHIRE), which found that African Americans were more likely to report allergies to food and to have anaphylactic reactions than European Americans, with the largest driver between the group differences being shellfish allergy and shrimp allergy in particular. The only foods that were more of a bother to white Americans were gluten and posh fruits like avocado and papaya. The report also mentioned ‘that socio-environmental determinants may play a role in these disparities’.

One of these ‘socio-environmental determinants’ may be where many black people live. A team of researchers on the Food allergy management & Outcomes Related to White and African American Racial Differences (FORWARD) project has also noted a higher rate of shellfish allergy and asthma in black children than in white children, and has also found that black children are more likely to be sensitised to cockroach and house dust mite, both of which contain tropomyosin proteins that cross react with shellfish. They point to research showing that people who live in inner city areas, where many African Americans are located, tend to have a higher sensitisation to cockroach, and think that this might explain the link between being African American and being more likely to have an allergy to shellfish.

Which brings us neatly to the last risk factor: the creepy-crawlies in our environments. One of the reasons that shellfish is thought to be the most common food allergen in Asia is the omnipresence of insects like dust mites and cockroaches in the environment. Proteins found in these insects are regularly breathed in by people living in the region and are believed to cross-react with proteins in shellfish, making them potential sensitisers for shellfish allergy, much in the same kind of way that birch pollen in Northern Europe sensitises thousands of people living there to an array of fruits and nuts.

Supporting this idea is the fact that research has linked shellfish and house dust mite sensitisation in schoolchildren and adults living in southern China, adults living in mainland Malaysia, adults living in southern India and Iranians living in coastal cities.

Asians are not the only people to suffer from concomitant shrimp and dust mite sensitisation; a similar link has also been discovered in Norway, Poland, Italy, Algeria, Canada and Colombia.

Similarly, a link has also been noted between house dust mites and molluscs, namely snails in the snail-eating countries of France, Italy and Portugal, as well as the Netherlands, and limpets in mainland Spain and the Canary Islands.

A correlation has also been found between shellfish and cockroach sensitisation in children living in rural China, children living on the island of Singapore and adults living in mainland Malaysia.

Having a high exposure to cockroaches in the home has also been linked to higher shrimp sensitisation in American inner city children living in New York, St. Louis, Washington, D.C., Baltimore, Chicago, Cleveland and Detroit. In the Canary Islands, the predominance of sensitisation to house dust mite has been blamed for the high prevalence of both shellfish and cockroach allergy in adults.

Finally, link between a sensitisation to shellfish and cockroach and moth has also been reported in allergic people in Southern China.

In fact, there are so many examples of people living with co-allergies to insects—notably house dust mite and/or cockroach—and shellfish that some experts are wondering whether the life-long shellfish allergy that so many people seem to suffer from is due to a booster effect from the cross-reacting insect allergens that they’re exposed to in daily life.

Image by Nadine Doerle on Pixabay

Cross reactions to shellfish

Technically-speaking, a person can be allergic to a shellfishand another food (or foods, or aeroallergen(s)) either by cross-reactivity—the immune system mistakes the proteinin one allergen for aprotein with a similar structure inthe other—or by an independent sensitisation to each food and/or aeroallergen(a co-sensitisation or co-allergy), in which case the immune system has developed specific IgE antibodies against each allergen. It can be difficult to determine whether reactions are caused by cross-reactions or co-allergies,but the end result is the same; problems, problems.

Shellfish is considered to be one of the most cross-reactive food families, and this cross-reactivity hampers efforts to both estimate the true rate of allergy to different types of shellfish and its diagnosis.

Crustaceans are the most allergenic group and the evolutionary family tree places them closer to insects and arachnids than to molluscs, something which is reflected in the reactions of the shellfish-allergic; basically, people who are allergic to crustaceans like shrimp are more likely to suffer cross-reactions with other crustaceans (e.g. crab) and arachnids (e.g. house dust mites) and insects (e.g. cockroaches) than with molluscs (e.g. squid or snails) or fish.

Between shellfish

The major cause of cross reactivity between shellfish and insects are the proteins tropomyosin and arginine kinase (AK). Although AK is thought to play a major role in cross-reactivity between shellfish and edible insects, the similarity between mollusc and crustacean AK is quite low and, of the two, tropomyosin is by far the better researched.

The similarities between tropomyosins of the different shellfish species reflect the evolutionary family tree. Crustacean tropomyosin shows up to 98% amino acid sequence homology—i.e. 98% of the amino acids in the chains that make up the tropomyosin proteins in different crustaceans are in the same order.

Molluscan tropomyosin shows a high level of amino acid sequence homology within each class; cephalopods show 91–100% homology, bivalves, 70–100% and gastropods, 85–91%. Similarities between molluscan classes is lower—between 68 and 100%—and the amino acid homology between crustacean and molluscan tropomyosin is even lower, between 56–68%.

As a rule of thumb, a homology greater than 80% is regarded as potentially cross-reactive.

Research at the molecular level also suggests that this high level of similarity in the amino acid sequences results in a protein with a similar 3D structure, which would also increase potential cross-reactivity.

A study which compared shrimp tropomyosin epitopes (the surface part of the allergen that the antibody attaches itself to) with those of other shellfish found that around 91% of the epitopes were conserved between crustacean tropomyosins. By contrast, less than 20% were shared between shrimp and the different molluscs classes (which was less than a third of the epitopes shared between shrimp and insects), which probably accounts for the lower rate of cross-reactivity between those shellfish. Of all the molluscs, cephalopods (e.g. squid, octopus) were shown to have the greatest chance of cross-reacting with shrimp.

The similarities between shellfish arginine kinase were even fewer, but showed the same type of pattern: more similarities between shrimp/crustaceans and insects than between shrimp/crustaceans and molluscs.

Ultimately, what all this tells us is that someone with an allergy to a crustacean like shrimp has a relatively high chance of being allergic to another crustacean, but is quite likely to tolerate molluscs. And even when they are allergic to a mollusc in one class, like squid (cephalopod), they are probably not allergic to molluscs in the two others, such as clams (bivalve) or snails (gastropod).

Most studies on cross-reactivity are based on results obtained using the blood of allergic people and seeing how it reacts to food allergens in test tubes—so-called in vitro studies. These types of studies tend to suggest very high levels of cross-reactivity among and between crustaceans and molluscs.

These lab-based results, however, are often not matched by actual reactions of people eating shellfish—so called in vivo reactions. In fact, people who are allergic to crustaceans often tolerate molluscs and vice versa, probably because of the differences in the tropomyosins between each class of shellfish and the fact that each class has its own specific proteins.

Overall, the (rather limited) in vivo data tells us that it’s more common to be allergic to crustaceans only than to be allergic to both crustaceans and molluscs. About 3 in 4 people with a crustacean allergy tend to react to more than one type of crustacean. Just under half (45%) of people with a crustacean allergy are also allergic to molluscs.

The proportion of people with mollusc allergy who are also allergic to crustaceans is much larger. Among the American mollusc-allergic, 70–80% children and adults also experience allergic reactions to crustaceans, the most common co-allergy being an allergy to shrimp (probably because that’s the most popular edible crustacean).

Just over 1 in 10 people allergic to any shellfish are thought to be allergic to both crustaceans and molluscs.

Cross-reactivity between molluscs is less well researched but the studies that have been done suggests that it is less common than cross-reactivity between crustaceans. According to one (old) American telephone-based survey, around 50% of people allergic to molluscs reported reactions to more than one species of mollusc.

Research among people actually diagnosed with an allergy reveals a much lower number. A Texas-based study of 159 adults with seafood allergy found that just 16.7% were allergic to more than one type of mollusc. Similarly, a study of 45 patients from northern Spain with mollusc allergy found that only 7 (15.5%) of them were allergic to several classes of mollusc (cephalopods or bivalves) and only 3 were allergic to several molluscs from the same group. Lab tests revealed a high level of reactivity between cephalopods and bivalves, however, and the authors of the study speculated that the fact that they had found such low numbers of people allergic to both groups might have been because, when they discovered their allergy for one type of mollusc, they simply avoided them altogether. There was no cross-reactivity detected between gastropods and cephalopods or bivalves; basically, if you can’t eat the escargots, you can still probably order the calamari or the oysters.

A panel of European experts who examined the available data on mollusc cross-reactivity concluded that it was limited to a small number of species, and often unpredictable.

After all this talk of cross-reactions, it’s good to remember that it’s possible to be allergic to one type of species of shellfish and not to others, a so-called species-specific allergy. This type of allergy has been demonstrated to both crustaceans—like shrimp, crab and lobster—and molluscs—such as razor shell and clam.

Species-specific shellfish allergy happens when someone’s immune system recognises a species-specific epitope of a major allergen, or when they are allergic to a species-specific minor allergens like haemocyanin.

Although an allergy to a specific shellfish may be quite unusual, it’s not vanishingly rare; for example, a study carried out in Bangkok Thailand found that roughly around 1 in 5 children were specifically allergic to either giant freshwater shrimp or marine tiger shrimp.

Between shellfish and fish

Although fish and shellfish are both called seafood, these two groups are very different in evolutionary terms and they contain different types of food allergens; while the major allergen in shellfish in tropomyosin, in fish it’s parvalbumin. As a result, they are much less likely to cross-react with crustaceans and molluscs than insects and arachnids, which are evolutionary closer to shellfish.

That said, fish do contain tropomyosin and, although fish tropomyosin has historically been regarded as non-allergenic, recent research suggest that the tropomyosin from some types of fish can cause problems for people with fish allergy, and might be responsible for the gastrointestinal complaints that some people have after eating fish.

Their potential to cross-react with shellfish is still small, though. Their amino acids share only about 55% sequence similarity and show very little cross-reactivity with shellfish, and indeed little ability to cause reactions in general, whether in raw or boiled form.

That said, American research has revealed that people can be allergic to both types of seafood. A 2004 phone survey carried out among American households, for example, revealed that 0.2% of those surveyed reported an allergy to both fish and shellfish and, 15 years later, the percentage of self-reported fish allergy among shellfish-allergic American adults was revealed to be 18.4%, which seems rather high, but most self-reported numbers usually are.

A review of 13 years’ worth of medical records of adult patients with confirmed seafood allergy seen at the Texas Medical Center revealed that just 6% were allergic to both fish and shellfish.

In Australia, a study of 167 seafood-allergic children reported that at least half of the crustacean-allergic children could tolerate fish and only 5% of the fish-allergic children showed symptoms when they ate crustaceans.

Research suggests that the mollusc-allergic may be more likely to react to fish than the crustacean-allergic; about 1 in 3 American children with an allergy to molluscs (versus about 1 in 4 with an allergy to crustaceans) and around 1 in 5 mollusc-allergic adults, This finding has recently been corroborated by a study of young Vietnamese adults which also found that a co-existing fish allergy was more likely among the mollusc-allergic. People in this group were also more likely to have an allergy to peanuts and nuts and other foods compared to those who were only allergic to shellfish.

Despite the relatively high numbers of potential cross-reactions revealed in those surveys and reviews, however, there are very few case reports of actual cross-reactions. In 2018, doctors at a Portuguese children’s hospital reported a case of cross-reactivity between shrimp and various types of fish in an 11-year-old boy. He had been admitted to the emergency department after eating a shrimp patty for the first time. His case history included mild reactions to different white fish and his blood tests were positive for antibodies to a variety of shellfish and fish. Further blood tests showed that he was reacting to tropomyosin and not to parvalbumin. His doctors concluded that he was primarily sensitised to shellfish tropomyosin and that the previous, mild reactions he had had to fish were due to shrimp tropomyosin cross-reacting with fish tropomyosin.

In 2019, doctors at the Texas children’s hospital reported a case of potential cross reactivity between shellfish and anchovy in a 16-year-old girl who had had three allergic reactions all involving hives and swelling. Whereas the first reaction involved only shrimp, the last two involved both fish and shellfish and included shortness of breath and wheezing. Her blood tests showed that she had IgE antibodies to both anchovy and various shellfish and her doctors suspected that her positive IgE levels to anchovy might be due in part to cross-reactivity with shellfish.

More research is needed before the idea of cross-reactivity between fish and shellfish is convincingly demonstrated to be a phenomenon that’s not highly unusual.

Between shellfish and arachnids, insects & nematodes

Mites, insects and, to a much lesser extent, Anisakis simplex have a relatively high likelihood of being involved in cross-reactions with shellfish, often because of the pan-allergen tropomyosin, although lesser-known allergens are also implicated.

Cross reactions between shellfish and house dust mites are quite common, especially in environments with high humidity, in which mites thrive. In those kinds of surroundings, house dust mites have been shown to sensitise infants, children and adults to shellfish (notably, shrimp) without those people ever having eaten any. It’s such a common phenomenon that some scientists refer to the house-dust mite-crustaceans-molluscs-syndrome. But not many, because it’s a bit of a mouthful.

Sensitisation to shrimp via inhaled house dust mite allergens rather than through the gut may also explain why people tend to be older when they become allergic to shellfish and why the majority of the shellfish-allergic tend to suffer from oral allergy syndrome.

The fact that one’s immune system can become sensitised to shellfish purely because of cross-reacting allergens in mites also explains the presence of specific IgE to shrimp in Orthodox Jews (who do not eat shellfish) and vegetarians.

This also works the other way around: the sensitisation to house dust mites found in about 9% of young adults from Iceland, a country in which house dust mites are rarely found, is thought to be due to a sensitisation to shrimp.

There is also a relatively big chance of cross-reactions between house dust mites and molluscs, primarily snails and gastropods in general (such as periwinkles and abalone). One French study reported that most cases of snail allergy appeared to be cases of secondary sensitisation in people who were primarily allergic to house dust mites. Another one found that around 4 in 5 snail-sensitised children were also sensitised to mites and that about 1 in 3 mite-sensitised children were also sensitised to snails (and had never eaten any). However, there are always exceptions to prove the rule.

A link has also been noted between mite sensitisation/allergy and squid allergy, barnacle allergy and limpet allergy.

In many of cases of shellfish and house dust mite cross reactions, tropomyosin is the culprit cross-reacting pan-allergen. Tropomyosins from house dust mites show an amino acid sequence similarity ranging between 81 and 83.5% with crustaceans, but less than 66% with molluscs, which is why there are fewer reports of cross-reactions involving mites and molluscs.

However, because the main culprit allergens differ between the different allergies, more people who are allergic to shrimp tend to be sensitised to mites than the other way around. People allergic to shrimp (or other crustaceans) are often allergic to shrimp tropomyosin, which makes them more likely to react to the tropomyosin in house dust mites. However, compared to shellfish, mites actually contain very little tropomyosin and people who are allergic to house dust mites tend to be allergic to another allergen in the mites, which is less likely to cross-react with the equivalent allergens in shrimp.

A Canadian study of 95 shrimp-allergic people found that 86 (90.5%) were sensitised to house dust mite. The higher their level of IgE antibodies to shrimp, the more likely they were to also be sensitised to mites.

Conversely, a lot fewer people who are primarily sensitised to house dust mite show secondary sensitisation to shellfish. However, when they do, quite a few tend to be symptomatic when eating shellfish. For example, a study from Colombia examined 229 people allergic to mites and found that 48 (21%) were sensitised to shrimp. Of those, a quarter (25%) showed symptoms.

An Algerian study examined 446 people allergic to mites and found that 91 (20.4%) were sensitised to shrimp. Of those, almost two thirds (58, or 63.7%) reacted to shrimp (and, of those, almost two thirds had symptoms limited to itchy mouths).

People allergic to both shrimp and mites are very likely to be sensitised to tropomyosin and, therefore, to be sensitised to other shellfish. In the previous study, 95.5% were also sensitised to crab and 89.5% to squid, although none had ever eaten any.

However, ubiquitous though it may be, tropomyosin is not the major allergen in every shrimp-allergic population. In a Polish study, for example, researchers analysed the blood of 35 people allergic to house dust mite who also had high levels of IgE antibodies to shrimp and found that only 11 (31.4%) of them were sensitised to tropomyosin.

An Austrian study found that only 23 of their 67 (34.3%) shrimp-allergic subjects were also sensitised to house dust mite, leading the researchers to question the close causal relationship between house dust mite sensitisation and shellfish allergy. They may have been looking at the wrong kind of dust mite allergens; research has found that cross reactions between shellfish and mites could be due to unknown allergens, whose involvement may also differ depending on the climate.

In one unusual case report, a 30-year-old mite-allergic Spanish man was described as developing a rare selective allergy just to lobster, due to a cross-reactive allergen that was not tropomyosin. The authors of the report proposed that there could be 2 sensitisation profiles to shellfish in people allergic to mites: one in which tropomyosin is involved as a panallergen, resulting in allergies to several types of shellfish, and one in which other allergens are involved and result in a selective sensitisation to one kind of mollusc or crustacean.

Neither is tropomyosin necessarily the most important allergen when it comes to cross-reactions with molluscs (whose tropomyosin is quite different to that of crustaceans and house dust mites); several studies of mollusc-allergic people have noted its minor role as the cross-reacting allergen in their subjects and have instead found a wide spectrum of possibilities and noted that no single allergen is responsible for the cross-reactivity.

Cockroaches have been suspected of causing both asthma and cross-reactions with shrimp for quite a long time.

Tropomyosins from cockroaches share a high amino acid sequence similarity to house dust mites (around 81%) and shrimp (82%) and other allergens like myosin light chain arginine kinase and hemocyanin are also similar enough to be potentially cross-reactive.

In China, the higher sensitisation to shrimp among children living in rural areas is thought to be driven by sensitisation to cockroach.

In the US, puzzlement as to why poor black people seemed to be at higher risk of sensitisation to shellfish led to a study which determined that inner city children with asthma who were exposed to cockroach allergens floating around in the bedrooms and TV rooms were more likely to be highly sensitised to cockroaches and, as a result, to shrimp.

A separate study analysing American National Health and Nutrition Examination Survey (NHANES) data also linked a decrease in sensitisation to shrimp observed between 1988–1994 and 2005–2006 with a decrease in sensitisation to cockroaches, reinforcing the probable link between cockroach and shrimp sensitivity.

Edible insects like mealworms, silkworms, crickets, moths and dragonflies are widely consumed in some Asian countries and their potential as eco-friendly meat substitutes is increasingly being recognised in the West; mealworms, crickets and locusts were recently authorised to be used as food in Europe. However, a recent study investigating the sensitisation of people allergic to shrimp found that mealworm, giant mealworm, lesser mealworm, house cricket, African grasshopper, large wax moth and black soldier fly all have the potential to cause problems for people allergic to shrimp and, therefore, probably other crustaceans, although the digestive process may potentially reduce their ability to cause allergic reactions.

A 2016 study found that the majority of shrimp-allergic people seemed to be allergic to mealworm, too; 15 shrimp-allergic subjects underwent a food challenge and 13 failed it. Symptoms ranged from oral allergy syndrome and hives to stomach cramps, vomiting and difficulty breathing.

A case report from around the same time described four cases of people who developed allergic respiratory symptoms during professional or domestic mealworm breeding. One them was also sensitised to house dust mite and another to shrimp, although they could consume shrimp without any allergic reaction.

Another report described the case of a shellfish-allergic man who had downloaded a recipe from the Internet, sautéed about 30 cicadas in some butter and garlic and had subsequently suffered an itchy all-over body rash.

A recent study evaluating the risk of potential allergy to cricket in patients with shrimp allergy found a strong correlation between shrimp- and cricket-specific IgE levels, suggesting that crickets have the potential to induce allergic reactions in the crustacean-allergic. Another one warned that people with crustacean shellfish allergy were at risk of life-threatening anaphylaxis after eating crickets. Indeed, anaphylaxis to grasshopper has been reported in people who are allergic to dust mite, cockroach, and crustaceans. Arginine kinase (AK) and hemocyanin (HC) have been identified as the potential cross-reactive allergens.

A recent study carried out on Italian children who had had anaphylactic reactions to shrimp found that all of them were sensitised to one insect or another. The authors of the study warn that the rising popularity of ‘novel foods’ containing insect flours, such as yellow mealworm and cricket, could pose a risk for people with shrimp allergy.

That said, anyone suffering from shellfish allergy who wants to adopt a more eco-friendly diet might like to known that the cross-reactivity of shrimp and different insects is variable, and mealworm, superworm and waxworm seem to be less allergenic than other insect species.

Finally, the potential for the parasitic nematode Anisakis simplex to cross-react with crustaceans has also been mentioned, primarily because of its tropomyosin which has been shown to have an amino acid sequence that is 75% identical to that of shrimp.

However, apart from one Norwegian study, in which 3 of 29 (10%) people with self reported shrimp allergy had a positive skin test to Anisakis, and 25% a positive basophil activation test, data describing this kind of potential for a co-allergy is thin on the ground.

As for cross reactions with molluscs, based on one study reporting a reaction in someone who was allergic to Anisakis and shellfish and who experienced an anaphylactic reaction to white fish, a European scientific panel looking into food labelling with regards to mollusc allergy wrote that ‘Individuals reacting clinically to fish because of Anisakis infestation may also react to molluscs, which would appear as fish–mollusc cross-reactivity.’

It’s entirely possible that a lot of these kinds of cross-reactions have been hidden by the horrible case of food poisoning that the parasite itself is probably going to give anyone who accidentally eats it.

Image by Joshua Chekov on Unsplash

Symptoms of shellfish allergy

Shellfish allergy can be IgE-mediated, non-IgE-mediated or mixed; a combination of both. These variations generally present different types of symptoms.

Immediate reactions to shellfish

Immediate allergic reactions are caused by IgE antibodies. These antibodies bind to certain immune system cells—mast cells and basophils—and trigger the release of histamine and other inflammatory chemicals that cause the characteristic symptoms of allergy.

Immediate reactions are the most common type of allergic reaction to shellfish and they range from superficial rashes to life-threatening anaphylaxis. Reactions are different for different people and they can also be different for the same person, varying in severity from episode to episode.

There are three main routes to developing an IgE-mediated allergy to shellfish, and the route taken affects the type of allergy that you get and the principal symptoms that go with it.

1. You can become allergic to shellfish via gastrointestinal sensitisation—i.e. by eating it. This tends to produce the ‘classic’ signs of allergy.

In this case, sensitisation to heat- and digestion- resistant shellfish allergens are the basis of subsequent moderate-to-severe systemic reactions—i.e. reactions that spread from a localised area on the body to the whole body, often involving more than one organ—when small amounts of (processed) shellfish products are eaten.

The most common symptoms by far involve the skin, especially among people allergic to crustaceans, and include hives, swelling (oedema) and oral allergy syndrome (OAS)—mild itching and/or light swelling confined to the lips, tongue and mouth—which is by far the most common symptom, affecting about 9 in 10 people in tropical Asia and children in western countries. Although the vast majority of Western adults suffer from skin symptoms, OAS seems to be much less common; they affect about 1 in 3 adults with a crustacean allergy and 1 in 5 with a mollusc allergy. OAS is especially common in people who are also allergic to house dust mites.

Respiratory symptoms are the second most common type of symptom suffered by the shellfish-allergic. They include throat tightness, shortness of breath, coughing and wheeze.

Studies have shown that the steam of boiling shrimp or squid can induce asthmatic reactions in children and in adults when they’re at home or while taking part in clinical studies. Although people with food allergy often have asthma, you don’t actually have to have asthma to suffer from asthma-like symptoms to food.

When shellfish allergens are breathed in, the reactions they provoke have been reported to be worse than those produced by eating the shellfish, and can even produce rare cases of anaphylaxis.

That said, it’s not uncommon for people who react when they breathe in shellfish allergens to actually be able to eat shellfish without problems.

Sometimes there are reports of people having allergic reactions when breathing in shellfish allergens even when they have never eaten shellfish before. These people may be having cross-reactions to shellfish because of a primary allergy house dust mites.

Severe asthmatic symptoms are more likely to be reported in reactions to molluscs—namely snails and limpets—in people who are also allergic to house dust mites and cockroaches.

Hay-fever-like eye and nasal symptoms (rhinoconjunctivitis) are the least common respiratory symptoms reported by both adults and children.

Gastrointestinal symptoms, including nausea, vomiting and stomach pain, tend to be less common in shellfish-allergic children than they are in adults (affecting just over 1 in 3 children and 1 in 2 adults), and about twice as likely to be experienced by adults allergic to molluscs than those allergic to crustaceans.

Other symptoms that tend to be more commonly experienced by people with a mollusc allergy include headaches and dizziness.

Symptoms generally occur within a few minutes to a couple of hours after eating the shellfish. On rare occasions, however, people can suffer delayed onset IgE-mediated reactions—in these cases, symptoms can happen up to 8 hours after eating and have been reported after after eating shrimp, snow crab, cuttlefish, limpet and abalone.

A global review of all food allergens published in 2021 found that milk and shellfish (notably, crustaceans)were the most important causes of anaphylaxis globally; they were the biggest cause of anaphylaxis in European, American and Australasian adults and the most common cause of anaphylaxis in Asian children, but the 7^th most common cause of anaphylaxis in European, American and Australian/New Zealander children. However, they did not cause as many fatalities as nuts, milk and peanuts in Western countries, and very few fatalities in Asia.

The symptoms of acute anaphylaxis tend to develop within minutes of eating the offending shellfish and involve:

• the skin, producing hives, itching and swelling of the lips and tongue (almost always involved)
• and/or the respiratory system, producing symptoms of asthma, including shortness of breath, wheezing and bronchospasm
• and/or the cardiovascular system, with a lower blood pressure resulting in feeling of dizziness and faintness and maybe loss of consciousness (more common in adults)

Research suggests that adults are more likely to have anaphylactic reactions to shrimp than children, with nearly half (45%) of shellfish-allergic American adults reporting a visiting to the emergency department for their food allergy. However, adults of over 60 are apparently less likely to have a severe reaction than other age groups.

In contrast, ‘only’ 12% (American) (1) to 20% (Australian) shellfish-allergic children are reported as having a history of anaphylaxis. A more recent survey of American parents, however, reported that about half (47.1%) of their shellfish-allergic children had a history of life-threatening reactions. Curiously, children with eczema are apparently less likely to have a severe reaction.

Data from the American National Electronic Injury Surveillance System and a New York City pediatric emergency department show that shellfish (notably, crustaceans) are the most likely food to provoke anaphylactic reactions in children over 6 years old. Data from the North American paediatric intensive care units (PICUs, including both American and Canadian children) puts the average age of crustacean-triggered anaphylaxis among children at about 13 and a half years old. It also finds that American children in the Midwest and the South are more likely to suffer anaphylactic reactions to shrimp than children in the Northeast or in the West, showing the importance of regional eating patterns.

Don’t panic: To be clear, the official definition of anaphylaxis is probably not what you think it is.

This means that many of the cases of anaphylaxis reported in medical studies are not actually life-threatening—when dealing with an emergency, however, since it’s impossible to predict which reactions will become life-threatening, every case of anaphylaxis should be treated as if it is potentially deadly.

Crustaceans have a worse reputation than molluscs when it comes to causing anaphylactic reactions, with (extremely rare) cases of deadly reactions to minute amounts of shellfish on record as well as (less rare) cases of crustacean allergy-induced hearts attacks (a reaction otherwise known as Kounis syndrome).

That doesn’t mean that molluscs can’t be dangerous. Reactions to molluscs are often systemic and involve more than one organ system, and there are several reports of anaphylactic attacks to snails in France and Italy (often in people also sensitised to house dust mites), as well as limpets in Spain and limpets and abalone in Singapore. Ultimately, it may be that these particular molluscs are just as dangerous as shrimp and crab, it’s just that fewer people eat them.

People sensitised to the heat-stable pan-allergen allergen tropomyosin are the most likely to have severe reactions to crustaceans and molluscs.

Reactions can be very different even in the same person; in Asia, people who have reacted to prawns often continue eating them because their previous reactions have been quite mild. Some of these individuals, however, can then go on to develop a life-threatening reaction on occasion. This may be because some prawn species provoke more serious reactions than others, or it could be because they have eaten different parts of the prawn; allergens in the abdomen, for example, are often less powerful than those in the cephalothorax—the part which comprises the brain, heart, stomach and bladder. It could also be due to cofactors like exercise or medicine (see ‘Threshold for reactions’ later).

Those at greatest risk of a worse reaction are probably those who are undergoing immunotherapy treatment. Immunotherapy against dust mite allergy, for example, has been shown to provoke worse reactions in people allergic to shrimp or snails (or to cause new allergies, see Good to know section later).

For example, in 2002, 4 children who developed asthma whenever they ate snails were given allergy shots by a team of Italian doctors to try and desensitise them to dust mites. Within 8 months to 2 years after beginning their treatment, all of the children, who had initially suffered from mild symptoms when they ate snails, experienced life-threatening reactions and their skin test reactions to snails also worsened.

Another example of immunotherapy treatment resulting in a serious reaction involves a 61-year-old man who was being given a new type of immunotherapy treatment designed to help his body fight cancerous tumours. Before his fourth cycle of treatment he ate a shrimp and developed a severe anaphylactic reaction which resulted in a heart attack. He had no history of allergy but was show to have developed a new allergy to shellfish and shrimp. He recovered from his reaction and continued with his cancer treatment without further problems (or shellfish-eating).

Immunotherapy can help people with shellfish allergy (see Good to know section later), all you can really do is be aware of the possible complications if you decide to try it.

2. You can become allergic to shellfish via inhalation—i.e. by breathing allergens in. This generally results in respiratory symptoms, although it can eventually lead to the ‘classic’ symptoms of food allergy.

This route to allergy is commonly due to occupational exposure and crustaceans are one of the two foods most likely to cause sensitisation in this manner (the only one being cereal flours).

This is probably because getting shellfish ready for the rest of us to eat involves many processes that result in lots of food allergens flying through the air, including tailing lobsters, blowing prawn meat through shells, cracking and boiling crabs, opening mussels or scallops, chopping squid and the general butchering, grinding, washing, scrubbing and mincing of shellfish, as well as cleaning the processing line or storage tanks with high-pressured water hoses.

Occupational asthma has been reported in workers processing king crab, edible crab, snow crab, Atlantic shrimp, prawn, king scallop and queen scallop. Crab seems to be more likely to cause allergies than other shellfish, which may at least partly be due to the processing techniques involved, which can involve more manual work.

Air sampling in crab processing environments has found high levels of heat-stable pan-allergens like tropomyosin and arginine kinase, as well as other proteins in minute sizes capable of being inhaled. Some of the highest allergen concentrations have been found on crab-processing vessels at sea, because the processing occurs in confined spaces with poor ventilation.

The reported prevalence of asthma in the seafood industry—involving both fish and shellfish—is between 7% to 36%, and tends to be higher in those working with crustaceans. And this is bearing in mind that most of the studies have been cross-sectional and probably incorporate a healthy worker effect—the fact that workers with symptoms will probably have left the industry or moved from jobs with high exposure to low exposure to try and control their symptoms, so have not been counted in the studies.

Typically, the manifestation of occupational shellfish allergy starts with hay-fever-like symptoms and builds up to asthma-like symptoms including chest tightness, cough, shortness of breath and wheeze. They may go away when a worker is away from the job, only to come back when that person returns to work, but often remain even when a worker has not been exposed to shellfish for several months, although they may show some improvement.

Some research has found that the length of time spent in a job affects how likely a person is to get symptoms, and some has not, but chronic exposure can lead to a permanent impairment of lung function.

In some cases, classic symptoms of food allergy such as gastrointestinal symptoms may also follow. The good news is, symptoms are rarely systemic. The bad news is, they tend to reduce the quality of life of those who have them, and many people in these types of jobs tend to stay in them as long as possible (2) in order to reduce the economic impact of their illness.

With all of this in mind, high-level meetings are now taking place among industry representatives of several countries to try and come up with safety regulations that will improve the working conditions and quality of life of seafood workers.

Occupational asthma caused by shellfish is not limited to the seafood processing industry; it’s also been reported in aquaculture industry workers making food powder from shrimp and clam, cooks preparing squid and a restaurant worker exposed to too many boiling scallops and shrimp.

Shellfish has also caused respiratory allergy in at least one person working in the pet food industry, namely a 34-year-old man who worked for a company that made and packaged animal feed, including Gammarus shrimp as turtle and fish feed. After working for 2 years packing and supervising the packaging of feed, he developed symptoms including a runny nose, shortness of breath, coughing and wheezing. 6 months later, he began to suffer from an itchy mouth when he ate shrimp, so he removed them from his diet. He was eventually diagnosed with an allergy caused by inhaling shrimp allergen and quit his job. His symptoms disappeared.

3. You can become allergic to shellfish by touching it and developing a contact allergy. This generally results in rashes and can later lead to respiratory and ‘classic’ symptoms of allergy.

There are several types of contact allergy, with 2 of the most common allergy-related forms being allergic contact urticaria (CU) and protein contact dermatitis (PCD).

Allergic contact urticaria is an immediate-type allergy often caused by seafood, meat, fruits and vegetables. It’s basically like hives; an itchy, a weal and flare reaction that occurs within 10 to 60 minutes of touching the offending substance and completely resolves within 24 hours. This occurs on whatever part of the body is in contact with the allergen, often the hands in the case of food.

Other systemic symptoms including anaphylactic shock may eventually develop, by which point it’s become ‘contact urticaria syndrome‘. It can also be associated with the development of protein contact dermatitis.

Protein contact dermatitis is a term that was introduced to describe allergic (or non-allergic) eczema-like reactions to food proteins experienced by food handlers. The mechanism by which it develops is unknown, and some allergists believe that it’s an immediate-type allergy to large food molecules that penetrate damaged skin to cause IgE-mediated allergic reactions, while others believe that it’s a combination of immediate- and delayed-type allergies involving IgE antibodies, and yet others think that it may be a delayed cell-mediated reaction.

The most common symptom is chronic or recurrent eczema on the hands, wrists and arms with periods of intensified reactions and feelings of itching or tingling a few minutes after repeated contact with the allergen.

Sometimes symptoms of both contact urticaria or protein contact dermatitis can extend to the face, either because of touching with contaminated hands or because of airborne allergens.

Most cases of contact urticaria and protein contact dermatitis are caused by occupational exposure, with people who handle food being at the highest risk of developing these types contact allergies; seafood processors and delivery people, cooks, catering workers and fishmongers.

Symptoms have been reported to shrimp, prawn, crawfish, crab, squid, cuttlefish and scallops. They can start months to years after the person starts their job and tend to go away during vacations or rest days only to return immediately when the person gets back to work.

A review of English- and German-language studies published between 1 January 1990 and 31 December 2014 revealed that seafood accounted for two thirds of reports of occupational contact urticaria, most of which involved people working in the food and fishing industries.

As for occupational contact dermatitis, the reported prevalence in the seafood industry (involving both fish and shellfish) is 3–11%.

Of course, not all cases of contact allergies occur in the workplace—people who handle seafood in their own kitchens get it too—but the vast majority of the cases are occupational because of the risk factors associated with getting a contact allergy and the nature of jobs involving food.

Risk factors for contact allergies—and factors that are often encountered in food industries—include continued exposure to food allergens and things that affect skin integrity, such as pre-existing skin conditions like eczema, chronic scratching, cuts from processing machinery, exposure to (high-protein) shellfish juice and skin irritants like salt and cleaning agents, wet working conditions, and frequent hand washing without the use of hand cream. Additionally, factors like time-pressure can also result in workers not wearing protective clothing.

A French study looking into whether seafood-induced occupational contact dermatitis was more likely to affect cooks or fishermen reported that chefs were more likely to be affected by contact allergies. However, the low numbers of people working in the fishing industry with allergies surprised them and the authors of the report admitted that their numbers might have been affected by the fact that fishermen, who require a clean bill of health to work, may simply not report their health problems, and that the job turnover in the fish processing industry was rather high; people who really don’t like their job or find that it affects their health simply move to another job. However, chefs do tend to run higher risks than workers in fishing and fish-processing jobs because they have to handle the food bare-handed.

Continued contact with trigger allergens can lead to other symptoms of allergy. Sometimes, those symptoms are quite mild, such as oral allergy syndrome (i.e. itchy mouth and/or some facial swelling) and sometimes they are quite severe and include respiratory symptoms, gastrointestinal symptoms and even anaphylactic shock. The severity of the symptoms seems to depend on how much exposure a person has had to the seafood allergens on the job.

Unfortunately, the only solution for people with worsening symptoms is a new job, and it’s been estimated that up to a third of people who handle food for a living end up having to find alternative employment because of the debilitating symptoms that they contract at work. Some, however, will find their symptoms never go away.

Delayed reactions to shellfish

Delayed allergic reactions can occur hours or even days after exposure to an allergen, unlike IgE-mediated reactions that often happen within minutes. These reactions either involve diseases that rely on cell-mediated mechanisms (immune responses that do not rely on the production of IgE antibodies but instead involve the activation of T cells and macrophages which leads to inflammation and tissue damage) or by ‘mixed’ diseases that rely on both IgE- and cell-mediated mechanisms.

The three most common forms of delayed reactions to shellfish are eczema (atopic dermatitis), food protein induced enterocolitis syndrome (FPIES) and eosinophilic oesophagitis (EoE).

Eczemais a chronically relapsing inflammatory allergic condition that specifically affects the skin and look like this. It’s classified as a ‘mixed’ form of allergy that can produce either immediate or delayed reactions that can occur up to 48 hours after eating a trigger food.

Food Protein Induced Enterocolitis Syndrome (FPIES) is a delayed allergic reaction to food that affects the gastrointestinal (GI) tract. There are two main types of FPIES, chronic and acute.

Chronic FPIES is quite rare and occurs mostly in infants who eat the trigger food on a daily basis. It can be recognised by intermittent vomiting and diarrhoea and, occasionally, failure to thrive (which means that a child is not getting in enough calories to reach a similar weight and size to other children of the same age and sex). Cases of chronic FPIES in adults are vanishingly rare, but not unheard of.

Acute FPIES is by far the most common form. In children, symptoms often occur within 2 to 4 hours after eating the offending food and can include:

• vomiting
• pallor
• lethargy
• dehydration
• diarrhoea
• shock or hypotension (i.e. low blood pressure) which can manifest as dizziness, fainting or blurred vision (as well as pallor and lethargy)

Symptoms of shellfish FPIES take longer to appear that symptoms of FPIES to other foods; on average, about 2.5 hours after eating the offending food.

Sometimes children with chronic FPIES can develop an acute version of the disease. 1 in 5 children with FPIES may develop an IgE sensitisation to their trigger food as well—known as ‘atypical FPIES’—which can be more persistent.

In adults, the typical symptoms of acute FPIES are not the same as those seen in children. They can also appear faster. They include:

• stomach pain and cramps
• diarrhoea
• vomiting
• dehydration
• transient weakness and shivering (possibly hypothermia)
• lethargy
• weight loss

Although symptoms in FPIES suffered by adults and adolescents tend to be milder than those suffered by children, cases of adult FPIES can also occasionally result in anaphylactic shock, which is more likely to happen with molluscs than crustaceans.

Another difference between FPIES in adults and children is the onset and duration of symptoms; although most children suffer a reaction within 4 hours of eating shellfish, symptoms in adults fall within a larger range—anything from 3 minutes to 6.5 hours—and last, on average, for 9.5 hours, with a range of less than a minute to over 48 hours.

The vague nature of the symptoms combined with the fact that they look a lot like food poisoning means that adults normally suffer through several episodes of FPIES before going to an allergist and it may take a while to get the right diagnosis.

People who experience severe symptoms of acute FPIES may have a longer-lasting form of the disease.

Eosinophilic oesophagitis (EoE) is an inflammation of the oesophagus caused by a food allergy, environmental allergens or acid reflux. It is characterised by symptoms including:

• food impaction; this is when food becoming stuck in the oesophagus which can lead to a sensation of squeezing in the chest, and can be accompanied by excessive salivation (unlike choking, a person can still breathe and talk, but they cannot eat or drink any more)
• difficulty swallowing (dysphagia)
• reflux (the flow of liquid back from the stomach into the oesophagus)
• vomiting
• heartburn (pyrosis)
• stomach pain
• food refusal

Symptoms are variable and often age-dependent. In infants, EoE tends to provoke general symptoms of oesophageal difficulties such as gagging, vomiting, feeding difficulties and weight loss, or so-called ‘failure to thrive’. Young and school-age children have symptoms that are indistinguishable from those associated with gastroesophageal reflux, such as abdominal pain, vomiting and an unpleasant taste in the back of the mouth that comes from regurgitating sour or bitter liquid. Older children and adults are more likely to have trouble swallowing and to get food lodged in their oesophagus (food impaction) and, less commonly, to suffer from heartburn.

Because symptoms of shellfish allergy can be severe, if you do suspect that you’re allergic, it’s important that you see your GP/family doctor and get a referral to an allergy clinic for further testing.

Threshold for reactions

A relatively large amount of shellfish is necessary to provoke a response in a shellfish-allergic person.

VITAL®, the Australian initiative for voluntary incidental trace allergen labelling, put out recalculated threshold doses for the ‘Big 14’ allergenic foods in 2020. Using a database containing datasets from studies carried out worldwide that used double-blind, placebo-controlled food challenges (DBPCFC), they calculated that the lowest threshold dose of protein that was needed to produce a reaction in 1% of the population allergic to shellfish is 26.2 mg. (Note: in this case, the ‘population allergic to shellfish’ is 75 people who were given a DBPCFC)

723 mg was the dose needed to produce a response in 10% of the test subjects, and 7906 mg was the dose needed to provoke a reaction in half of the test subjects.

That is approximately half a shrimp, which is actually quite a lot. In fact, shrimp is the least potent of all the major allergens, requiring more protein than any of the others to provoke a reaction. Some very sensitive people will still react to tiny amounts, however; in one study some people had subjective symptoms (for example, an itchy mouth) with a dose of just 3 micrograms (3 thousandth of a milligram) and there is one sensational case of a 20-year-old shellfish-allergic woman who had a deadly anaphylactic attack after kissing her boyfriend who had eaten shrimp less than an hour beforehand.

Ultimately, the threshold dose needed to provoke symptoms varies widely between people. It also varies per person, depending on the circumstances around the meal.

Your threshold can be lowered and your allergic reactions worsened by things called ‘cofactors’. Cofactors include things like how much you eat and whether those ingredients have been cooked or processed, as well as exercise, anti-inflammatory drugs, alcohol, infection and stress.

Shellfish has been specifically implicated in several cases of food-dependent exercise-induced anaphylaxis (FDEIA). In fact, it’s one of the most common triggers of this condition. Although the first recorded case was to oysters, shrimp is the most commonly reported crustacean, both alone and in combination with wheat.

Other reported cases have involved crab, abalone, squid, cuttlefish, scallops, limpet and horned turban. Sometimes physicians know that shellfish are involved, but they can’t pinpoint exactly which ones, because other factors are also involved.

Sometimes exercise isn’t involved at all, but another cofactor like nonsteroidal anti-inflammatory drugs (NSAIDs) is, as in this case of allergy to prawn and barnacles.

Cofactors are thought to play a role in about 14% to 30% of all anaphylactic reactions.

A 2013 study carried out in Germany identified seafood as the third most common cause of food-induced anaphylaxis, including in situations involving cofactors.

Please note: the amount of shellfish needed to provoke a reaction says nothing about how severe the reaction will be. And, even if your reactions have been mild in the past, it does not mean that they will continue to be mild.

Image by Ivan Samkov on Pexels

Diagnosing shellfish allergy

Diagnosing an allergy to shellfish is difficult because of the wide variety of edible species, the limited number of commercial extracts for those species and the fact that many of them cross-react with each other and with insects, so it can be difficult to tell which one is the root cause of the reactions (the primary allergy).

To make things more confusing, shellfish is often eaten together with other foods that are known to cause allergies so it can be difficult to pinpoint the culprit allergen, and it’s often difficult to identify the type of shellfish that you were eating in the first place; so many of them have different names. ‘Crayfish’, for example, are also known as ‘crawfish’, ‘crawdads’, ‘mudbugs’, ‘ditchbugs’, ‘red claws’ or ‘yabbies’, as well as ‘rock lobsters’, ‘spiny lobsters’ and ‘langoustes’. The latter look similar and are easily confused, but they’re not even in the same family.

Additionally, produce can be mislabelled and fraudulent substitution happens in fish markets and restaurants which buy a cheaper type of fish and sell it on as a more popular and expensive variety.

Still, you have to do your best to identify where you ate what, because a diagnosis of shellfish allergy will primarily be based on your clinical history—a record of consistent symptoms following the consumption of shellfish or shellfish-containing foods. This will require you to provide your allergist with answers to questions about your general medical background (including any other allergies you may have, such as hay fever symptoms—especially relevant if you’re having cross-reactions to dust mites or cockroaches—and relevant illnesses in your family) and your dietary history (what you ate to provoke your symptoms, what those symptoms were, how long they lasted, whether you had exercised or taken painkillers and many other details).

Your medical history determines what comes next; on the basis of your answers, the allergist will try to determine what type of allergy you have—a primary, immediate-type allergy, a cross-reactive allergy or a delayed-type allergy—or whether it could be something else, and this will determine the tests they ask for to come up with a diagnosis.

Diagnosing IgE-mediated reactions to shellfish

Skin tests

An IgE-mediated sensitisation to shellfish is typically confirmed by a skin prick test, which involves someone placing a small sample of shellfish extract onto your skin (generally the forearm of an adult/older child or the upper back of a young child) and pushing it through the top layer of skin by pricking it with a lancet. It takes about 15 minutes to see a reaction (or not).

This test is often carried out first because it is quick and simple to perform and gives rapid results, but it is generally used to rule out an allergy rather than to confirm one, because it has excellent negative predictive value—if the skin weal is under a certain size, you are highly unlikely to have an allergy—but poor positive predictive value—the skin weal has to be very large before an allergist can say with any kind of confidence that you probably have an allergy.

However, this type of test suffers from several problems. For a start, commercial skin testing extracts are not that reliable for people are sensitised to an allergen that is not the abundant, heat stable allergen tropomyosin, such as hemocyanin, which is also an important shrimp allergen.

Extracts are also affected by storage conditions; temperatures of over −20 °C for 2 weeks to a month can destroy the allergens and make the extract unusable.

As such, research has shown that skin tests extracts made by different commercial companies tend to show widely different allergen contents and provide completely different results as well as potentially dangerous false negative outcomes, giving results suggesting that people are not allergic to shellfish when, in actual fact, they are.

Commercial extracts are also very limited when you consider the wide variety of shellfish available to eat. For a start, they tend to include species of crustacean that are consumed in the northern hemisphere, so not the Asia Pacific region, where the majority of the shellfish-eating takes place. Allergens that are found in parts of the crustaceans that are not as popular in the West, such as the cephalothorax, are also under-represented.

There are also hardly any extracts available to test for sensitivity to molluscs, which puts people who are allergic to both crustaceans and molluscs at a disadvantage.

One solution to this problem is the prick to prick test. This test is very similar to the skin prick test, except first the lancet is used to puncture fresh food and then it is used to prick your skin. When the food is in liquid form, the technique is actually the same as the one used for the skin prick test and, when the food is solid, it’s often ground down and put in saline solution.

If your history suggests an allergy to shellfish and the skin testing has failed to produce a positive result, a prick to prick test can also be done using the actual food that seems to be causing the reactions—perhaps an old family recipe of Bouillabaisse or raw sushi from a certain restaurant—prepared in the same way as it was the day you had the reaction.

The prick to prick test often produces superior results to commercial extracts because the fresh food used theoretically contains all of the allergens a person can react to—that said, because different allergens are present in different quantities in different parts of a crustacean’s body, the person making the extract needs to know what part of the body to target, or vital allergens may be missed.

Blood tests

Sometimes, the doctor may decide to order a blood test, aka an immunoassay. Perhaps the skin prick test was inconclusive, or the suspected allergen is not available for skin prick testing, or you’re unable to undergo the test for some reason.

A blood test involves having a small sample of blood drawn so that it can be sent to a lab where technicians will use allergen extracts to check whether there are IgE antibodies in your blood that react to them. It can take 1 or 2 weeks to get the results.

Blood tests can be less sensitive or specific than skin tests, but they have other advantages: they are perfect for people who cannot stop taking certain medications or have extensive skin disease or tattoos, and they can safely be used on infants, squirming toddlers and people who are at risk of suffering an anaphylactic reaction.

Blood test panels also typically include a whole range of potential allergen extracts including other foods or aeroallergens that the allergist may want to check your reaction to.

For more specific information, a component blood test—aka Component Resolved Diagnosis (CRD)—can be carried out. Instead of using extracts of whole foods containing only (heat-stable, plentiful) allergens, the CRD tests the reaction of IgE antibodies in your blood to isolated, individual proteins. This improves the diagnostic sensitivity of the test as allergens that would otherwise be missing from the whole food extract or exist only in tiny amounts are present in concentrated form in the CRD test.

This type of test enables the doctor to see exactly which allergen(s) you react to, which allows them to determine whether you are sensitised to cross-reactive allergens that are unlikely to produce symptoms, and whether you are sensitised to certain allergens that could affect your management plan. For example, if you’re sensitised to a heat- and digestion-stable protein like tropomyosin, you are at risk of reacting to both raw and cooked shrimp and you may have to watch out for cross-reactions with other shellfish, arachnids and insects.

Component blood tests are also made up of very large panels of allergens which include many other foods and aerollergens that the allergist may want to check your reaction to and can help to determine whether or not a sensitisation to a cross-reactive allergen will be symptomatic or not.

Unfortunately, although CRD could potentially reduce the need for oral food challenges and contribute to tailored management plans, it’s not yet considered a routine diagnostic method and it’s not comprehensive; the most widely used tests neither contain all of the identified allergens (which are also not all of the possible allergens), nor are they universally available. Testing for certain specific shellfish allergens would require special preparation and is therefore only likely to be done for research purposes.

Additionally, as it is with skin and standard blood tests, CRD is better at confirming an allergy than at eliminating the possibility of one. And, because sensitisation patterns differ according to geography and populations, with different allergens being more important in different regions and in people of different ages, allergists need to understand their patient populations so that they interpret the results of the tests correctly.

A positive skin or blood test does not mean that you are allergic to something. While skin prick tests and blood tests help with diagnosis, positive results only show sensitisation to specific allergens. Being sensitised to a food doesn’t mean that you’re allergic to it and that you will develop any symptoms.

For example, in a Spanish study of 35 people with suspected allergic reactions to shrimp, all but 2 had positive blood tests to shrimp extract but, based on oral food challenges or a history of reactions to shrimp, only 20 ended up being diagnosed with shrimp allergy.

And in an American study of children who were avoiding certain foods because many had been diagnosed with a food allergy based primarily on positive blood test results, oral food challenges were able to show that, of the 16 children who were avoiding shellfish because of a positive skin or blood test, all could actually eat shellfish without reacting.

A positive result simply means that your immune system is specifically aware of an allergen or allergens in that food. Why some people later develop an allergy to that food, and some do not, is not yet known.

Some of the false positive skin and blood tests to shellfish also come about due to cross-reactions in people who are, for example, sensitised to house dust mites or cockroach.

There can also be false negatives, which is potentially more worrying as people with shellfish allergy are mistakenly told that they can eat shellfish when they actually can’t.

That said, skin tests producing large weals and/or blood tests showing relatively higher IgE levels to shrimp may indicate a potential allergy, and high levels of IgE antibodies to shrimp or tropomyosin in the blood seem to indicate that the symptoms will be severe, although skin test results show no such correlation.

Ultimately, however no skin nor blood test is capable of detecting all people allergic to shellfish, even when combined with the medical history which itself can be misleading.

Food challenge

The only way to get a definitive diagnosis of shellfishallergy, and to have some idea of how severe your reactions may be and how much shellfish—and which shellfish—is needed to provoke them, is to undergo an oral food challenge. This involves eating a very small amount of shellfish—5 mg is the recommended initial dose, but doses can range from 3 micrograms (0.003 mg) to minimise any risk of reaction in cases of high sensitivity, all the way to 500 mg—waiting for an objective–visible—reaction, and then doing it again, gradually increasing the dose until a reaction occurs or a maximum dosage is reached. It can take around 4 hours, depending on the type of challenge undertaken and the length of observation time needed.

You can read more about oral food challenges here.

Oral food challenges are generally undertaken either when someone’s history and their test results disagree (i.e. they have negative tests results but their history strongly suggests an allergy, or vice versa) or to check whether someone has outgrown their allergy to ensure that they don’t unnecessarily restrict their diet or worry about hidden allergens in processed foods.

Because of the risk of severe reactions, oral food challenges should only be done by an experienced consultant in a medical setting.

Although food challenges help to diagnose food allergies and identify a suspect food, there are other reasons to undergo food challenges, namely:

• to identify culprit foods in cases of allergies to multiple unknown foods
• to determine a patient’s threshold—how much of a certain shellfish they can eat without reacting—so that dietary advice based on the outcome of the challenge can be given
• to confirm the development of tolerance to shellfish

Practically speaking, most people do not undergo this kind of test since it requires a lot of time and resources. And oral challenges are rarely, if ever, offered to people whose history includes severe reactions to a suspected food. Whenever possible, allergy diagnoses are based on a combination of medical history and lab tests.

If you fail your food challenge—an allergy is proven—the doctor can then advise you on the dietary precautions to take to avoid further reactions. The general advice is to avoid all shellfish. However, it may be that you are only allergic to certain species of shellfish and that you can eat others without problems. If you want to find out, you will need to undergo further challenges.

Other tests

In cases of respiratory allergy, you may have to undergo other types of testing including pulmonary function testing which is an umbrella-term for various types of tests designed to measure your well your lungs work; how well they exchange air, how well they absorb oxygen into your blood, how exercise affects them.

If the initial tests like spirometry and FeNO haven’t given clear results, you may be asked to undertake a bronchial provocation test which is designed to measure the sensitivity of your lungs. It’s often carried out to diagnose asthma or measure how severe the asthma is.

This test can take the form of a methacholine challenge, which is when you breath in increasing amounts of chemical metacholine. It irritate the airways and causes them to get slightly narrower like how an asthma trigger would work. Low doses will cause constriction among people with asthma and high doses in people without asthma.

Or it could be a mannitol challenge, which works in a similar fashion except using mannitol, which is is a type of sugar alcohol generally used as a sweetener or medication.

Or it could take the form of an exercise challenge, using a treadmill or stationary bicycle to see if physical exertion makes your airways react.

Sometimes, tests specifically tailored to your condition can be carried out, as in the case of a man with a suspected allergy to Gammarus shrimp-containing feed. His doctors mixed Gammarus shrimp powder with lactose and tipped the concoction from one tray to another 30 centimetres away from his nose for 5 minutes. This produced a reduction in his lung function. Another test carried out using just lactose produced no response, and his occupational asthma to shrimp was effectively diagnosed.

Finally, some allergists will use the nasal allergen provocation test, which involves using a syringe, bottle dropper, micropipette or pump action spray to deliver some drops/spray of allergen into the nose and measuring the resulting nasal airway resistance with equipment like an acoustic rhinometer.

Diagnosing non-IgE-mediated and mixed reactions to shellfish

Non IgE-mediated diseases are difficult to diagnose for several reasons, not least of which is the fact that IgE testing is often of no use. This makes the clinical history especially important for the diagnosis of these types of conditions. Even then, the symptoms are not easy to connect to the actual meals because of the time delay, and the symptoms associated with digestive allergies lack the skin and respiratory signs that doctors usually associate with allergy.

Skin tests

Eczema is diagnosed based on personal and family history of allergy and a skin examination. While there are no standard diagnostic criteria, there are certain features that a doctor can look for to diagnose it.

That said, these criteria are based on the characteristics of paediatric eczema, which is not the same as the manifestation of eczema in adolescents or adults, making diagnosis of eczema in older age groups more challenging. Sometimes people with eczema in these age groups will have to undergo additional tests to rule out other diseases first, and a skin biopsy may be needed before a diagnosis of eczema is made. However, these differences are now being taken into account and guidelines are being updated.

Once the diagnosis of eczema is made, efforts will first be made to try and get the skin condition under control using topical skin creams and drugs before any further testing is done. Generally, only if the skin is not getting any better will tests be carried out to see whether allergens, like food, could be aggravating the condition.

The identification of potential food allergens is generally done by looking for specific IgE antibodies to a food using skin prick tests or blood tests (the latter is often used if the skin condition is too bad for a skin test, or medications are being taken that will interfere with the results, or if the tests involve a young infant).

In cases of delayed symptoms, doctors may use the atopy patch test (APT). This test generally involves walking around with food (either fresh or in solution) contained in tiny aluminium capsules taped to your back for up to 3 days and having your skin checked for a reaction after 48 hours and 72 hours.

The atopy patch test has also been used to try and diagnose delayed digestive allergies, with mixed results; in the case of food protein–induced enterocolitis syndrome (FPIES), for example, it has proven itself to be both ‘a promising diagnostic tool for the diagnosis of FPIES’ and ‘not helpful in identifying the [trigger] foods’, while showing ‘poor utility in the follow-up prediction of outgrowing FPIES in children’, and with eosinophilic oesophagitis (EoE), it has shown that it can ‘identify potential causative foods’. For these diseases, it is not the diagnostic instrument of choice.

Diagnosing contact allergies can be tricky because they can look similar; some like contact urticaria are immediate type allergies which can be diagnosed by prick testing, and some are delayed-type allergies which may be diagnosed by APT.

When there is confusion as to what type of contact allergy you may have, several tests may be carried out to get a diagnosis, as in the case of a 52-year-old woman who was referred to an allergist with a contact allergy on her hands after handling raw shellfish. As her doctors were unsure whether she had protein contact dermatitis (an allergy caused by large molecules) or contact urticaria (an allergy caused by smaller molecules), as well as a patch test and a skin prick test and a skin biopsy, they performed a rub test on her skin (healthy skin will not let large allergens through) which was negative and therefore ruled out contact urticaria. The prick to prick test was positive for prawn and her doctors diagnosed her with protein contact dermatitis to raw shellfish.

A skin application food test (SAFT) may be used instead for children under the age of 4. It’s basically the same thing, but the capsule of food is only applied to the skin for 10 to 30 minutes. It’s had mixed results, having been described as reliable and child-friendly and also as not being as good as the APT in diagnosing certain food allergies.

Elimination diets and food challenges

While skin tests may provide an indication of sensitisation, they cannot diagnose a food allergy; that has to be done with a food challenge during which the doctor can see whether or not, in addition to any immediate reactions, the suspected food produces a worsening of the skin symptoms within the next 48 hours. If it does, the food can then be eliminated from a person’s diet and their skin condition will be monitored for the next few months to see if there is a persistent improvement. When more than one food is suspected, the next challenge will be done a few weeks after the first one.

The diagnosis of delayed digestiveallergies generally starts with exclusion; first other possible causes of the symptoms are eliminated and only then will the suspected food(s) be excluded from a patient’s diet—and, if they are breastfeeding, from their mother’s diet, too.

If the symptoms disappear, the first step towards a diagnosis involves reintroducing the foods one by one into the diet and seeing if the symptoms return. If the symptoms don’t disappear, it could be that the diet has not been restricted enough or that other foods should (also) be considered for elimination. Or it could be that something other than an allergy is responsible, in which case, the allergist’s job ends and another specialist’s begins.

The diagnosis of food protein induced enterocolitis syndrome (FPIES) mainly relies on a person’s clinical history and symptoms appearing when the offending food is reintroduced after an elimination diet.

In the case of chronic FPIES, an elimination diet should result in the symptoms going away within 3 to 19 days. Reintroducing the trigger food should produce the symptoms of acute FPIES—i.e. projectile vomiting—which should be enough confirmation.

In the case of acute FPIES, eating the offending food should be followed by symptoms that should fit specific diagnostic criteria including copious vomiting within 4 hours. Although confirmation of the diagnosis officially requires an oral food challenge, because it often produces nasty symptoms that the patient quite rightly has no wish to suffer through, in practice, this is rarely done and challenges for the diagnosis of chronic FPIES are more common.

However, since FPIES symptoms tend to be different for adults with acute FPIES and there are no strict diagnosis guidelines for them, oral food challenges are often necessary.

There are other reasons to undergo food challenges in cases of FPIES, including:

• identifying a culprit food in cases of allergies to multiple foods
• confirming the development of tolerance to a trigger food, which is often done between 12 to 18 months after the most recent reaction
• identifying alternative shellfish that can be eaten without symptoms in order to avoid an unnecessarily restrictive diet

Many clinics will only carry out a food challenge in an infant to see whether they have outgrown their allergy.

Although the majority of people with FPIES will have negative skin or blood tests to their trigger food, in some cases people do have an IgE sensitisation too. This is called ‘atypical FPIES’ and it affects between 1 in 4 and 1 in 8 people with FPIES. According to American research, the foods most commonly associated with this type of FPIES are egg, milk and peanut, but this may just be because those are the foods most typically eaten by American children (who make up the bulk of these studies). A person can have atypical FPIES to several foods, and those foods can include anything, from shrimp to avocado.

Some children with atypical FPIES may take longer to outgrow their condition (if, indeed, this happens at all) or may develop a classic IgE-mediated food allergy with potentially more dangerous symptoms. As such, periodic testing for an IgE sensitisation is advised in children who also have an IgE-mediated food allergy to other foods or suspected food-induced eczema.

When diagnosing eosinophilic oesophagitis (EoE), other conditions that produce similar symptoms, like gastroesophageal reflux disease (GERD), are first eliminated as a possibility before any intrusive testing is done. Then, if eosinophilic oesophagitis is still suspected, an upper GI endoscopy (aka an oesophagogastroduodenoscopy) and biopsies are carried out to look for specific levels of eosinophils in the oesophageal tissue (15 or more eosinophils per high-powered field, to be precise).

Standard elimination diets for cases of EoE are often based on the most common causes of the disease, either ‘2 food diets’ (dairy and wheat), ‘4 food diets’ (dairy, wheat, egg, and legumes) or ‘6 food diets’ (dairy, wheat, egg, legumes peanuts/tree nuts and fish/shellfish). These are called ‘empiric’ diets, i.e. diets that are based on observation and experience. The diet can be made less cumbersome by starting small, first with one food (i.e. milk) or two foods and then eliminating more foods if the symptoms don’t disappear.

The empiric diet approach is not the only approach. Sometimes the foods to be eliminated are determined using lab tests—atopy patch test and SPT and/or blood test—first (a targetted approach). Both methods work equally well for both children and adults although the targetted approach has the advantage of often requiring the elimination of fewer foods. That said, a lot of people with EoE don’t have any measurable IgE antibodies to their trigger food, so the targetted approach can only help some.

The diet normally takes about 6 weeks. If the symptoms go away and the tissue samples look good, the trigger is assumed to be one or more of the foods that was eliminated. In order to pinpoint the trigger(s), each food is reintroduced back into the diet one by one. If a reintroduced food causes symptoms to return and/or biopsy specimens to look abnormal, then it is identified as a trigger food and must be eliminated from the diet indefinitely. (In the real world, children may balk at undergoing so many intrusive tests or there may not be the capacity to perform them, in which case, they will probably not be required for a diagnosis.)

Elimination diets are best performed under expert guidance, because there is a risk that excluding a food from your diet because you think that you may be allergic to it or because it causes mild or delayed symptoms can lead to you developing an IgE-mediated allergy to that food, often with severe—sometimes fatal—reactions.

The good news is that, when a delayed digestive allergy is diagnosed, excluding offending food(s) from the diet leads to the resolution of symptoms and the repair of the underlying tissue damage in most children and adults, and improves quality of life, even though the diet may be difficult to stick to. When it comes to young children, if staple foods are being eliminated, dietary guidance may be needed to ensure normal growth and development.

Differential diagnosis

Not all adverse reactions to shellfish are due to allergy; some are due to bacteria, viruses, parasites or toxins contaminating the shellfish. Here are some of the most common alternative diagnoses that should be considered if eating shellfish has made you suffer from allergy-like gastrointestinal symptoms, but your tests have been negative or inconclusive.

Shellfish contamination

Shellfish can be contaminated by all sorts of things, including:

Viruses present in polluted seawater

Hepatitis A (HAV), Norovirus (NoV) and other virsuses can infect molluscs and crustaceans and then the people that eat them, producing gastrointestinal like nausea, vomiting, watery diarrhoea, stomach pain and fever that usually occur several hours after consumption and resolve within 48 to 72 hours.

Bacteria present in polluted coastal waters

Bacteria including Escherichia coli (E.coli), Salmonella and Vibrio can also infect molluscs and crustaceans and then the people that eat them. Although the first two proliferate due to faecal contamination, Vibrio is part of the natural flora of coastal environments.

The Vibrionaceae family contains a variety of bacteria, of which the most common is Vibrio parahaemolyticus and the most notorious—and dangerous—is Vibrio vulnificus. Around 84 000 Americans are thought to contract a food-borne Vibrio infection—vibriosis—a year and, each year, 150 to 200 cases of Vibrio vulnificus are reported to the CDC.

Most people with Vibriosis have diarrhoea, but V. vulnificus causes 95% of all seafood-related deaths in America. It enters the food chain by taking up residence in the stomachs of shellfish including clams, mussels, scallops, crawfish and crab, but mostly oysters—around 90% of V. parahaemolyticus and V. vulnificus infections stem from eating undercooked or raw oysters.

Symptoms of V. vulnificus—e.g. the sudden onset of fever, chills, watery diarrhoea, nausea and vomiting—are limited to the gastrointestinal tract in about 10 to 15% of cases, but most involve sepsis and the mortality rate in those cases is around 50%.

People can also get vibriosis by swimming in water with open wounds, which can result in cellulitis, blisters, muscles infections and tissue damage. Vibrio vulnificus likes warmish water, so most infections occur between May and October.

Staphylococcal food poisoning

This type of food poisoning can be caused by shellfish that has been contaminated by unwashed hands that have the bacterium Staphylococcus aureus (Staph) on them, and then left at room temperature. The bacteria gets in the food and produces Staphylococcal enterotoxin, which targets the intestines and causes nausea, vomiting, stomach pain and diarrhoea within 30 minutes to 8 hours after eating the contaminated food.

The parasite Anisakis simplex

Anisakis simplex is a parasitic nematode (roundworm) that mainly infects fish but can also be found in large crustaceans and cephalopods (notably squid) and causes health problems in humans who become accidental hosts when they eat raw, marinated or undercooked seafood that’s infected with live larvae. People who are sensitised to house dust mites (and/or mould) are a greater risk of allergic reactions.

There are actually several types of parasitic worm that can be found in seafood and cause reactions in humans—namely Pseudoterranova decipiens, Anisakis physeteris, Nybelinia surmenicola—but A. simplex is the most frequent culprit.

The vast majority (over 90%) of reported reactions to Anisakis come from Japan, where raw seafood is an integral part of the diet—over 2,500 cases are thought to occur annually. In Europe, the number is more like about 20 cases per country per year (but around 30 in Italy) and in America, around 10 cases a year.

Anisakis simplex can cause problems for people in 2 ways:

1. Tissue damage: the parasitic worm can penetrate your innards, forming a mass called a granuloma. This infection is called anisakiasis and it normally lasts about 3 weeks before the worm dies and is expelled from the body. If it isn’t naturally expelled, it can be removed through either endoscopy or surgery. Symptoms depend on where the worm ends up:
   • If it burrows into your stomach wall, it causes acute symptoms that occur 2–8 h after eating, often in the form of severe abdominal pain
   • If it burrows into your intestinal wall, it causes chronic symptoms that occur 5-7 days after eating and can manifest as mild stomach pain, nausea, vomiting, diarrhoea, intestinal obstruction and sometimes fever
2. Allergy: the worm itself contains allergens that people can become allergic to. One of the allergens (Ani s 3) is tropomyosin, which could be responsible for cross-reactions among the shellfish-allergic. Some experts believe that a person must first be infected with the worm before becoming sensitised and developing an allergy to it. Symptoms of allergy to A. simplex usually occur between 15 minutes to 6 hours after eating fish and include typical allergic symptoms such as gastrointestinal pain, rashes, facial swelling, asthma and, occasionally, anaphylaxis

Because the symptoms mimic other, more common, illnesses, anisakiasis (especially the chronic, intestinal version) is often misdiagnosed as a myriad of conditions including a wide range of cancers, acute appendicitis, intestinal endometriosis, intestinal obstruction, pancreatitis, peritonitis, gastritis, gastric ulcers, ovarian cysts, tumours, hernia, Kounis syndrome or Crohn’s disease.

Although the Anisakis larva can be killed by cooking, freezing and salting its allergens are resistant to heating or freezing and can still trigger allergies after the larva is killed, so people with Anisakis allergies should avoid seafood altogether.

The anisakis-allergic need to be particularly careful when it comes to eating squid, cuttlefish, oyster, whelk or scallop, which are often found in their raw form in the traditional Japanese dishes of sushi and sashimi and can sometimes, when undercooked, be a cause of allergic reactions to hidden allergens in processed foods like surimi.

Shellfish poisoning

Shellfish poisoning can often be found masquerading as allergic reactions. Filter-feeding bivalve molluscs like clams, oysters, mussels and scallops take in toxin-producing algae as they feed and are, in turn, eaten by people who then get sick. Although the symptoms of shellfish poisoning often include gastrointestinal symptoms that look like they could be caused by allergies, they also include things like fever and neurological symptoms which can help to distinguish them from allergic reactions.

Five types of poisoning have been identified:

1. Paralytic shellfish poisoning (PSP) – caused by the potent neurotoxin saxitoxin produced by Alexandrium algae. Symptoms are both neurological and gastrointestinal and usually start within 2 to 3 hours of eating the contaminated shellfish, but can begin anywhere between 15 minutes and 10 hours afterwards. They classically begin with a tingling/burning sensation or numbness around the lips which gradually spreads to the face and neck, accompanied by a prickly sensation in fingertips and toes. Symptoms are dose-dependent and can progress to headache, nausea, vomiting, diarrhoea, lack of coordination in the limbs (ataxia) and respiratory difficulty. Other symptoms include paralysis and cranial nerve dysfunction—which causes intense pain, vertigo, hearing loss, weakness and paralysis. Paralysis can occur within 2 to 12 hours and persist for as long as 3 days.
2. Diarrhoetic shellfish poisoning (DSP) – the mildest and most benign of the shellfish poisonings, caused by okadaic acid and dinophysis toxins, the most abundant and recurrent toxins found in Europe, produced by Dinophysis algae. Typically associated with stomach and intestinal symptoms that begin between 30 minutes and 6 hours after exposure. The most common symptom is severe diarrhoea followed by nausea, vomiting and stomach cramps. Can sometimes include chills, fever or headache.
3. Neurotoxic shellfish poisoning (NSP) – caused by brevetoxins produced by the marine algae Karenia brevis. Has been likened to a mild case of Paralytic Shellfish Poisoning (i.e. PSP without paralysis) Symptoms are both neurological and gastrointestinal and begin between 30 minutes and 3 hours after consuming toxic shellfish. They include gastroenteritis (producing nausea, cramps, diarrhoea and vomiting), chills, sweats, low blood pressure (hypotension), irregular heartbeat (arrhythmia), numbness, pins and needles, muscle pain, vertigo, inability to distinguish hot/cold sensation and, in severe cases, bronchoconstriction (producing wheezing and shortness of breath), seizures, paralysis and coma.
4. Amnesic shellfish poisoning (ASP) – caused by domoic acid produced by planktonic diatoms. Symptoms usually appear within 24 hours of exposure and generally start off with nausea, diarrhoea, vomiting and stomach cramps, followed by headache and neurological symptoms such as confusion, short-term memory loss, disorientation and coma. Although the first reported outbreak in 1987 in Canada was deadly, no more deaths have been reported since, both due to increased monitoring and, possibly, lack of awareness of the condition.
5. Azaspiracid shellfish poisoning (AZP) – the most recently discovered and least well known toxic syndrome. Symptoms appear within hours of exposure, persist for 2 to 3 days, and include nausea, vomiting, diarrhoea and stomach cramps, similar to diarrhoetic shellfish poisoning.

Several species of algae involved in shellfish poisonings are part of the natural phenomenon known as ‘red tides’ because of the red colour of the seawater:

• Alexandrium algae that cause PSP
• Dinophysis algae responsible for DSP
• Karenia brevis responsible for NSP

Unlike other shellfish toxins, brevetoxins can become airborne thanks to surf and wave action along shorelines, causing ‘aerosolized red tide respiratory irritation’ (ARTRI). These aerosols can irritate the eyes and cause sneezing and runny noses that can resemble an allergic reaction. People with underlying asthma or chronic lung diseases who are exposed to these toxins can experience shortness of breath, coughing and wheezing.

Algal blooms are more common during the summer and, therefore, so are cases of shellfish poisoning.

Image by VK bro on Unsplash

Image by Jakub Żerdzicki on Unsplash

Good to know

Airborne shellfish allergens can get everywhere and they may prolong your allergy.

Several studies on fish allergens have found them in the mattresses in our bedrooms and floating around fish markets. Since shellfish allergens are just as sturdy as fish allergens (hence the multiple studies that have detected them in the air at processing plants), there’s no reason to believe that they are not lurking in our mattresses or in the air at our seafood markets, too—the researchers carrying out those particular studies just weren’t looking for them.

Avoiding these airborne hazards is advisable if at all possible because they might delay the development of tolerance. Unfortunately, being exposed to the cross-reactive allergens of dust mites or cockroaches can also mean that your immune system remains primed to attack anytime it comes across shellfish.

Having an allergy to shellfish does not increase your risk of having a reaction to iodine.

There is a persistent myth that people with shellfish allergy are more likely to react to things that have iodine in them, including antiseptics like Betadine or Povidine and X-ray radio contrast agents (substances used to improve medical images). Surveys carried out a couple of decades ago found that, not only did laypeople tend to believe that iodine is linked to seafood allergy, so did doctors. This is not true.

Doctors’ misconceptions were probably caused by the fact that shellfish contain relatively high levels of iodine and they linked this in their minds to the fact that the iodine-containing radio contrast agents they gave patients before a medical scan were know to occasionally cause allergic reactions. But these reactions were subsequently found out to be due to the agents’ hyperosmolarity—the fact that they contained more salts and other substances than the blood, which led to problems when the blood then tried to draw water out of the body’s organs. Early radio contrast agents were also severe irritants.

The first radio contrast agents were called High-Osmolarity Contrast Media (HOCM) and they had a tendency to cause mast cells to degranulate and release histamine and other inflammatory chemicals. Newer contrast agents—so-called Low-Osmolar Contrast Media (LOCM)—introduced about 30 years ago are about half to a third as osmotically active and have significantly reduced the number of reactions. They are now used practically everywhere.

You can still have a reaction to radiological contrast media, but those at risk include people with any type of food allergy, people with asthma and people with heart problems and/or taking beta-blockers. A recent study found that being female and being older might pose a risk, but having a food allergy did not.

It’s worth stressing again: anaphylactic reactions to radio contrast agents are due to hyperosmolarity, not iodine. Iodine allergy is not a thing.

Iodine is a vital micronutrient. We need it to grow and develop properly. Iodine deficiency is considered to be the most commonly preventable cause of mental retardation. It’s not made by the body so we have to get it from foods like seafood, dairy and eggs. Iodine is even added to table salt to prevent iodine deficiency. We wouldn’t survive without it.

A permanent solution for shellfish allergy is being worked on.

Scientists have been working on finding a way to desensitise people with shellfish allergy for a couple of decades.

In 2014 in Egypt, the usefulness of sublingual immunotherapy (SLIT aka allergy drops or tablets) for shrimp allergy was evaluated in 60 allergic patients (who also had asthma, hay fever symptoms or hives) and 20 healthy controls. They all received daily allergy drops, gradually increasing the concentration and amount of shrimp allergen. Six months later, the team measured a significant reduction in allergic symptoms and shrimp specific IgE levels. Although the study was not rigorously controlled, it did find the procedure to be safe and well tolerated.

In 2016, a clinical trial was carried out with American patients who also received omalizumab—a synthetic anti-IgE antibody used to treat symptoms like asthma and chronic hives and reduce the potential side-effects of the therapy. It included 3 shrimp-allergic people who also had asthma, hay fever and eczema. A successful outcome was recorded, with all 3 patients reaching the maintenance dose of 1g (a dose that would allow them to accidentally ingest some shrimp without reacting) without suffering from any serious reactions during therapy.

A follow-up of 2 of the 3 patients (one did not turn up) found that they could both eat 12 g of shrimp (the equivalent of about 3 medium-sized white prawns) without reacting. It was just a tiny number of people and no other immunological measures (like shrimp specific IgE) were taken, but it was a promising start.

In 2021, a rare case of sustained unresponsiveness—a ‘cure’—was reported in a 39-year-old American woman who had a history of severe anaphylactic reactions to shrimp. She was 2 years into her treatment, which consisted of eating 4 medium-sized shrimp every day, when she had to have oral surgery to deal with an infection. After the operation, her doctor used a relatively new type of blood test—the basophil activation test (BAT)—to see how her immune system reacted to shrimp so that he could advise her on how to continue with her treatment, and there was no response.

One month later, having avoided her daily doses, the woman passed an oral challenge to shrimp without reacting. She now eats some shrimp regularly and can eat all forms of shellfish ‘without restriction or symptoms’. That is a relatively unusual outcome; generally people who undergo immunotherapy do not lose their tendency to react to their food trigger, it’s simply somewhat suppressed. So, while I’d like to give people with shellfish allergy hope, I don’t want to give you false hope. You could think of that patient as someone who won an immunotherapy lottery; maybe you could do it too, but probably not. That said, you’re probably more likely to achieve sustained unresponsiveness than you are to win the Powerball.

Unfortunately, immunotherapy for shellfish is complicated—people with allergies to different allergens could have different responses to therapy. It can also be dangerous—something illustrated by a study carried out in mice that showed that medium and high doses of shrimp allergens could lead to serious symptoms (and death).

Therefore scientists are looking into several ways of making it a safer procedure, for example by modifying allergens to produce hypoallergenic variants or developing DNA vaccine-based immunotherapy.

As far as the general allergic public is concerned, then, immunotherapy for shellfish allergies is still a few years off, although it might be possible for some people to treat allergic reactions to shellfish that are a result of cross-reactions with insects like house dust mites by having treatment to those insect allergens instead. That said………I

Immunotherapy treatment for house dust mite allergy may induce shellfish allergy.

Some studies suggest that the cross-reactivity between dust mites and shellfish might be responsible for the development of shellfish allergy in people who are receiving immunotherapy for their dust mite allergy.

In one study, the blood of 17 patients who were undergoing treatment for their allergy to dust mites was tested for IgE antibodies against snail and shrimp at the start of their treatment and over a year later. Most of them ended up testing positive for a sensitivity to snail and three to shrimp. 2 of them eventually developed oral allergy symptoms when they ate shrimp. Another study reported that a 12-year-old girl who was undergoing immunotherapy to house dust mite had an anaphylactic reaction after eating snails during her treatment.

However, other studies have reported no shrimp sensitisations among their patients or that the treatment against the house dust mite allergy either improved the symptoms of people who were also allergic to shrimp or even made their allergy disappear.

Sadly, there’s no way of telling which way it will go, so each person has to be evaluated by a doctor who will use test results to decide whether or not the benefit of immunotherapy is worth the risk and discuss it with their patients(’ parents).

You can develop an allergy to shellfish after a blood transfusion or an organ transplant.

Cases involving the development of new food allergies after operations that involve getting someone else’s blood or organs are not as rare as you might think.

In the case of blood transfusions, we’re talking about a passive transfer of food allergy—when the allergy is simply transferred from one person to another because there are IgE antibodies present in the donor blood. For example, the case of a 64-year-old man who developed an allergy to shrimp after a transfusion. In these cases, the allergy is only temporary and generally goes away within a few months.

Food allergy involving organ transplants is a different matter; once you develop this type of allergy, you’re likely to keep it, with only about 1 in 4 people becoming tolerant to their food trigger(s). Although it can happen after different types of transplant, the liver is by far the organ most often associated with the acquisition of a new food allergy, and shellfish is the 8^th most common transplant-acquired food allergy (TAFA) according to a large review, although a new shellfish allergy is more likely in countries where shellfish is more commonly eaten, like Thailand.

Children are much more likely to develop an allergy after a liver transplant than adults; in fact, there are case reports describing children and adults getting a liver from the same donor and the child developing a food allergy while the adult does not. Why this is so is not known; it could be something about the liver itself, or the fact that the child needs a liver transplant in the first place, or it could be because a child’s immune system is too immature to be able to suppress the expression of newly acquired food allergies.

Symptoms of liver transplant-acquired food allergies (LTAFA) generally manifest within 18 months of the operation and typically involve the skin; facial swelling (angio-oedema) and hives (urticaria) occur in around 4 to 5 in 10 people. Gastrointestinal manifestations (diarrhoea, vomiting, stomach pain) also affect around half of people with LTAFA, while respiratory symptoms affect about 1 in 10. Anaphylaxis affects around 16 in 100 children with LTAFA, but no fatal cases have been reported so far. Although the majority (around 4 in 5) of the new food allergies are IgE-mediated, some people also get mixed or non-IgE-mediated allergies like Eosinophilic Gastrointestinal Disorders (EGIDs).

Transplant-acquired food allergy can also happen with other types of transplant, such as cord blood transplants (when blood stem cells are collected from umbilical cord blood and given to someone who needs a stem cell or bone marrow transplant but doesn’t have a matched donor), like the case of a 3-year-old boy who developed allergies to several foods, including shrimp, 4 months after his transplant. This produced relatively mild symptoms (hives and facial swelling) and lasted for around 2 years before resolving.

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