Shellfish allergy
Information provided in co-operation with the Food-INFO.net website.

Shellfish is a culinary term generally used for aquatic invertebrates used as food: molluscs, crustaceans, and echinoderms. Crabs, crayfish, lobsters, prawns and shrimps are crustaceans. Food allergy to crustaceans is relatively common, symptoms ranging from mild oral allergy to severe symptoms such as anaphylaxis.


 

Cooking does not remove the allergen. Crustacea are the third most important cause of food induced anaphylaxis after peanuts and tree nuts (cashews, almonds, pecans, walnuts, etc.). Thus crustacea and products thereof are listed in annex IIIa of the EU directive on labelling of foods and must be labelled when used as ingredients in pre-packaged food.

Molluscs include many of the most important seafood such as abalone, clams, mussels, octopus, oysters, squid, and scallops and also include terrestrial snails. Allergy to molluscs has been reported less frequently than allergy to crustaceans. Thus the EU labelling directive does not currently demand labelling of products derived from molluscs when used as ingredients in pre-packaged food.

Echinoderms such as sea cucumber and sea urchins are rarely eaten in Europe or North America. There have been few or no reports of allergies to these foods and echinoderms are not included in the EU labelling directive on allergenic foods

 

Specific information on allergies to:

 

1.) Mollusc (shells) Allergy

Molluscs include many of the most important seafood such as abalone, clams, mussels, octopus, oysters, squid, and scallops as well as terrestrial snails. Marine molluscs and crustacea (shrimps, prawns, crabs, crayfish and lobsters) are both known as shellfish. There is a sharp legal distinction because annex IIIa of the new EU labelling directive makes the listing of crustaceans and crustacean products on labels mandatory but does not currently specify labelling molluscs. There is a clear biological difference between molluscs and crustacea but some limited cross-reactivity has been reported. Thus shrimp allergic individuals are more likely to react to molluscs than to an unrelated allergen such as a plant food.

Molluscs are eaten all over the world but the species eaten and the quantity consumed differs greatly from region to region. In many countries molluscs are eaten occasionally rather than as part of the staple diet. Southern Europe, especially Spain, consumes more cephalopods as calamari and shellfish such as mussels than Northern Europe. The coastal regions of Asia are also major areas of mollusc consumption which are often included in Chinese dim sum, soup and rice porridge, making molluscs important hidden allergens in those dishes. More squid is consumed than any other seafood in Japan. Terrestrial snails are mostly eaten in France, Italy, Spain and Portugal.

 

Symptoms

Adverse reactions following the ingestion of molluscs are similar to those reported for allergic reactions to other foods. They range from mild oral allergy syndrome (itching of the lips, mouth, or pharynx, and swelling of the lips, tongue, throat, and palate), through urticaria (hives), which is probably the most commonly reported symptom, to life-threatening systemic anaphylaxis (difficulty breathing, drop in blood pressure, and even death). Symptoms may involve the skin (itching, swelling), the gastro-intestinal tract (nausea and diarrhoea) and also respiratory symptoms (asthma, rhinitis). Most reported symptoms occurred within 90 minutes of ingesting the food. However, delayed reactions have been reported. The symptoms reported following the ingestion of molluscs may be unusual in two ways. Firstly, there may be a larger fraction of serious reactions than for most foods. Secondly, allergic reactions to snails very frequently involve asthma. This may be associated with allergy to dust mites which is involved in many cases of asthma and also in allergic reactions to snails.

Symptoms generally occur after ingestion. However, symptoms have been reported on handling or inhaling steam from cooking molluscs and asthma has been associated with workers opening mussels. These reactions have been less frequently reported and less studied than similar occupational allergy to crustacea.

Severe symptoms sometimes only occur when exercise follows shortly after eating molluscs (abalone or snails) as has also been reported for several other foods.

 

Who, when, how long, and how often?

Studies based of questionnaires in the USA suggest that allergy to molluscs is less frequent than allergy to crustacea by a factor of approximately 3 but that a significant number of individuals suffer adverse reactions (>50 reacting to clam from 14,948 individuals). The frequency of food allergy probably follows the regional pattern of consumption, so that allergy to molluscs may be proportionally more important in France, Spain, Hong Kong or Singapore and less frequent in the United Kingdom or Germany. Food allergies to molluscs have been reported in both children and adults and little is known about the persistence of allergy to molluscs.

Molluscs caused 5.6% and 11.7% of serious reactions in France in 2002 and 2003 and an article reported that 36% of cases of anaphylaxis at a clinic in Singapore were due to molluscs. A possible explanation could be that reactions might be attributed initially to bacterial contamination rather than food allergy so that consumption is continued.

 

Related foods (cross reactivity)

Information on cross reactions is imperfect because oral challenge with molluscs has rarely been reported. Thus cross-reactions are identified from clinical histories, skin tests and tests for specific IgE from sera.

Cross-reactions are found between molluscs especially within the same class (bivalves, cephalopods or gastropods). For example, in a group of patients with a history suggesting shellfish allergy, patients with a positive skin test to clam had a 0.55 (55%) probability that they have a positive skin test to scallop, whereas those with a negative result with clam had only a 0.05 (5%) probability of a positive skin test to scallop. The same study found that those testing positive to abalone had only a 25% probability of a positive skin test to scallop but a 79% probability of a positive skin test to limpet. Thus those allergic to bivalves (clams, mussels, oysters, and scallops) are likely to react to other bivalves while those reacting to gastropods (abalone, limpets, snails, winkles and whelks) are likely to react to other gastropods. One case of fatal anaphylaxis on consumption of terrestrial snails has been reported of a patient who had been diagnosed as allergic to abalone showing that the cross reactions relate to biological families (gastropods) rather than to sea food versus land animals. However, cross-reactions between the classes of molluscs are also quite common.

There is good evidence for cross reactions between molluscs and arthropods (the phylum contains insects, mites and crustacea) and two different cross-reactivities are reported. Allergy to crustacea (shrimps, lobster, crawfish, or crab) is generally due to allergy to a highly conserved muscle protein called tropomyosin. In vitro studies show that IgE antibodies (the class involved in allergy) from shrimp allergic patients also frequently bind to tropomyosins from molluscs. However, as noted above, this does not necessarily demonstrate clinical allergy. An alternative estimate come from the observation that a positive skin test with shrimp was associated with a 26%-41% probability of a positive skin test to scallop, clam, oyster, abalone or limpet. Tropomyosin is also involved in cross-reactions between insects and crustacea and it is possible that individuals with allergy to cockroaches and other arthropods might show cross-reactions with molluscs.

The second well established cross-reaction is between dust mites and gastropods such as snails and limpets. This does not seem to depend on tropomyosin as only 2/28 patients in one study showed IgE binding to tropomyosin. In this case, the cross-reaction has been confirmed by oral challenges. For example, out of 51 children with dust mite allergy, 36 showed positive skin prick tests with snail. 15 of the children who had shown a positive skin test were challenged with cooked snails and 11 reacted. This suggests that as many as 30-40% of dust mite allergic individuals might react to snails although other studies have suggested that only approximately 15% might react. Allergy to limpet has also been associated with dust mite allergy. The reactions reported or observed were also unusual in that asthma was the most frequently reaction. Thus if you are dust mite allergic, have not recently eaten snails or related shellfish but intend to try them in future, it may be prudent to undergo a skin prick test to the relevant molluscs. The effects of desensitation therapy to dust mites on the probability and severity of reactions to molluscs have provoked controversy. This may be a topic to rise with your doctor if you are considering such therapy.

No cross-reactions have been reported between molluscs and foods from fin fish, birds or mammals.

 

Diagnosis

Allergies to molluscs are diagnosed using the same procedures as for other food allergies. A detailed patient history is first taken. Skin and laboratory tests are then used to identify the foods likely to cause allergic reactions, which can then be eliminated from the diet. Commercially available extracts from molluscs include abalone (Haliotis spp.), blue mussel (Mytilus edulis), clam (Ruditapes spp.), octopus (Octopus vulgaris), oyster (Ostrea edulis), pacific squid (Todarodes pacificus), scallop (Pecten spp.), snail (Helix aspersa) and squid (Loligo spp.). There are some pitfalls in investigating allergy to molluscs as the identification of the species eaten may not be simple, especially if a restaurant has substituted a cheaper mollusc such as limpet for an expensive one such as abalone.

 

Avoidance

Avoidance is the only therapy after diagnosis of an allergy to any mollusc. Because of the high probability of cross-reaction, it is probably sensible to eliminate from the diet all molluscs unless a negative skin test suggests that a species may be safely consumed. Even in that case, the difficulty of identifying molluscs after cooking suggests that care should be taken. Care should also be taken to avoid molluscs as ingredients in soups and dishes such as dim sum. Fortunately, there are no nutritional consequences to eliminating molluscs from the diet and no substitutes are required.

 


2.) Abalone / perlemoen allergy

Abalone is normally eaten as a cooked dish. It may be found in soups or Chinese dishes such as dim sum. It is sometimes used in rice porridge but this is not common as it is one of the most expensive shellfish. Its scientific name is Haliotis midae.

 

Abalone allergy

Allergy to shellfish such as abalone which are molluscs is less well known than allergy to shrimps (crustaceans). As with most food allergies, symptoms are usually mild such as oral allergy syndrome but severe symptoms such as anaphylactic shock can also occur after consumption. Note that abalone is not currently listed in annex IIIa of the EU directive on labelling of foods and thus individuals allergic to molluscs must be alert to the possibility of abalone or other molluscs as hidden allergens.

Allergy to abalone is most commonly associated with allergy to other related shellfish such as limpet, snails, winkles and whelks (gastropods). Reactions can also be triggered by eating more distantly related molluscs such as clams, mussels, oysters, and scallops (bivalves) or cuttlefish, octopus and squid (cephalopods). Thus after a diagnosis of allergy to one mollusc, patients are normally advised to avoid all molluscs. Whilst most individuals with allergy to shrimps (crustacea) can tolerate molluscs, individuals with allergy to both types of shellfish have been reported. However, individuals allergic to finfish (such as cod or salmon) do not generally have allergies to shellfish.

The two main allergenic proteins in abalone are tropomyosin and the Hal m1 protein.

 

Further information:

 


3.) Oyster allergy

Oysters are eaten either raw or cooked. They can be used as ingredients in soups and sauces, such as Chinese oyster sauce. Their scientific name is Crassostrea gigas.

 

Oyster allergy

Allergy to shellfish such as oyster is less well known than allergy to crustaceans. As with most food allergies, symptoms are usually mild such as oral allergy syndrome, but severe symptoms such as anaphylactic shock can also occur after consumption. Note that oysters are not currently listed in annex IIIa of the EU directive on labelling of foods and thus individuals allergic to molluscs must be alert to the possibility of oyster or other molluscs as hidden allergens.

Since oysters are bivalve molluscs, allergy to oyster is most commonly associated with allergy to other bivalves such as clams, mussels, and scallops. Reaction is also likely after ingestion of more distantly related molluscs such as limpet, snails, winkles and whelks (gastropods) or cuttlefish, octopus and squid (cephalopods). Thus, after a diagnosis of allergy to one mollusc, patients are normally advised to avoid all molluscs. Whilst most individuals with allergy to shrimps (crustacea) can tolerate molluscs, individuals with allergy to both types of shellfish have been reported. However, individuals allergic to finfish (such as cod or salmon) do not generally have allergies to shellfish.

The main allergenic protein in oyster is tropomyosin.

 

Further information:

 


4.) Snail allergy

Snails are eaten cooked with consumption being highest in France, Italy, Spain and Portugal.

 

Snail allergy

Allergic reactions have been reported to several species of snails including the brown garden snail (Helix aspersa), the burgundy snail (Helix pomatia), and the vinyard snail (Cernuella virgata). Helix terrestre may be a synonym for one of these. Allergies to the white garden snail Theba pisana (synonyms Euparipha pisana, Euparypha pisana or Helix pisana) and to the sea snail Bolinus brandaris have also been reported.

Allergy to molluscs such as snail is less common than allergy to shrimps. Snail allergy is associated with an unusual distribution of symptoms with asthma being very frequently reported. However, mild symptoms such as oral allergy syndrome, urticaria (hives) and severe symptoms such as anaphylactic shock can also occur after consumption.

Allergy to snail is frequently associated with allergy to dust mites and this may account for the high frequency of asthma and rhinitis seen as symptoms. There are also concerns that desensitization therapy with dust mite extracts may cause a more severe reaction to snails. Some individuals with allergy to shrimp (crustacea) may also suffer associated allergy to snail. For others, their allergy to snail is associated with allergy to other shellfish (molluscs) such as abalone and limpet which can include serious or fatal reactions.

The main allergenic protein in snails is tropomyosin.

 

Further information:

 


5.) Squid allergy

Squid are a type of octopus, also known as calamare or calamari. They are normally eaten cooked or fried. Consumption varies greatly between regions with high consumption in Spain and in Japan. There are over 300 species of squid, but only a few species are widely consumed.

 

Squid allergy

Allergy to molluscs such as squid is less common than allergy to shrimps (crustaceans). Although squid has no shell it is related to shellfish such as clams and mussels. As with most food allergies, symptoms are usually mild such as oral allergy syndrome but severe symptoms such as anaphylactic shock can also occur after consumption. Note that squid are not currently listed in annex IIIa of the EU directive on labelling of foods and thus individuals allergic to squid must be alert to the possibility of squid or other molluscs as hidden allergens.

Squid are a type of mollusc called a cephalopod and thus allergy to squid is associated with allergy to other cephalopods such as cuttlefish or octopus. Reaction is also likely after ingestion of more distantly related shellfish (molluscs) such as limpet, snails, winkles and whelks or clams, mussels, oysters and scallops. Thus after a diagnosis of allergy to one mollusc, patients are normally advised to avoid all molluscs. Whilst most individuals with allergy to shrimps (crustacea) can tolerate molluscs, individuals with allergy to both types of shellfish have been reported. However, individuals allergic to finfish (such as cod or salmon) do not generally have allergies to shellfish.

The main allergenic protein in squid is Tod p1.

 

Further information:

 


6.) Shrimps and other crustaceans allergy

Crustaceans are among the most commonly consumed seafood. Crustaceans belong to the Arthropod family. Crustaceans are divided into six major subgroups that include 44,000 species. Amongst these is a variety of commonly and less commonly eaten sea foods like langoustine, lobster, crayfish, and crab. Other sea foods like clams, mussels, oysters, scallops, abalone and squid are no crustaceans but molluscs. Crustacean allergy is a so-called IgE-mediated food allergy. IgE (Immunoglobulin E) is the allergy antibody.

 

Symptoms

Reactions range from mild oral allergy syndrome (itching of the lips, mouth, or throat, and swelling of the lips, tongue, throat, and palate) to life-threatening systemic anaphylaxis (difficulty breathing, drop in blood pressure, and even death). Symptoms occur within one to one hour of ingestion (90% of cases). However, urticaria (hives) is the most frequent symptom. Others symptoms affecting the skin (itching, swelling), gastro-intestinal tract (nausea, cramping heartburn, and diarrhoea), respiratory symptoms (asthma, hayfever), and eyes (conjunctivitis) have been reported. Shellfish are frequently implicated in fatal anaphylactic reactions and are the third most important cause of anaphylaxis after peanuts and tree nuts (cashews, almonds, pecans, walnuts, etc.).

Symptoms most often occur when the seafood is ingested (generally require higher doses to elicit a reaction), but can also appear when raw seafood is handled and even after inhaling steam while crustaceans such as shrimp are being cooked (may cause respiratory reactions). Cooking oil contaminated with seafood residues has also been reported to cause adverse reactions. Allergic reactions in workers at every stage of seafood processing also occur and are a serious public health problem in countries with major shellfish industries.

 

Related foods (cross-reactions)

It has been estimated that 75% of individuals who are allergic to one type of crustacean (shrimp, lobster, crawfish, or crab) are also allergic to another type. This is referred to as cross-reactivity. Cross-reactions between crustaceans and molluscs (oysters, squid, scallops) are also possible involving the same type of proteins in these foods. In fact, the molecules responsible for the cross-reactions can also cause cross-reactions between crustaceans, dust mites, cockroaches, and chironomid (used as fish food). However, no cross-reactions have been reported between crustaceans and fish such as Pollock, salmon, tuna, mackerel, trout, and anchovies.

 

Who, when, how long and how often?

Food allergies are most frequent in children, but adults are not exempt. Crustaceans, peanuts, fish, and tree nuts are the most common causes of food allergies among adults. Fish and shellfish allergies are estimated to affect approximately 1% of the general population. Countries where large amounts of crustaceans are consumed, such as the Scandinavian countries, have higher rates of crustacean allergies, although no species-specific studies have been conducted. Little is known about their persistence, but all evidence indicates that crustacean allergies are usually not outgrown.

 

How much is too much?

Symptoms most often occur when the seafood is ingested (generally require higher doses to elicit a reaction), but can also appear when raw seafood is handled and even after inhaling steam while crustaceans such as shrimp are being cooked (may cause respiratory reactions). Cooking oil contaminated with seafood residues has also been reported to cause adverse reactions. Allergic reactions in workers at every stage of seafood processing also occur and are a serious public health problem in countries with major shellfish industries.

While one shrimp is often enough to induce an allergic reaction, some people react to even smaller amounts. Lowest threshold dosages remain to be elucidated.

 

Diagnosis

A detailed patient history is first taken. Skin and laboratory tests are then used to help eliminate allergic foods from the diet. In the case of crustaceans, a positive skin test combined with elevated antibody levels allow shrimp allergies, for example, to be diagnosed with 87% certainty. There is no established threshold value for crustacean specific IgE.

However, food challenges (giving increasing amounts of potentially allergic food in graduated steps to patients to determine whether they will have an allergic reaction) are the most effective way of determining whether a person is truly allergic to a certain food. Initial food challenges are always conducted as double-blind, placebo controlled tests. In the case of crustaceans, for example, food challenges involve double-blind, placebo-controlled tests using increasing doses of the boiled crustacean in vanilla ice cream containing grape flavouring. If the double-blind placebo-controlled food challenge is negative, an open food challenge is generally performed. Food challenges should only be performed in a hospital setting with highly trained personnel used to dealing with anaphylactic reactions. Patients with a recent history of severe reactions should not be challenged. In addition, patients that are allergic to one type of crustacean should be tested for allergies reactions to all the other crustaceans because of the potential for cross-reactions.

 

Where do I find crustacean?

Crustaceans or crustacean residues may be present in certain processed foods. It is thus very important that people with allergies to crustaceans develop the habit of carefully reading ingredient labels and be aware of the words, terms, and indicators used on labels that indicate the presence of crustaceans or crustacean residues. Food products can contain hidden allergens.

 

Non-food products

Crustaceans are used in few non-food products such as creams. Contact with these products can induce allergic reactions.

 

Avoidance

For patients diagnosed with a crustacean allergy, avoidance of crustaceans is the only proven therapy, especially since adverse reactions to very small amounts of crustaceans are not uncommon. It is thus very important that people with allergies to crustaceans develop the habit of carefully reading ingredient labels and that they are aware of words, terms, and indicators used on labels that indicate the presence of crustaceans or crustacean residues. Food products can contain hidden allergens. Eating out especially in restaurants with South-East Asian cuisine is a risk factor for unexpected contact with crustaceans. Annex IIIa of the new EU labelling directive makes the listing of crustaceans and crustacean products on labels mandatory.

 


7.) Crab allergy

There are many species of edible crabs. Crabs are eaten cooked crab but also cooked in mixed seafood dishes such as Spanish paella. Crabsticks and surimi contain both finfish meat and some crab for flavour.

 

Crab allergy

Crabs along with crayfish, lobsters and shrimps are crustaceans. Food allergy to crustaceans is relatively common, symptoms ranging from mild oral allergy to severe symptoms such as anaphylaxis. Cooking does not remove the allergen. Crustacea are the third most important cause of food induced anaphylaxis after peanuts and tree nuts (cashews, almonds, pecans, walnuts, etc.). Thus crustacea and products thereof are listed in annex IIIa of the EU directive on labelling of foods and must be labelled when used as ingredients in pre-packaged food.

Most allergies to crustacea seem to involve a muscles protein called tropomyosin, which is very similar in a wide range of crustacean foods. As a result someone with allergy to tropomyosin from one kind of crustacean is likely to react to others. Thus individuals with allergy to one kind of crustacean are usually advised to avoid all types of crustacean foods.

In addition, some individuals with allergies to insects such as cockroach or moths can suffer food allergy to crustacean foods. Whilst most individuals with allergy to shrimps (crustacea) can tolerate molluscs, individuals with allergy to both types of shellfish have been reported. However, individuals allergic to finfish (such as cod or salmon) do not generally have allergies to shellfish.

The main allergenic proteins in crabs are tropomyosin and Cha f1.

 

Further information:

 


7.) Lobster allergy

There are many species of lobsters, but only a few are consumed widely, most of these are of the genus Homarus. Lobsters are generally eaten cooked, either alone or in mixed seafood dishes such as Spanish paella.

 

Lobster allergy

Lobsters along with crabs, crayfish, and shrimps are crustaceans. Food allergy to crustaceans is relatively common, symptoms ranging from mild oral allergy to severe symptoms such as anaphylaxis. Cooking does not remove the allergen. Crustacea are the third most important cause of food induced anaphylaxis after peanuts and tree nuts (cashews, almonds, pecans, walnuts, etc.). Thus crustacea and products thereof are listed in annex IIIa of the EU directive on labelling of foods and must be labelled when used as ingredients in pre-packaged food.

Most allergies to crustacea seem to involve a muscles protein called tropomyosin, which is very similar in a wide range of crustacean foods. As a result someone with allergy to tropomyosin from one kind of crustacean is likely to react to others. Thus individuals with allergy to one kind of crustacean are usually advised to avoid all types of crustacean foods.

In addition, some individuals with allergies to insects such as cockroach or moths can suffer food allergy to crustacean foods. Whilst most individuals with allergy to shrimps (crustacea) can tolerate molluscs, individuals with allergy to both types of shellfish have been reported. However, individuals allergic to finfish (such as cod or salmon) do not generally have allergies to shellfish.

The main allergenic proteins in crabs are tropomyosin, Pan s1 and Hom a1.

 

Further information:

 


This article was written in co-operation with the Food-Info.net website, an initiative of the Wageningen University, The Netherlands . We gratefully thank the authors behind the Food-Info website for providing us with the information for this article. For further information please visit http://www.food-info.net.