Shellfish Allergy: What You Need to Know?

By Jeanette Bradley, About.com Guide

Shellfish Allergy Overview:

Shellfish allergy is the most common food allergy among adults in the United States. About two percent of American adults have a shellfish allergy, and 0.1 percent of children have a shellfish allergy.

Unlike many food allergies, shellfish allergy is more likely to develop in adulthood than in early childhood. Shellfish allergies tend to be severe, lifelong food allergies.

Symptoms of Shellfish Allergy:

Symptoms of shellfish allergy usually appear within minutes to two hours of eating shellfish. Symptoms may include:

Skin reactions such as hives or eczema.
Allergic conjunctivitis: Itchy, red, watery eyes.
GI reactions such as nausea, abdominal pain, vomiting, or diarrhea.
Airways symptoms such as wheezing or coughing or runny nose.
Angioedema: Swelling of lips, tongue, or face

Shellfish allergies may cause a severe reaction called anaphylaxis. Anaphylaxis is a medical emergency that requires immediate medical care.

Shellfish allergy is the most common cause of Exercise-Induced Anaphylaxis, in which the combination of eating a food allergen and exercising causes anaphylaxis.

Seafood Allergy Categories

The types of seafood that cause allergies can be divided into three groups. The mollusca group contains three different sub-classes of seafood covering species as diverse as oysters, abalone and squid. The crustacea group includes prawns, crabs, crayfish, and shrimps. The third group, the chordata, includes all common edible fish, including salmon, cod, tuna, and flounder. Cod is the most common type of fish allergy, followed by species like trout, salmon, mackerel, herring, haddock, and halibut. Sometimes people with a seafood allergy relating to one particular species are able to eat other species of seafood without any problems.

The following table categorises the most common edible seafood that can cause an allergic reaction into three groups.

Classification of the types of seafood that cause allergies:

INVERTEBRATES

Group
Class
Species

Mollusca
Gastropoda
Bivalvia (Shellfish)
Cephalopoda
Abalone (Perlemoen), Snails (Escargot)
Mussels, Oysters, Clams, Scallops, Cockles
Squids (Calamari), Octopus, Cuttlefish

Arthropoda
Crustacea
Crabs, Lobsters, Shrimp, Prawn,
Saltwater Crayfish (Rock Lobsters),
Freshwater Crayfish (Yabbies)

VERTEBRATES

Group
Class
Species

Chordata
Condrichthyes
Osteichthyes
Sharks, Rays Salmon, Trout, Pike,
Cod, Haddock, Hake, Snapper, Mackerel,
Tuna, Bonito, Grouper,
Sole, Flounder, Halibut, Plaice


(Note that the degree of relation between each can be judged from the distance apart.)

Most seafood allergens belong to a group of muscle proteins. For example, parvalbumin is the allergen protein in codfish, while the proteins in crustaceans are called tropomyosin. While people with seafood allergy may only be sensitive to a few specific proteins, those proteins may be present in multiple species, which is why they may be at risk of allergic cross reactivity.

The dominating allergen in fish is the allergenic molecule Gad c1 found in cod and its homologues, and previously referred to as allergen M. Some individuals with cod allergy are known to have cross-reactivity with other fish species, including herring, mackerel and plaice. Other people with an allergy to shrimp will also exhibit cross-reactivity with other crustaceans, including crab, lobster, and crayfish. There is some evidence to suggest that the protein tropomyosin is a cross-reactive allergen for both crustacean and mollusc allergy.

Seafood Allergy Cross-Reactivity

It has been estimated that if someone has an allergy to fish then they have about a 50% chance of being allergic to at least one other species of fish. If they are allergic to shellfish then they will have approximately a 75% chance of having an allergy involving another shellfish. There appears to be little, if any, cross-reactivity between fish and shellfish but concomitant allergies are always possible.

Research has ascertained some common cross-reactivity clusters. Among these are:

Salmon, sardine, and mackerel.
Cod and tuna.
Octopus and squid.
Crab and shrimp.



These groups give an indication of groups of allergens to test for IgE reactivity when screening for seafood sensitisation.

There are incidences where food allergies have been linked to what has been referred to as “house dust mite-crustaceans-molluscs-syndrome”. It has been estimated that 80% of food allergies in adults are preceded by a clinical or sub-clinical sensation to inhaled allergens. This is because the food allergy is caused by cross-reactivity between ingested food and inhaled particles.



Living With a Shellfish Allergy:

Since there is no cure for shellfish allergy at this time, managing your shellfish allergy involves avoiding all shellfish and being prepared for future reactions. If you have been diagnosed with a severe shellfish allergy, your doctor will prescribe an epinephrine auto-injector (commonly called an Epi-Pen) that you will need to carry with you at all times.

Avoiding shellfish may seem easy, but food allergens can lurk in surprising places. You will need to learn to read labels to avoid shellfish, and learn to ask questions when you eat in restaurants.

Shellfish and Labeling Laws:

The Food Allergy Labeling Law (FALCPA) defines crustacean shellfish as one of the big eight allergens, but not mollusks. This means that manufacturers are not required to list the presence of clams, oysters, mussels, scallops or other mollusks in ingredient lists.

If you are allergic to crustacean shellfish, there is a likelihood that you may have a sensitivity to mollusks as well. Allergy testing can help you determine if mollusks are safe for you to eat.

You should always read ingredient labels carefully if you have shellfish allergies.

Shellfish Allergy and Iodine:

There are unsubstantiated concerns of a cross reactivity between shellfish allergy and iodine/contrast allergy — some old medical forms still list this incorrectly as a contraindication. If you are allergic to shellfish, you do not need to avoid iodine or radiocontrast material. It is possible to be allergic to iodine or radiocontrast material, but those allergies are not related to shellfish allergies, so you do not need to worry about cross-reactions.

Shellfish Poisoning:

The symptoms of shellfish poisoning (also called paralytic shellfish poisoning and red tide) usually occur within 30 minutes of eating tainted shellfish, and may be confused with an allergic reaction. Shellfish poisoning is caused by a very potent toxin called saxitoxin that is released by algae-like organisms that live in two-shelled mollusks, such as clams and oysters. As such, this kind of toxin only affects mollusks, and not fish or lobster. Symptoms may include tingling or burning in the mouth or extremities, nausea, vomiting, and diarrhea. Shellfish poisoning can be very serious or even fatal. If you experience any of these symptoms after eating shellfish, seek emergency medical care.



Sources: treatallergies. http://www.treatallergies.com/dyn/320/Seafood-Allergies.html 2002

FAAN. Shellfish Allergy. http://www.foodallergy.org/page/shellfish-allergy Accessed 2/10/2011.

NIAID-Sponsored Expert Panel. Guidelines for the Diagnosis and Management of Food Allergy in the United States: Report of the NIAID-Sponsored Expert Panel. The Journal of Allergy and Clinical Immunology. Volume 126, Issue 6, Supplement , Pages S1-S58, December 2010

Sheerin, Kathleen A. “Seafood Allergy.”Allergy and Asthma Advocate. Winter 2006. 9 June 2007.