Shellfish Allergy: Symptoms, Treatment and Foods to Avoid

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Crab is a crustacean shellfish. David Trood/Getty Images

Shellfish allergy is the most common food allergy among adults in the United States, although it's rare among children. Unlike many food allergies, shellfish allergy is more likely to develop in adulthood than in early childhood. In fact, around 60% of people with shellfish allergy experience their first reaction to shellfish during adulthood.

About 2% of American adults have a shellfish allergy, and 0.1% of children have a shellfish allergy. Shellfish allergies tend to be severe, lifelong food allergies.

Types of Shellfish

Shellfish are divided into two types: mollusks and crustaceans. Mollusks include clams, oysters, mussels, and scallops. Crustaceans include shrimp, lobster, and crayfish. Shellfish may live in fresh or salt water, and even can live on land.

Out of the many different types of shellfish, shrimp, crab, and lobster cause most shellfish allergies.

Allergy Symptoms

Allergic symptoms of shellfish allergy usually appear within minutes, although they may appear up to two hours after eating shellfish. Symptoms may include:

  • skin reactions such as hives or eczema
  • allergic conjunctivitis (itchy, red, watery eyes)
  • gastrointestinal reactions including nausea, abdominal pain, vomiting, or diarrhea, or a combination of these symptoms
  • airway symptoms including wheezing, coughing or a runny nose and/or a combination of these symptoms
  • swelling of lips, tongue, or face.

Shellfish allergies can be life-threatening because they can cause a severe reaction called anaphylaxis. Anaphylaxis is a serious medical emergency and requires immediate medical care. Many people with shellfish allergy carry a source of epinephrine, such as an Epi-Pen, in the event of a severe allergic reaction. Epinephrine can curb the allergic reaction quickly, enabling you to breathe again.

Shellfish allergy is the most common cause of exercise-induced anaphylaxis, where the consumption of a food allergen such as shellfish, coupled with exercise, may trigger an anaphylactic reaction.

Diagnosis and Treatment

Shellfish allergy is diagnosed by a medical doctor (usually, but not always, an allergist). Your doctor will talk to you about your medical history, and will perform a physical examination and food allergy testing.

The only treatment for shellfish allergy is to avoid shellfish and foods made with shellfish.

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. If you mistakenly consume shellfish, you will need to inject yourself with the Epi-Pen to stop the reaction.

How to Avoid Shellfish

Since shellfish allergy is one of the most common food allergies in the United States, federal law requires that manufacturers declare shellfish in clear language on food labels, either in the list of ingredients or following the word "contains" after the ingredient list. Most manufacturers do both, listing the shellfish-containing ingredient in the ingredients list, and adding a "Contains: shellfish" statement in bold type under that list.

Be sure you know how to read an ingredients list so that you can spot potentially dangerous ingredients.

Federal law does not require manufacturers to label mollusk shellfish. This means that manufacturers are not required to list the presence of clams, oysters, mussels, scallops or other mollusks in their ingredient lists. Those with mollusk shellfish allergy must be very careful with food products and err on the side of caution.

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.

Foods to Avoid with a Shellfish Allergy

Some of the most common types of crustacean shellfish include:

  • crab
  • crawfish (also called crayfish or crawdads)
  • langoustines (lobster)
  • lobster
  • prawns
  • sea urchin
  • shrimp

People who are allergic to mollusks should avoid all mollusks, which include:

  • abalone
  • clam (quahog)
  • cockle
  • limpet
  • mussels
  • octopus
  • oysters
  • scallops
  • snails (also known as escargot; both sea and land snails should be avoided)
  • squid (calamari)
  • surimi (imitation shellfish)
  • whelk

Cross-Reactivity

People who are allergic to one type of crustacean, such as shrimp, are generally allergic to all other crustaceans. However, if you are allergic to crustaceans, you may not be allergic to mollusks. Allergy testing is the safest way to determine which shellfish, if any, you will be able to eat. Consult with your doctor before eating any new type of shellfish.

The allergenic protein in shellfish is called tropomyosin. Tropomyosin is also found in land snails, dust mites, cockroaches, and other insects. People with shellfish allergies may also experience symptoms to these substances and insects.

Shellfish Allergy and Iodine

There have been concerns that people with shellfish allergy would also be allergic to iodine and possibly to certain so-called radiocontrast agents used to enhance x-rays, and in fact, some old medical forms still mention shellfish allergy as a potential problem for iodine and radiocontrast agents.

However, the most recent medical research shows that people who are allergic to shellfish 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.

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. However, they're not the same condition.

Shellfish poisoning is caused by a very potent toxin called saxitoxin that is released by algae-like organisms that live in bivalve mollusks, such as clams and oysters. This kind of toxin only affects mollusks, and not fish or lobster. Symptoms of shellfish poisoning may include tingling or burning in the mouth or extremities, nausea, vomiting, and diarrhea. Shellfish poisoning can be very serious and even fatal. If you experience any of these symptoms after eating shellfish, seek emergency medical care.

A Word from Verywell

Managing your shellfish allergy can seem daunting, especially if you develop the condition as an adult. You may be forced to give up foods you enjoyed, and to monitor what you eat much more carefully. However, reading ingredients lists and restaurant menus should quickly become second nature, and you'll be able to avoid most or all reactions simply by avoiding the foods that you know trigger those reactions.

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Article Sources
  • Joneja JV. The Health Professionals Guide to Food Allergies and Intolerances. 2012.

  • Sicherer S. Food Allergies: A Complete Guide for Eating When Your Life Depends on It. 2013.

  • Food Allergy Research & Education. Shellfish allergy fact sheet.