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Shellfish Allergy

Introduction
Signs and symptoms
Causes
Risk Factors
Screening and diagnosis
Treatment
Prevention

Shellfish includes animals with shells, such as clams, lobster and shrimp, as well as octopus and squid. You may only be allergic to some kinds of shellfish, or you may need to avoid all shellfish. While over-the-counter and prescription medications can help control symptoms, the best way to prevent an allergic reaction is to avoid the type of food that causes it.

If you or your child has a reaction to shellfish, tell your doctor about it, no matter how mild the reaction may have been. Tests can help confirm a shellfish allergy, so you can take steps to avoid future and potentially more-severe reactions.

Signs and symptoms of a shellfish allergy include:

• Hives, itching or eczema
• Swelling of the lips, face, tongue and throat, or other parts of the body
• Wheezing, nasal congestion or trouble breathing
• Abdominal pain, diarrhea, nausea or vomiting
• Dizziness, lightheadedness or fainting
• Tingling in the mouth

A severe allergic reaction to shellfish called anaphylaxis is rare — but can be life-threatening if it interferes with breathing. An anaphylactic reaction is a medical emergency that requires treatment with an epinephrine (adrenaline) injection and a trip to the emergency room. Signs and symptoms of anaphylaxis include:

• Constriction of airways, including a swollen throat or a lump in your throat, that makes it difficult to breathe
• Shock, with a severe drop in blood pressure
• Rapid pulse
• Dizziness, lightheadedness or loss of consciousness

A shellfish allergy is distinct from an adverse reaction caused by toxins or bacteria. This type of food poisoning is common and can cause symptoms similar to a shellfish allergy. But unlike an allergy, it does not involve the immune system and only occurs when you eat food that has been contaminated. An allergic reaction to shellfish usually occurs every time you eat the type of shellfish that causes the reaction.

All food allergies are caused by an immune system malfunction. Your immune system identifies certain shellfish proteins as harmful, triggering the production of antibodies to neutralize the shellfish protein (allergen). The next time you come in contact with proteins in shellfish, these antibodies recognize them and signal your immune system to release histamine and other chemicals that cause allergy symptoms.

Histamine and other body chemicals cause a range of allergic signs and symptoms. Histamine is partly responsible for most allergic responses, including runny nose, itchy eyes, dry throat, rashes and hives, nausea, diarrhea, labored breathing, and in some cases, anaphylactic shock.

There are several types of shellfish, and each kind contains different allergy-causing proteins.

Crustaceans include crab, lobster, crayfish, shrimp and prawns.

Mollusks include:

• Bivalves such as clams, mussels, oysters, scallops and abalone
• Gastropods such as limpets, periwinkles and snails (escargot)
• Cephalopods such as squid, cuttlefish and octopus

Some people are allergic to only one type of shellfish, but can eat others. However, some people with a shellfish allergy react to and must avoid all shellfish.

Your doctor will ask about your symptoms and may perform a physical exam to find or rule out other medical problems. He or she may also recommend one or both of the following tests:

• Skin test. In this test, your skin is pricked and exposed to small amounts of the proteins found in shellfish. If you're allergic, you develop a raised bump (hive) at the test location on your skin. Allergy specialists usually are best equipped to perform and interpret allergy skin tests.
• Blood test. Also called allergen-specific IgE antibody test, radioallergosorbent (RAST) test or allergy screen, this test can measure your immune system's response to shellfish proteins by measuring the amount of certain antibodies in your bloodstream, known as immunoglobulin E (IgE) antibodies. A blood sample is sent to a medical laboratory, where it can be tested for evidence of sensitivity to shellfish proteins.

A history of allergic reactions shortly after exposure to shellfish can be signs of a shellfish allergy, but allergy testing is the only sure way to tell what's causing your symptoms. Adverse reactions to shellfish are also sometimes caused by a nonallergic reaction such as food poisoning or a bacterial or viral infection.

The only sure way to prevent an allergic reaction is to avoid shellfish altogether.

Medications such as antihistamines may reduce signs and symptoms of a shellfish allergy. These drugs can be taken after exposure to shellfish to control your reaction and help relieve discomfort.

Despite your best efforts, you may still come into contact with shellfish. If you have a serious allergic reaction, you may need an emergency injection of epinephrine (adrenaline) and a trip to the emergency room. If you're at risk of having a severe reaction, you may need to carry injectable epinephrine (such as an EpiPen) with you at all times.

If you know you are allergic to shellfish, the only sure way to avoid an allergic reaction is to avoid all shellfish or products that might contain shellfish. Even trace amounts of shellfish can cause a severe reaction in some people. Shellfish is seldom a hidden food ingredient, so it may be easier to avoid than some other allergy-causing foods.

Avoiding shellfish

• Eating at restaurants poses the biggest danger of mistakenly eating shellfish. When you eat at restaurants, always check to make sure the same pan, oil or utensils used for shellfish are not also used to prepare other foods. This is called "cross-contamination."
• Fish and shellfish are biologically distinct, so fish will not cause an allergic reaction if you have a shellfish allergy — unless you are also allergic to fish. But when eating at a seafood restaurant, there is a higher risk of cross-contamination of your food with trace amounts of shellfish.
• Cross-contamination can occur in stores where food may be processed or displayed along with shellfish, and during manufacturing. Be sure to read food labels carefully. Companies are required to clearly label any product that contains even small amounts of shellfish or other foods that often cause allergic reactions.
• You may need to completely avoid environments where shellfish are prepared or processed. Some people even have a reaction after touching shellfish or inhaling steam from cooking shellfish.
• Do not take coral calcium if you're allergic to shellfish. Coral calcium, which is obtained from ocean coral reefs, can trigger allergic reactions, such as hives, breathing problems and swelling, in people with shellfish allergies.

Some people mistakenly believe that allergy to iodine or allergy to radiocontrast dye used in some lab procedures can cause reactions in people with a shellfish allergy. Reactions to radiocontrast material or iodine are not related.

Glucosamine, a supplement used to prevent and treat arthritis, is made from crab, lobster or shrimp shells. While it does not appear to cause an allergic reaction in most people who have a shellfish allergy, more studies need to be done to determine whether it is safe for people allergic to shellfish.

If you are at risk of a serious allergic reaction, talk with your doctor about carrying emergency epinephrine (adrenaline). If you have already had a severe reaction, wear a medical alert bracelet or necklace that lets others know that you have a food allergy.