Shellfish allergy is the most common food allergy among adults in the United States. It's actually much more common in adults than children.

Unlike many food allergies, shellfish allergy is more likely to develop in adulthood than in early childhood. The majority of people who have shellfish allergy have their first reaction as adults. Once you develop a shellfish allergy, it tends to be severe and lifelong.﻿﻿

Symptoms

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

Skin reactions such as hives or eczema

Allergic conjunctivitis: Itchy, red, watery eyes

Digestive reactions such as nausea, abdominal pain, vomiting, or diarrhea

Airways symptoms such as wheezing or coughing or a 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 also is the most common cause of exercise-induced anaphylaxis, in which the combination of eating a food allergen and exercising causes an anaphylactic reaction.

What Are Shellfish?

Shellfish are divided into two families: mollusks and crustaceans. Mollusks include clams, oysters, and squid. Crustaceans include shrimp, lobster, and crayfish. Shellfish may live in fresh or salt water, or even on land — land snails, for example, are shellfish.

People who are allergic to one type of crustacean, such as shrimp, are generally allergic to all other crustaceans as well. If you are allergic to crustaceans, you may or may not be able to eat mollusks, such as clams or oysters. Allergy testing is the safest way to determine which shellfish, if any, you will be able to eat.

The allergenic protein in shellfish (tropomyosin) is not only found in sea creatures. People with shellfish allergies may also have reactions to dust mites, cockroaches, or other insects.

Living With a Shellfish Allergy

Since there is no cure for shellfish allergy, managing your condition 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.

The U.S. Food Allergy Labeling Law (FALCPA) includes crustacean shellfish as one of the big eight allergens that must be called out on food labels.﻿﻿ However, mollusks aren't included, which means 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, you're likely to also have a sensitivity to mollusks. Allergy testing can help you determine if mollusks are safe for you to eat, or if you have to avoid them.

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

Is Iodine a Problem?

Years ago, doctors believed there was a possibility that people who were allergic to shellfish might also react to iodine, including iodine used in medical imaging. In fact, some old medical forms still list this as an issue.

But it's not true—we now know that if you're allergic to shellfish, you do not need to avoid iodine.

That being said, it's possible to be allergic to iodine itself, or to the formulations of iodine used in medical imaging. But if you do have that allergy, it's not related to a shellfish allergy, so you don't need to worry about cross-reactions.

Shellfish Poisoning Is Not an Allergy

Allergies aren't the only medical conditions related to shellfish. Shellfish poisoning (also called paralytic shellfish poisoning and red tide) is a condition 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.

Symptoms may include tingling or burning in the mouth or extremities, nausea, vomiting, and diarrhea, and usually, occur within 30 minutes of eating tainted shellfish. These symptoms can be mistaken for an allergic reaction.

Shellfish poisoning can be very serious or even fatal. If you experience any of these symptoms after eating shellfish, seek emergency medical care.