A shellfish allergy occurs when the body’s immune system mistakenly identifies proteins in certain marine animals as harmful. This triggers the release of chemicals like histamine, leading to reactions. These reactions can range from mild symptoms, such as hives or a stuffy nose, to severe and potentially life-threatening conditions like anaphylaxis, which requires immediate medical attention and an epinephrine injection. For individuals with this allergy, strict avoidance of shellfish and products containing them is necessary.
Defining Shellfish Allergens
Shellfish encompasses two primary categories: crustaceans and mollusks. Crustaceans include animals with segmented bodies and hard exoskeletons, such as shrimp, crab, lobster, crayfish, and krill. Mollusks are soft-bodied invertebrates, often enclosed in a shell, and include clams, oysters, mussels, scallops, squid (calamari), octopus, and snails (escargot).
The major allergen in most shellfish allergies is a muscle protein called tropomyosin, which is similar across various shellfish species. Due to potential cross-reactivity between proteins, allergists often advise avoiding all shellfish, even if allergic to only one type. For example, someone allergic to crab may also react to lobster or shrimp, and a person allergic to clams might react to mussels or scallops. Cross-reactivity can even extend to unrelated invertebrates like dust mites and cockroaches due to similar tropomyosin structures.
Common and Concealed Food Sources
Shellfish can be found in many obvious dishes and ingredients. Common examples include shrimp scampi, crab cakes, lobster bisque, clam chowder, paella, and sushi containing shellfish. Seafood mixes are also a clear source of shellfish.
Beyond obvious dishes, shellfish can be present in less apparent forms or through cross-contamination. Seafood flavoring or stock, derived from shellfish, may be incorporated into broths, soups, sauces, and gravies. Surimi, often labeled as imitation crab meat or “sea legs,” typically contains fish but can also include shellfish or flavoring.
Asian cuisine frequently uses ingredients like fish sauce, oyster sauce, and shrimp paste, which contain shellfish and can be hidden sources. Cross-contamination poses a risk, especially when dining out, as shared fryers, cooking surfaces, or utensils can transfer shellfish proteins to other foods. Given these hidden sources, it is important to inquire about ingredients and preparation methods when consuming food prepared outside the home.
Strategies for Daily Avoidance
Careful label reading is an important strategy for managing a shellfish allergy. In the United States, the Food Allergen Labeling and Consumer Protection Act (FALCPA) mandates clear labeling of major allergens, including crustacean shellfish. Food labels will declare crustacean shellfish in plain language, either within the ingredient list or in a “Contains: Shellfish” statement. It is important to note that FALCPA does not require labeling for molluscan shellfish, so vigilance is still necessary for ingredients like clams, mussels, or scallops.
Dining out safely requires communication. Inform restaurant staff about the allergy, inquire about ingredients in dishes, and ask about cooking methods to prevent cross-contamination. Requesting separate preparation areas or dedicated utensils can help minimize risk.
Beyond food, non-food sources of shellfish exposure exist. Glucosamine and chondroitin supplements, commonly taken for joint health, are frequently derived from shellfish shells. Individuals with shellfish allergies should consult their doctor before taking such supplements. Some pet foods, particularly those with fish flavors, may also contain shellfish. Contact with shellfish, such as at seafood counters or during fishing, can lead to reactions, and airborne allergens from cooking shellfish can also trigger symptoms.