Shellfish allergy

What it is

Shellfish allergy is an immune reaction to proteins in shellfish, mainly muscle proteins called tropomyosin.⁵ Shellfish fall into two groups: crustaceans, such as shrimp, prawns, crab and lobster, and mollusks, such as clams, mussels, oysters, scallops, octopus and squid.⁶ Crustaceans cause the greatest number of shellfish reactions.⁴ Shrimp is the most common shellfish allergen in both children and adults.⁶

About 2% of people in the US report a shellfish allergy.⁶ It is more common in adults than in children. AAAAI puts this down to eating habits: young children often don’t eat shellfish, so the allergy may not show up until later in life.⁵ About 60% of people with shellfish allergy have their first reaction as adults, and the allergy is usually lifelong.⁶

Symptoms it causes

Shellfish allergy can affect the skin, breathing, the gut and the heart and blood vessels, and symptoms can differ from one reaction to the next.⁴ ACAAI lists hives; swelling of the tongue or lips; vomiting, stomach cramps, indigestion and diarrhea; wheezing, shortness of breath, a repeated cough, a tight throat or a hoarse voice; and a weak pulse, pale or blue skin, dizziness or confusion.⁴

Shellfish can cause anaphylaxis, a severe reaction that affects the whole body. It can begin within seconds or minutes of exposure and get worse quickly.⁴ A tiny amount of shellfish can be enough to cause a severe reaction.⁵ Between 10% and 20% of life-threatening severe allergic reactions have no skin symptoms.⁴

Eating is not the only way in. Some people react when they are close to shellfish being cooked, because vapors and steam can carry small particles of shellfish protein.⁴⁶

Reading food labels

US law treats crustacean shellfish as a major food allergen, but not mollusks. According to FDA guidance, crustaceans such as crab, lobster and shrimp, and ingredients that contain their protein, are major food allergens; molluscan shellfish such as oysters, clams, mussels and scallops are not.³

For crustaceans, the label must name the species. FDA guidance says a label cannot broadly declare “crustacean shellfish”; it has to say shrimp, crab, lobster or the species used.³ The name goes either in parentheses after an ingredient or in a “Contains” statement right after or next to the ingredient list.¹ This rule comes from the Food Allergen Labeling and Consumer Protection Act of 2004 (FALCPA), which named crustacean shellfish among eight major food allergens. The FASTER Act, signed in 2021, added sesame as the ninth from January 1, 2023.¹ FDA enforces these rules for the foods it regulates; poultry, most meats, certain egg products and most alcoholic drinks fall under other agencies.²

Mollusks sit outside these rules. FARE notes that US labels do not have to fully disclose mollusks, so they may be in a food unexpectedly.⁶

Statements such as “may contain shellfish” are not required by law. FDA says they must be truthful and not misleading, and they cannot stand in for good manufacturing practices.¹

Shellfish is rarely hidden in foods, but it can be in fish stock, seafood flavoring such as crab extract, sushi and surimi.⁴ FARE adds bouillabaisse, cuttlefish ink and fish sauce, which is sometimes made from krill.⁶ Shellfish are often stored together in restaurants and markets, so one kind can come into contact with another.⁴