Shellfish Allergy

Shellfish allergy is an IgE-mediated reaction to crustaceans (shrimp, prawn, crab, lobster, crayfish) or molluscs (clam, mussel, oyster, scallop, squid, octopus, snail). It usually starts in adulthood and usually persists.

Pathophysiology. The major allergen is tropomyosin, a muscle protein that is heat stable and conserved in many invertebrates. Cross-reactivity is high within crustaceans and variable between crustaceans and molluscs, and with house dust mite, cockroach and Anisakis.

It affects roughly 2% of adults. Reactions may follow ingestion, inhalation of cooking vapours, or handling.

Allergy to iodine or radiocontrast is not a consequence of shellfish allergy.

• Urticaria, angio oedema, flushing within minutes
• Oral itching, vomiting, diarrhoea, abdominal pain
• Wheeze, rhinitis, throat tightness
• Anaphylaxis, sometimes with exercise as a cofactor
• Reactions to vapour in cooking
• Occupational asthma and rhinitis in seafood workers
• Reactions to molluscs in patients allergic to house dust mite, without crustacean allergy

• History: which shellfish, amount, timing, cofactors such as exercise or alcohol.
• Skin prick test or specific IgE to shrimp and to tropomyosin (Pen a 1): positive results indicate sensitisation and cross-reactivity, but do not predict severity. Fresh food prick-to-prick testing may be more sensitive.
• Supervised oral food challenge when tolerance of molluscs or other crustaceans is in doubt.
• House dust mite sensitisation: consider because it explains positive tests without clinical allergy.

Differential diagnosis.

• Fish allergy and Anisakis allergy
• Scombroid poisoning and histamine toxicity
• Food additives (sulphites) in shellfish dishes
• House dust mite related cross-sensitisation without clinical allergy
• Food poisoning and toxin ingestion
• Idiopathic anaphylaxis

• Avoidance of the shellfish group that causes reactions. Many patients avoid all crustaceans, since cross-reactivity is common. Molluscs can be tested and challenged separately.
• Adrenaline auto injector: 150 microgram for 7.5 to 25 kg, 300 microgram for 25 to 50 kg, 500 microgram for over 50 kg, with a written action plan and training.
• Anaphylaxis: adrenaline 0.5 mg intramuscularly (children 0.01 mg/kg, maximum 0.5 mg) into the anterolateral thigh, repeated after 5 minutes if no improvement, plus oxygen and intravenous fluids, and observation for at least 6 hours.
• Mild reactions: cetirizine 10 mg orally once daily in adults (children 1 to 5 years 2.5 mg twice daily; 6 to 11 years 5 mg twice daily).
• Avoid cooking vapour exposure and shared fryers where allergic to vapour.
• Occupational disease: remove from exposure or control with ventilation and masks.
• Contrast media and iodine are not contraindicated by shellfish allergy.

Special groups.

• Adolescents and adults: exercise and alcohol raise the risk of reactions to even small doses.

Monitoring. Review every 12 months. Check auto injector dose and expiry. Repeat testing is not routine because adult onset allergy persists.

Referral and escalation. Refer to allergy for anaphylaxis, multiple shellfish reactions, or occupational exposure. Emergency for airway swelling, wheeze, or collapse.