Fish Allergy

Fish allergy is an IgE-mediated reaction to bony fish proteins. It may begin in childhood or adulthood and usually persists, with reactions that can be severe.

Pathophysiology. The main allergen is parvalbumin, a heat stable muscle protein that is highly conserved across fish species, so cross-reactivity between species is common. Collagen and enolase are additional allergens.

It affects roughly 0.5 to 2% of the population. Cod, salmon and tuna are common triggers, and reactions occur by ingestion, inhalation of cooking vapours and skin contact.

Some patients tolerate certain species, in particular tuna or swordfish, which have less parvalbumin, and canned fish may be tolerated or more allergenic depending on processing.

• Urticaria, angio oedema, flushing within minutes
• Vomiting, abdominal pain, diarrhoea
• Wheeze, rhinitis, throat tightness
• Anaphylaxis
• Reactions to vapours or handling
• Contact dermatitis and occupational asthma in fish workers
• Atopic eczema flares in children

• History: species, preparation, amount, timing and cofactors.
• Skin prick test or specific IgE to fish extracts and to parvalbumin (cod Gad c 1): widespread sensitisation suggests a species wide reaction. A negative parvalbumin result with a positive result to one species suggests species specific sensitisation, for example to collagen.
• Supervised oral food challenge to test species that have not been eaten, because many patients tolerate some fish.
• Exclusion of Anisakis allergy, if reactions follow raw or undercooked fish.

Differential diagnosis.

• Scombroid poisoning (histamine toxicity from spoiled fish)
• Anisakis allergy and parasite infection
• Shellfish allergy
• Ciguatera and other fish toxin illness
• Food additives and sulphite sensitivity
• Idiopathic anaphylaxis

• Avoidance of fish proven to cause reactions, and of dishes that cross-contact them (shared fryers, fish sauce, Worcestershire sauce, Caesar dressing). Patients with extensive sensitisation avoid all fish until challenge testing clears some species.
• 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).
• Fish oil supplements are usually purified of protein, but a cautious approach is needed in severe allergy.
• Nutrition: omega-3 intake from non-fish sources (flaxseed, walnut), and vitamin D and iodine through diet, with a dietitian.
• Vaccines: no fish proteins in routine vaccines.

Special groups.

• Children: persistence is common, so review with challenge at intervals, but resolution is less frequent than in milk or egg allergy.

Monitoring. Review every 12 months. Repeat testing every 1 to 2 years in children. Check auto injector dose and expiry.

Referral and escalation. Refer to allergy for any suspected fish allergy, since cross-reactivity and species tolerance need specialist testing. Emergency for anaphylaxis.