Fish allergy

After a reaction: identify the exact fish

• Date and time, exact species if possible, name of dish, restaurant or brand, photo of the packaging and ingredients list.
• Fish alone or in a mixture, amount, raw/smoked/marinated/cooked, cooking method, delay before symptoms and exact symptoms.
• Medicines used, any adrenaline, emergency-department attendance and other fish eaten without a reaction.

Parvalbumin: the main allergen

Parvalbumin is a small, highly stable muscle protein. It is relatively resistant to heat and digestion. Beta-parvalbumin is the major allergen in many bony fish and explains much of the cross-reactivity between species.

Why are some fish sometimes better tolerated?

The amount of parvalbumin varies greatly between species and even between muscles. It is often more abundant in white than dark muscle. Tuna, swordfish and some large migratory fish may contain relatively little parvalbumin, which contributes to better tolerance in some patients.

Bony and cartilaginous fish

Are there allergens other than parvalbumin?

Raw or cooked fish?

Cooking fish is not enough to remove its allergenic potential because parvalbumin is heat stable. However, some allergens such as enolase and aldolase are more heat sensitive. Tolerance of raw or cooked fish therefore depends on the individual profile.

Can cooking vapours cause a reaction?

Yes, in some patients. During cooking, aerosols or droplets may carry fish proteins and cause rhinitis, cough, asthma or hives, especially with heavy or occupational exposure. A smell alone does not necessarily mean exposure to an allergenic dose.

Fish ↔ chicken: fish-chicken syndrome

Cross-reactivity between fish and chicken meat has been described through related proteins, particularly parvalbumins, enolases and aldolases. This remains rare.

Fish ↔ crustaceans

What about fish gelatin?

Fish gelatin is made from fish collagen. A minority of patients have IgE directed against this collagen, but clinical allergy to fish gelatin appears much rarer than allergy to fish flesh.

Sweets and capsules: no general ban

What about omega-3 supplements or fish oils?

Highly purified oils normally contain very little protein. A fish allergy therefore does not automatically prohibit every omega-3 supplement. In a patient with a severe allergy or a previous reaction to a supplement, the product should be discussed with a doctor or pharmacist.

A reaction after tuna is not always an allergy: scombroid syndrome

Poorly stored fish may accumulate large amounts of histamine and cause sudden redness, flushing, headache, palpitations, hives or digestive symptoms. This is histamine poisoning, not an IgE-mediated allergy. An important clue is that several people who ate the same fish may become unwell.

What about Anisakis?

Anisakis is a parasite found in some fish and cephalopods. A person may be allergic to the parasite rather than the fish itself, particularly after eating raw, marinated or insufficiently cooked fish. Tests to fish proteins may then be negative while Anisakis assessment is positive.

Fish-triggered FPIES

FPIES is a non-IgE-mediated food allergy. The typical presentation combines repeated vomiting, pallor, lethargy and sometimes diarrhoea, often 1 to 4 hours after eating, without hives or the respiratory symptoms typical of an IgE-mediated reaction. Skin prick tests and specific IgE may be negative.

What happens during allergy assessment?

• History: exact species, amount, cooking method, delay, symptoms, treatment and other fish already tolerated.
• Skin prick tests: commercial extracts and, in some centres, prick-to-prick testing with fresh or cooked fish when this provides additional information.
• Specific IgE: available for several species; a positive result does not automatically prove a clinical allergy to the species tested.
• Molecular allergology: parvalbumins such as Gad c 1/Gad m 1, Cyp c 1 and Sal s 1, and in some profiles enolases, aldolases or collagen.

OFC with another species

An oral food challenge may check tolerance of a fish, particularly when the patient has avoided every species for a long time, has reacted to only one or two, or when tests and history are discordant. It remains the reference test in uncertain situations.

Is fish allergy permanent?

Fish allergy is often more persistent than milk or egg allergy, but it is not necessarily permanent. Many people tolerate at least one other species, and reassessment may allow the diet to expand over time.

What should be avoided?

• First, avoid the species actually responsible and preparations containing them.
• Keep fish that are already clearly tolerated when the allergist confirms this is appropriate.
• Remain alert to cross-contact: shared deep fryers, griddles, broths, sushi, buffets and shared utensils.

Before your appointment: the SOS Allergo checklist

• Exact species if possible, photo or menu, brand and ingredients, raw/smoked/marinated/cooked, amount and delay before symptoms.
• Photos of the reaction, medicines used, emergency-department report, and any reactions to vapours or contact.
• List of other fish already eaten without a problem; sweets or gelatin, omega-3 or capsules taken without a reaction; whether chicken is tolerated; circumstances suggesting Anisakis or scombroid poisoning.