Fish and fish products play an important role in human nutrition. Fish is a valuable source of proteins and contains large amounts of healthy fats (so called polyunsaturated fatty acids) and fat-soluble vitamins. However, it also is one of the most common causes of food allergy. Fish allergy is a so-called IgE-mediated food allergy. IgE (Immunoglobulin E) is the allergy antibody.
Allergy to fish is caused by a reaction to protein in fish meat (muscle). The dominating allergen is a muscle protein called parvalbumin. In professional literature this allergen is often referred to as “Gad c 1” from the Latin name for cod fish Gadus callarias. This major allergen is extremely stable to heat, which means that boiling or frying of fish does not destroy the allergen. Other proteins in fish have been described as allergens, but most reactions to cod (and other fish; see under Related foods) are most likely caused by this one allergen
Reactions can be severe and even life-threatening. The severity of symptoms may vary according to the amount ingested and the sensitivity of the person. Often the first symptom is irritation and itching in mouth and throat appearing few minutes after the intake. It can be followed by other allergic reactions such as nausea, vomiting, stomach ache, diarrhoea, hives (also called urticaria or nettle rash), swelling under the skin (also called angioedema), itching and reddening of the skin, worsening of eczema, asthma (wheezing, breathlessness, coughing), hay fever (itchy nose and eyes, sneezing/runny nose), swelling of the airways, and sometimes fatal episodes of allergic shock. Usually a combination of several symptoms is seen.
Spoiled fish can contain a substance called histamine. This is the same substance that is produced by cells of an allergic patient during an allergic reaction. It is involved in the induction of symptoms. Spoiled fish can elicit symptoms in every person having eaten it. The reaction is similar to an allergic reaction, i.e. swelling, hives, wheezing etc., but it is poisoning.
Related foods (cross-reactions)
Most information on fish allergy is gathered on codfish. The variety of fish eaten around the world is immense. Despite this, fish species known to cause allergy belong to a few closely related orders: codfish and hake (Gadiformes), mackerel, tuna and perch (Perciformes), salmon and trout (Salmoniformes), plaice and sole (Pleuronectiformes), herring, anchovy and sardine (Clupeiformes), carp and catfish (Cypriniformes), and eel (Anguilliformes). Patients with allergy to codfish are often allergic to the other fish species as well. This can be explained by similarity of the allergen parvalbumin in all fish species. Allergic reactions based on such similarity are called cross-reactions. The cross-reactivity between fish species is certainly not complete. Some patients are allergic to one and tolerate other species.
Allergy to fish does not mean that other seafood like shellfish is not tolerated. Cross-reactivity is irrelevant between fish and shellfish. Of course, patients can develop allergy to both independently. Fish roe (or caviar) has been reported to cause food allergy but there is no relation to allergy to the fish from which the eggs originate.
Finally, it has been reported that parvalbumin in frogs legs can in some cases also cause allergy in fish allergic patients. This again is cause by cross reactivity.
Who, when, how long and how often?
Food allergy to fish is seen both among children and adults (approximately 0.1-0.2%). Varieties in food habits according to country influence the frequency patterns of fish allergy, with the number of fish allergics being higher in those countries where fish is a major component of the local diet. In general, fish allergy is not outgrown but it persists through life.
How much is too much?
Care has to be taken since very small amounts (few mg, in other words a tiny flake) of fish can elicit a reaction in very sensitive persons. A dose of only 5 mg of cod has been described to cause reaction. Furthermore, some fish allergic persons can get allergic symptoms due to the steam (airborne allergens) from cooking fish. Fish allergy is therefore sometimes a problem in the fish industry and among restaurant cooks, where handling and inhalation might cause eczema and asthma. Finally, even a kiss of somebody that has eaten fish can induce a reaction in a fish allergic person.
An indication for IgE-mediated fish allergy can be obtained from skin prick testing and from serum IgE testing. The presence of a positive skin prick test or of fish protein-specific IgE-antibody in serum is indicative of an IgE-mediated fish allergy, but both tests may be false-positive or false-negative. Therefore, a definitive diagnosis has to be based on strict, well-defined elimination and re-introduction protocols or on controlled fish challenge procedures. Fish allergy is confirmed if symptoms disappear after elimination and re-appear upon re-introduction or if a so-called double-blind placebo controlled food challenge gives a positive result. During such a challenge both doctor and patient do not know which challenge meal contains fish and which does not. Such challenge procedures are also helpful in determining the threshold dose of reactivity, and to verify if a person has outgrown the food allergy, although this is rarely seen with fish allergy.
Where do I find fish?
It is important to study the labels on processed foods since various products can contain fish: surimi (fish product imitating crabmeat), fish meal, animal fat, liver pâté, some sausages, crab salad, sushi, oyster sauce, Worcestershire sauce, Caesar salad, tapenade, and pizza toppings. Fish oils/animal oil can also contain fish proteins, depending on the degree of refining of the oil. Fish gelatine made from skin and bones and used in food products is not considered to present a risk to fish allergic persons at the doses typically used.
Fish gelatine is applied in pharmaceutical products like vaccines, but it is not considered a risk to fish allergic persons.
If suffering from fish allergy, strict avoidance of fish in any form and food containing fish-derived ingredients is the only way to prevent a reaction. This can sometimes be difficult because they can be hidden in food products. According to the new EU labelling directive (2003/89/EC) and the list of the Codex Alimentarius Commission, any fish-derived ingredient has to be listed on the label. Even if labels are carefully read, unintentionally and accidentally consumption of fish may happen. Fish allergic individuals should especially be cautious when eating away from home. When ordering a “non-fish meal” at a restaurant it may be contaminated with fish proteins from utensils, cooking oil or a grill exposed to fish.