Fish and shellfish allergy affects children less commonly than adults, and unlike other food allergies is often lifelong, particularly with shellfish. It is more commonly found in populations with a high consumption of fish, notably those from South East Asia. Fish and shellfish allergy are usually only associated with IgE mediated reactions and not normally seen in delayed reactions, and more commonly occurs in people with atopic conditions, sensitised via the respiratory system to aero-allergens or who have asthma or rhinitis. Reactions can be severe an include itching, hives or urticarial rashes, angio-oedema, tracheal obstruction, asthma, vomiting and shock. Cross-reactivity is common between fish and shellfish species, which in practical terms means that those diagnosed with a fish or shellfish allergy should avoid all fish or shellfish.
Food allergy testing including Skin Prick Testing and fish specific IgE blood tests are good indicators of the presence of food allergy with high positive predictive values. This makes testing useful making the diagnosis of fish or shellfish allergy. The type of reaction should be clarified with clinical and dietary history as fish allergy can often be confused with a bacterial contamination in particular fish such as anchovies, herring, mackerel, sardines and tuna. This is referred to as scrombroid poisoning and occurs when marine bacteria cause histamine levels to rise when those fish are poorly handled and refrigerated during transportation.
Avoiding fish species would appear straightforward once a diagnosis has been confirmed, although many products contain fish derivatives including many Chinese and Japanese products, sauces such as Worcestershire and Gentleman’s relish and fish gelatine. Food labels in the UK and Europe by law have to show fish and shellfish clearly the label and made clear if used as food ingredients in restaurants. Fish oil supplements, a source of Omega 3 fatty acids, should be avoided for those with severe reactions to fish and shellfish, due to the potential adverse reactions. Alternative Omega 3 supplements include flaxseed or algae oils.