Fish allergy is more likely to commence in early childhood, often in infancy, whereas shellfish are more likely to cause new-onset IgE-mediated food allergy presenting in adult life. The main fish provoking symptoms are the bony finned fish such as cod, haddock, plaice and sole, whereas cartilaginous fish such as shark, ray and swordfish rarely cause fish allergy. The prevalence of fish allergy ranges from 0% to 0.3%, and it can include a wide range of symptoms including anaphylaxis. Fish allergy can resolve, with tolerance to one or more types of fish possibly occurring in teenage years. The prevalence of allergy to shellfish is around 0.9% for challenge-confirmed allergy, with crustaceans, especially prawns, being the most common shellfish involved. The allergens involved in seafood allergy are heat stable, so generally present in reagents, although they can be species-specific, especially for some types of fish and also for prawns. Although some fish and shellfish share one common allergen, on the whole, the allergens in fish and shellfish are different, so usually people are allergic to either fish or shellfish. However, they might need to avoid both when eating out due to the problem of cross-contamination which is the major problem with a seafood allergy. For fish allergy, both SPT and SIgE tests for different species are useful, although cod is often the main fish tested as it contains the highest level of β parvalbumin, the major allergen. Specific IgE tests for individual shellfish are also useful, although it is important to use CRD if there is house dust mite sensitisation. In the case of a negative test to the shrimp reagent, a prick to prick test with different types of fresh prawns is recommended.
There are a number of differential diagnoses that need to be considered when reactions to seafood are reported. In some countries where fish are often eaten raw, then the reaction might be due to an allergy to the larvae of a common fish parasite, L3 Anisakis spp. However pseudo-allergic reactions to seafood can also occur to naturally high levels of histamine found in some fish species such as mackerel, or in seafood that is not completely fresh. Severe reactions can mimic an anaphylactic episode, and this is known as scombroid poisoning, with tuna being the most common cause of this worldwide. More rarely, reactions may be due to high levels of marine toxins present in the raw fish, or the reactions could be a manifestation of a non-IgE-mediated reaction such as FPIES.