The consumption of fish and seafood has increased significantly over the past few decades, partly because of concerns about the levels of dietary fat and cholesterol in our diet from meat. Fish and seafood is seen as a healthy alternative to meat, because it has much less saturated fat than meat and also has Omega-3 polyunsaturated fatty acids which can have health benefits.
As the consumption of fish and seafood has increased, so has the number of people found to be allergic to these foods. Allergy to fish affects about 1 in 200 people, while allergy to shellfish (for example, prawns and crabs) is more common, affecting about 1% in those populations with significant consumption of shellfish.
Seafood allergy:
is more common in adults than children
is not as common as milk egg or peanut allergy
tends to be lifelong
What are the symptoms of fish/seafood allergy?
The pattern of symptoms following ingestion of fish and seafood is similar to that reported for other foods, including, nausea, sickness, diarrhoea, abdominal cramps, wheezing, rhinitis, flushing, urticarial rashes and dramatic swelling. Fatal reactions have been reported.
Although some people complain that they can have significant allergic reactions to just the smell of a food, this is actually fairly uncommon for most foods. Fish and seafood is one important exception. According to recent reports, 15% of people with seafood allergy can react to vapours and steam produced during cooking (especially grilling and on the barbeque). This is because fish and seafood release very small proteins called amines during the cooking process, which can cause allergic reactions in the airways and lungs.
Surveys also indicate that after nuts, seafood is the next most common food causing severe allergic reactions (anaphylaxis). In a recent report of 167 children with seafood allergy, over one in five had experienced a previous anaphylactic reaction to the food.
What types of seafood and fish are there?
| Phylum | Class | Common Name |
| Molluscs | Gastropods Bivalves Cephalopods | Snail, Abalone Clam, Mussel, Oyster Octopus, Squid, Scallop |
| Athropods | Crustacea | Crab, Lobster, Shrimp, Prawn, Crayfish |
| Chordates | Cartilagenous Bony Fish | Fish Ray, Shark Cod, Salmon, Tuna etc. |
People who are allergic to one class of seafood can often tolerate those from another. For example, most people with an allergy to shellfish (crustacea) such as prawn are able to eat finned fish. Likewise, people allergic to tuna can often eat prawns. However, cross-reactivity within a class is common, so someone allergic to a fish like salmon must usually avoid all finned fish.
Crustacea-allergic individuals should also be cautious when eating squid and oyster as studies have shown that there may be some cross-sensitivity with these foods. Although a number of patients with fish allergy also report a sensitivity to shellfish, recent reports indicate that this is probably two different allergies existing concurrently and not a cross-reactivity between the fish and crustacea.
Cross-reactivity between finned fish and other seafood may be more common in children than in adults. It is therefore important that a child who is allergic to one type of fish or seafood is assessed by an Allergy Specialist to determine what other types of seafood they must avoid.
How does cooking affect the likelihood of an allergic reaction?
Some patients report that the smell of fish and shellfish during cooking causes allergic symptoms. As mentioned above, this is due to the release of amine vapours, which can trigger an allergic response in the airways.
The allergen which causes symptoms in crustacea is very resistant to cooking and may even persist on cutting boards etc. after washing. Patients with a history of severe reactions to crustacea should avoid locations where these are cooked and served, as these sensitive patients may react to allergens present either in vapours from cooking or remaining on surfaces after cleaning.
Interestingly, up to 20% of fish-allergic people can tolerate the fish when it is canned, e.g. tinned tuna. This is because the canning process involves extensive heating for a prolonged period of time, and this intensive cooking can change or even destroy the protein causing the allergy. However, 4 out of 5 people will still react, and this reaction can be severe. So never try out tinned fish at home, speak to your Allergy Specialist about whether it is appropriate to test for tolerance for tinned fish under medical supervision.
What foods can contain fish or seafood?
Seafood can be an ingredient in many foods, including sauces, salad dressings, pastes and some cracker biscuits. Fish and seafood can be processed into fish fingers, calamari rings, fish burgers and fish nuggets. It may be difficult to find out exactly what type of seafood is in a specific product and the seafood used may change from time to time.
Foods which can contain fish and seafood include:
Fish fingers, nuggets
Sushi and sashimi
Salads e.g. Caesar salad
Oyster sauce
Fish sauce including Worcester sauce (usually contains anchovies)
Fish oils
Pet food
Seafood dips
Prawn chips/crackers
Take special care when travelling. For example, in USA, many pasta sauces contain crustacea, so always check food labels.
What about fish oil and glucosamine supplements?
Fish oils are popular health supplements, with reports that they can be beneficial in neurological development, immune function and preventing cardiovascular disease and cancer. Glucosamine is a complementary medicine used to treat osteoarthritis, and is derived from the outer coatings of shellfish such as crustaceans. Sometimes chondroitin sulphate is added, usually derived from shark cartilage.
While people allergic to fish and shellfish are sensitive to protein and oils or constituents of the shell of crustacean, these products are not routinely tested to ensure no protein contamination has occurred. It is therefore important to avoid these supplements unless your Allergy Specialist has told you otherwise. Sometimes, it is possible to perform a skin test to the fish oil, so discuss this with your health professional.
Occupational disease to seafood
Not surprisingly, the prevalence of occupational disease in fish and seafood handlers is quite marked. Among these workers, skin reactions from contact with shellfish are some of the most commonly reported. It has been estimated that up to one-third of food handling caterers who suffer from skin disease of this type have to find alternative employment because of the severity of their symptoms. Workers who are employed in the processing of shellfish also often suffer from asthma, rhinitis and conjunctivitis.
Scientific investigations have shown that even five years after leaving the industry, symptoms still persist in a majority of workers with occupational asthma due to shellfish. The problems seem to be caused by either cooking steam or aerosolized tiny particles of shellfish. One group of prawn workers in England developed occupational asthma six weeks after the method of extracting the prawns was changed from hand peeling to air blowing by machine. Most of the symptoms disappeared again after the compressed air jets were replaced with cold water jets.
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