20 April 2007

Seafood allergy

In Southern Africa there are more than 2 000 species of fish, more than 5 000 species of mollusk and 2 500 species of crustaceans. Some are delicious – a few can be deadly.

In Southern Africa there are more than 2 000 species of fish, more than 5 000 species of mollusk and 2 500 species of crustaceans. Some are delicious – a few can be deadly if you’re allergic to them.

The major edible seafoods that can cause an allergic reaction fall into three categories.

  • Mollusks: alikreukel, abalone or perlemoen, back mussel, blue mussel, clam, flying squid, limpet, octopus, oyster, periwinkle, ribbed mussel, scallop, snail, squid, white mussel and white squid or chokka;
  • Crustaceans: crab, crayfish, deepwater Prawn, pink shrimp, lobster, langoustine, rock lobster, shrimp, tiger prawn abd zebra prawn;
  • Fish: achovy, angelfish or pomfret, Canadian salmon or geelbek, chub mackerel, cob or kabeljou, cod fish, eel, hake, hake, halibut, herring, jack mackerel or scad, jacopeve, John dorey, kingklip, mackerel, Maasbanker, megrim or whiff, monkfish, plaice, rainbow trout, salmon, sardine or Japanese pilchard, snoek, sole, tuna and anisakis, a fish parasite.

Often patients are only allergic to certain species but are able to eat other seafood species without problems.

But the evaluation of a patient can be difficult if the adverse reaction to a particular seafood was caused by a non-allergic reaction. A major cause is the presence of a toxin in fish and shellfish, which produce symptoms similar to allergic reactions.

Among fish species such as yellowtail, tuna and mackerel, scombroid fish poisoning is frequently encountered if the fish is improperly refrigerated or when refrigeration is delayed. In filter-feeding shellfish such as black mussels and oysters, toxins produced by the so-called Red Tide along the West Coast are found, mostly in later summer. Allergic reactions to a parasitic worm, anisakis, that is often found in fish, has been encountered in people eating fish, and can mimic a fish sensitivity.

How common is seafood allergy in South Africa?
Currently there is no exact data available on how common seafood allergy is in South Africa. A recent survey by the UCT Allergology Unit indicates that allergy to different mollusc and crustacea species is far more common that allergy to fish. The study of 80 students found that 44% of subjects reacted to crustaceans, 38% to one or more mollusks and 18% to fish. The allergens in seafood are very stable and are not destroyed by cooking.

What are the symptoms of seafood allergy?
Reactions are reported to be mostly within two hours of ingestion or handling of seafood, or even inhaling of cooking vapour. Reactions can also be delayed for up to six hours. The more common symptoms include skin, stomach and respiratory problems. Respiratory problems are very common in sensitive subjects following inhalation of fish or crustacean vapours, such as from cooking.

What is to be done?
If you have a seafood allergy you need to be aware that it’s likely to persist for several years. If you have a confirmed allergy it’s also recommended that patients with a confirmed allergy to a specific seafood are also tested for their reaction to other species.

Avoid any possible direct or indirect exposure to the offending seafood species including handling or inhaling cooking vapours. Take care when eating out, as some foods may have been in contact with the offending seafood species - french fries and fish may be prepared in the same oil, and so on. Also be aware that some seafood species are incorrectly named on menus, such as freshwater crayfish being billed as rock lobster.

Wear a Medic-Alert badge. Depending on how serious your allergy is your doctor may also suggest you should carry an adrenaline syringe.


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Dr Morris is the Principal Allergist at the Cape Town and Johannesburg Allergy Clinics with postgraduate diplomas in Allergology, Dermatology, Paediatrics and Family Medicine dealing with both adult and childhood allergies. obesity and diabetes societies and runs a trial centre for new drugs.

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