In Southern Africa there are over 2000 species of fish, more than 5000 species of molluscs and 2500 species of crustaceans. The major edible seafoods that can cause an allergic reaction belong to three groups (see Table 1). The Mollusc group includes three different classes of seafood with delicious species such as Abalone (Perlemoen), oysters, mussels, and squid (Calamari).
Table 1. Classification of seafood causing allergies
|Arthropoda||Crustacea||Lobsters, Shrimp, Prawn,
Rock Lobster (kreef)
The second group, the Crustacea, includes the rock lobsters ("crayfish"), prawns, crabs and shrimps; and the third important group of seafood includes all the common edible fish, such as Hake, Cod, Snoek, etc. Very often patients are only allergic to certain species but are able to eat other seafood species without problems.
However, 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 "Red Tide" along the West Coast are found, mostly in later summer. Recently allergic reactions to a parasitic worm, Anisakis, that is often found in various fish species, has been described 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 as to how common seafood allergy is in South Africa. Depending on regional diets, certain seafoods are more allergenic than others. For example, shrimp allergy is common in the southern USA while fish allergy is common in Spain and the Scandinavian countries, affecting up to 1:1000 individuals. In addition, international studies have demonstrated that occupational asthma or eczema can effect up to 30% of individuals in the various seafood-processing industries. A recent survey conducted in South Africa by the UCT Allergology Unit indicates that allergy to different mollusc and crustacea species is far more common that allergy to fish. In our study at UCT of 80 patients, we found 44% of subjects sensitised to one or more Crustacea, 38% to one or more Molluscs and 18% to fish.
What are the allergens?
Very few allergens of seafoods have been characterised, according to the medical and scientific literature. The best known allergens in seafood are the Allergen M from cod fish, and the major allergen found in crustacea. In the group of molluscs, very recently a new allergen from South African, Abalone (Perlemoen) has been found. All of these allergens are very stable and are not destroyed by cooking. As yet it has not been ascertained if some or all of these allergens are responsible for the fact that some patients are allergic to different seafood species in the different groups at the same time (possibly due to cross-allergenicity).
What are the symptoms of seafood allergy?
Reactions are reported to be mostly within 2 hours after ingestion or handling of seafood, or even inhaling cooking vapours. Reactions can also be delayed for up to 6 hours, as has been frequently reported for species of the Mollusc groups such as Abalone and squid. 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.
How is seafood allergy diagnosed?
A precise and detailed patient history and food history is very important in the diagnosis of allergy to seafood to indicate if a reaction is really an allergy or rather caused by a toxin in the food. Information regarding the suspected offending seafood species is very important, but it often fails to provide adequate information. This is often because of the confusion about the common name used for different seafood species in South Africa (e.g. "Rock Lobster" versus "Crayfish") or deceptive marketing practices (seven different tuna species have been labelled as "Tuna"). The clinical evaluation should be supported by the CAP RAST test performed at a laboratory on a small blood sample. In the case of a negative CAP RAST test result, a skin test with extracts of the suspected seafood species can give additional information. In Southern Africa we have many seafood species which are not found in Europe or the United States. This can mean that commercial tests are not available. Therefore, we at UCT Allergology Unit have improved the diagnostic process by developing and using in-house RAST tests and Western Blot tests and skin tests with locally important seafood allergens.
What is to be done?
Seafood Allergens for Blood Tests (Table II.)
There are numerous laboratory tests for allergy to different species of Fish, Crustacea and Molluscs. No all of these tests are commonly encountered in South Africa, as these species are not often consumed as food. However, tehre is an increasing trend in South Africa towards the use of unusual species of seafoods in restaurants, as well as visits by South Africans overseas where they will be exposed to these other seafood species. Therefore, the list of tests is comprehensive.
Those tests marked with an asterisk (*) are part of the Pharmacia ImmunoCAP RAST assay system, manufactured by Pharmacia & Upjohn Diagnostics of Upsala Sweden, and are used at all hospitals and private commercial laboratories in South Africa. Call Labspec at (011) 792-6790 for more information.
#Those tests marked with a has (#) have been developed inhouse by The Allergology Unit at UCT Medical School in Cape Town and those tests are available only from that Unit. These untilise local seafood species that are most usually not available as Pharmacia manufactured tests. Cal (021) 406-6320 for more information.
(Cod fish, Shrimp, Mussel, Tuna, Salmon)
Deepwater Prawn / Pink Shrimp
Rock Lobster, East Coast (SA)#
Rock Lobster, South Coast (SA)#
Tiger Prawn (SA)#
Zebra Prawn (SA)#
For more information: