Andreas Lopata Allergology Unit, Department of Immunology, Groote Schuur Hospital, University of Cape Town
Seafood constitutes an important food for humans, particularly as a source of animal protein. Global fish production now exceeds 100 million tonnes per year, and about 70% is available for direct human consumption. In addition, aquaculture of seafood is becoming a major source of income in many countries. The current global seafood consumption is 14 kg/capita/year. Increased consumption due to the high nutritive value of seafood and the promotion of a healthy diet has also lead to the more frequent reporting of adverse reactions, including immunological mediated reactions. What precisely causes these allergies?
Classification of Important Seafood
Seafoods that induce allergic reactions belong to three phyla. The phylum Mollusca containing the Gastropoda such as snails and abalone, Bivalvia such as. Mussels and oysters and Cephalopoda such as octopus, squid/calamari. The other two large groups are the class of Crustacea (Shrimp, Lobster, Crab) and Osteichthyes (bony fish). Patients identify offending foods by common names such as lobster, as opposed to the species names. These names often vary from place to place as is the case with the Lobster/Crayfish classification. These completely different animals look a bit alike and names are sometimes switched for marketing reasons. Consequently, careful in-vitro or in-vivo testing is essential to prove specific sensitivity to a food.
The prevalence of true seafood allergy is difficult to estimate accurately. In Norway 1/1 000 people exhibit fish allergy symptoms. Our observations in South Africa demonstrated that allergy to molluscs is very common, with 45% of subjects with seafood allergy demonstrating specific IgE to an indigenous abalone species. The prevalence of occupational asthma varies from 7-36% among different groups of workers including seafood processing and fishmeal workers, fishermen and restaurant cooks.
Clinical Manifestation of Seafood Allergy
Allergy to seafood can elicit almost any allergic symptom. Symptoms may be divided into four main groups. · Generalised Reactions Anaphylaxis which can cause death (Caused by the widespread release of histamine and other chemicals in the body in response to the presence of the allergy causing antigen. The result is that blood vessels dilate and blood pressure drops to such an extent that the heart can stop). · Respiratory Reactions Asthma and rhinitis (runny inflamed nose) · Cutaneous Reactions Skin reactions such as flushing and urticaria and/or angiodema are common. · Gastrointestinal Reactions Abdominal pain, nausea, vomiting, diarrhoea and/or abdominal cramping A study conducted in our clinic on over 100 subjects with perceived seafood allergy showed that over 50% exhibited respiratory symptoms. Adverse symptoms can also be caused by inhalation of vapours, such as would occur during cooking or food processing.
Seafood Allergy Diagnosis
The diagnosis of seafood allergy consists of a detailed medical history and various laboratory methods to determine the presence of seafood specific immunoglobulin E (IgE) · History · Skin testing (skin prick test) · Radioallergosorbent test (RAST) · Western-blot · Double-Blind Placebo-Controlled Food Challenge (DBPCFC)
Seafood contains a wide variety of proteins, yet only a few are known allergens identified with sophisticated molecular and biochemical techniques. Most seafood allergens are stable molecules, which resist the effects of cooking and processing. Proteins that occur only in minor amounts can also be major food allergens, as has been shown for an allergen from codfish. This muscle tissue protein known as a parvalbumin controls the flow of Ca2+ in and out of cells. It is only found in the muscles of amphibians and many fish species such as hake, yellowtail and pilchard. On the other hand, major structural proteins such as tropomyosin can also be major allergens in invertebrates. Similar proteins from vertebrates such as chickens, cattle etc. are not allergenic. Allergic reactions to molluscs, and consequently mollusc allergens, have not been as well studied as those of fish or crustacea. However, it has recently become apparent that molluscs such as abalone, oyster, squid or limpet are significant allergens and that these mollusc allergens are heat stable.
Allergic Reactions to other substances contaminating Seafood Parasites:
Sea squirt The sea squirt (Hoja), a parasite commonly attached to oyster shells, causes asthma, rhinitis and conjunctivitis among oyster farmworkers in Japan. Anisakis Worldwide increasing allergies to a foodborne parasite, Anisakis simplex, have been linked to the ingestion of this nematode, which causes infections in humans called Anisakiasis ('herring or cod worm' disease) and induces immune reactions. Anisakis is found around the gut of fish and causes allergic reactions in humans. These range from skin reactions to anaphylactic reactions.
Toxic Seafood Reactions
These are different to allergic reactions in that they are caused by a chemical toxin, not an immune response to a chemical as is the case in an allergy. In the United States seafood is responsible for at least 1 in 6 food poisoning outbreaks and to an even larger extent in Japan. Seafood toxins are very stable and their toxicity is not reduced by preparation procedures e.g. cooking or pickling. Scombrotoxism Scombroid Fish Poisoning occurs from the ingestion of spoiled fish with a high content of red meat e.g. tuna. Histidine is converted to histamine by bacteria causing symptoms including flushing, urticaria, vomiting and diarrhoea which are similar to a real allergic reaction. Shellfish Poisoning by Marine Algal toxins A major cause of adverse seafood reactions is the presence of numerous toxins in fish and shellfish produced by toxic algae. The phenomenon of "Red Tide" is a significant contributor. Toxic algae are taken up by filterfeeders such as mussels and oysters, accumulated in their tissue and in turn consumed by humans. Ciguatera Fish Poisoning This poisoning by fish causes more human illnesses than any other seafood toxicity affecting up to 50 000 individuals annually. Ciguatoxin, produced by dinoflagellates, is a fat-soluble and heat stable neurotoxin, which concentrates via the food chain in more than 400 different reef fishes. Symptoms present primarily as an acute neurologic disease with gastrointestinal symptoms. Bacterial and Viral Contamination Seafood poisoning outbreaks have frequently been associated with bacterial contamination (such as Salmonella) of shellfish harvested in waters containing untreated sewage. In many cases of poisonings after consumption of raw or undercooked shellfish, small round structured viruses (SRSVs) were associated as well as hepatitis A.
Symptoms may be caused by pharmacological properties of the ingested seafood such as vasoactive amines. Large amounts of histamine and tyramine found in canned and pickled fish (27) may cause severe headaches. However, these substances are also found in fresh fish such as herring, tuna and mackerel whereas shellfish contains rather histamine liberators. In addition, dyes (e.g. tartrazine) and food additives such as monosodium glutamate (MSG) can cause syndromes such as 'Chinese restaurant syndrome',and other adverse reactions. UCT's Allergology Unit has conducted the first African studies on allergic health problems and related factors after exposure to seafood in the domestic and occupational environment. This team is set to begin the process of identifying and characterise the specific allergens found in indigenous seafood species, which have not been described in the literature before. This brings good news which will as this will allow the development of more specific diagnostic tests. What about exposure to seafood in the work place? Lopata and his team will conduct additional epidemiological studies that will focus on quantifying seafood allergen exposure to identify environmental and host-associated risk factors. This will result in the implementation of appropriate procedures to minimise inhalation and dermal exposure to seafood agents commonly processed in Africa and the Southern African sub-region.