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Warnings to Salmon Fishermen and Consumers of Underprocessed Fish - 19/02/2008
 
An increase in parasite infestations in popular angling waters is prompting official warnings to anglers of the possible deadliness of their catches, eating of which may cause heart and lung problems....



1. An increase in parasite infestations in popular angling waters is prompting official warnings to anglers of the possible deadliness of their catches, eating of which may cause heart and lung problems. The parasite, Anisakis simplex, is being found increasingly in wild salmon from rivers and estuaries. Eating a live worm on the fish can cause severe symptoms, including erratic heartbeat, unconsciousness, breathing difficulties, and respiratory failure, possibly leading to death. Pregnant women and the elderly are particularly vulnerable.

2. The fish posing the greatest risk are likely to be those caught by anglers and netsmen who like to eat their own catch or sell it to local restaurants, shops, or friends. About 33,000 salmon fishing licences are issued each year in the UK, out of a total of 1.4 million rod licences. The Food Standards Agency has issued a specific alert for fishermen to be aware of the dangers.

3. Wild salmon sold commercially is governed by strict hygiene and food safety laws and is unlikely to pose a threat to human health. Most wild salmon on sale in supermarkets is from Alaskan waters. In advice to anglers the Agency says that all fresh wild salmon must be either frozen or heated at 70ºC for at least 2 minutes to kill the parasite and its larvae. However, even after cooking a health risk may persist owing to severe anaphylactic reactions in some people.

4. Scientists at the Environment Agency and the Centre for Environment, Fisheries, and Aquaculture (CEFAS) are now urgently investigating the increase in the number of cases of wild salmon found with swollen and bleeding vents. Male and female fish are affected. The parasite has been known about for some years. Salmon ingest it when they are feeding at sea and then carry it to fresh water. Last year fishermen reported record numbers of affected salmon on some of the most popular salmon-fishing rivers. Three years ago 5% of salmon caught on the River Dee had the condition but last year the prevalence had risen to at least 15%. On the River Itchen the prevalence was even higher - at least 30% - of salmon carrying the parasite. Affected salmon were also found on the Exe, Taff, Camel, Tamar, Eden, Tyne, Ribble, Exe, Wye, and Lune, as well as in Scotland and Irish rivers. There is no evidence of the condition in wild trout nor in farmed salmon or trout.

5. The FSA is advising any consumers of wild salmon in the UK to inspect the fish and to remove any parasites. Any fish that is obviously contaminated should not be eaten. If it is intended to be eaten raw or almost raw it should be frozen for at least 24hr at minus 20ºC or colder. This should ensure the destruction of any non-visible parasites and larvae. These precautions apply especially to young women persuaded by the eat-more-fish campaigns of the FSA and of consumers persuaded of the preference of wild caught over farmed fish.

6. Any salmon destined for cold smoking or to be eaten after marinating or salting such as gravadlax, must also be frozen. Wild salmon to be hot-smoked (at temperatures exceeding 60ºC) is safe to eat without freezing. Cooking at 70ºC for 2 minutes will kill any parasites. Heating or freezing should suffice to kill any parasite, “but if anyone sees any sight of the parasite they should not eat it but return it to their supplier”.

7. No wild salmon is being fished at the moment. The season begins soon in Scotland and in England and Wales on 01 June 2008. Salmon caught by rod and line may be removed from rivers after 16 June 2008. Last year’s catch figures are not yet available, but in 2006 49,389 salmon were killed for home consumption from rivers and a further 38,467 were netted along the coast. Because salmon spawn through the vents, there could be a looming year of collapse for the industry. Last year the Scottish Execute introduced a ban on the sale of rod-caught wild salmon from rivers. The Environment Agency may apply a similar ban this year in England and Wales. Anglers are still allowed to take some of their catch home to eat.

8. Anisakis is a genus of parasitic nematodes, which have a life-cycle involving fish and marine mammals. They are infective to humans and cause anisakiasis (now called anisakidosis - previously known as anisakiasis); fish infected with Anisakis species can produce an anaphylactic reaction in people who have become sensitized to Immunoglobulin E (IgE). Anisakis species have a complex life-cycle, which passes through a number of hosts in the course of their life. Eggs hatch in seawater and larvae are eaten by crustaceans, usually Euphausids. The infected crustacean is subsequently eaten by a fish or squid and the nematode burrows into the wall of the gut and encysts in a protective coat, usually on the outside of the visceral organs, but occasionally in the muscle or beneath the skin. The life cycle is completed when an infected fish is eaten by a marine mammal, such as a whale or dolphin.

9. The nematode excysts in the intestine, feeds, grows, mates, and releases eggs into the sea water in the host’s feces. As the gut of a marine animal is functionally similar to a human’s, Anisakis species, of which there are a dozen, are able to infect humans who eat raw or undercooked fish.

10. The diversity of the genus has increased greatly over the past 20 years, with the advent of modern genetic techniques in species identification. Each final host species is home to its own biochemically and genetically identifiable sibling species of Anisakis, which is reproductively isolated. This finding has allowed the proportion of different sibling species in a fish to be used as an indicator of population density in fish stocks.

11. Anisakiasis (anisakidosis), the disease caused by infection with Anisakis worms, is frequently reported in areas of the world where fish is consumed raw, lightly pickled, or salted. The areas of highest prevalence are Scandinavia (from cod livers), Japan (after eating sushi and sashimi), and the Netherlands (by eating infected herring, such as Maatjes), and along the Pacific coast of South America (from eating ceviche). Within hours of consumption of infective larvae violent abdominal pain, nausea, and vomiting may occur. The occasional larva may be coughed up. If the larvae pass into the bowel, a severe eosinophilic granulomatous response may occur 1 to 2 weeks after infection, causing symptoms mimicking Chron's disease. Diagnosis can be made by gastroscopic examination, during which the 2cm larvae are visualised and removed, or by histopathological examination of tissue removed at biopsy or during surgery.

12. Humans are thought to be more at risk of Anisakiasis (anisakidosis) from eating wild fish than farm-raised fish because the grinding process used for pelleted food for farmed fish kills the parasites. A study in 2003 by the FAO of the United Nations found no parasites in any farm-raised salmon, in contrast to wild salmon, in which parasites were frequently found. Even when thoroughly cooked Anisakis can pose a health risk to humans. Anisakis (and related species such as the sealworm, Pseudoterranova spp, and codworm Hysterothylacium aduncum) release a number of biochemicals into the surrounding tissues when they infect a fish. They may also be consumed whole accidentally, inside a fillet of fish.

13. People sensitized to nematodes may have severe anaphylactic reactions after eating fish infected with Anisakis spp., which may be confused with a fish or shellfish allergy, as the allergenic components of the Anisakids are difficult to test for and can produce a reaction in tests for other allergens. Several tests for toxins in fish and shellfish are crude experiments with live mice that end in a gruesome death of non-human animal. This is yet another difficulty to the Food Standards Agency and toxicologists are facing in trying to persuade the British public to the healthier aspects of fish consumption without being able to suppress misgivings over matters of the environment and animal welfare.

14. Humans are a dead-end host for Anisakis and Pseudoterranova larvae, which cannot survive in humans and eventually die. Treatment is usually symptomatic, accompanied by heavy doses of reassurance. It is always as well to check that gut function is not compromised and absorption of essential nutrients is unaffected (one marine worm that crops up in Scandinavian waters nullifies the absorption of vitamin B12, for instance, and gut upsets and ill thrift are associated with terrestrial infestations of domestic and farmed animals and development of resistance to the few drugs farmers and vets can resort to). The only indication for treatment for Anisakis larvae is obstruction of the small bowel, which may require emergency surgery, although success has been reported recently with treatment with albendazole alone, avoiding surgery. Fragments of dead worms may still elicit allergic reactions.

15. Larval Anisakis are common parasites of marine and anadromous fish, and can also be found in squid and cuttlefish, so travellers may encounter them when visiting unfamiliar territories or cuisines. They are absent form fish in waters of low salinity, owing to the physiological requirements of Euphausids, which are needed to complete their lifecycle. Anisakids are also uncommon in areas where cetaceans are rare (such as the southern North Sea). There is still a lot to learn from the movements of animals of many species and the spread by them and human travellers of zoonotic diseases transmitted by microorganisms, some “primitive” perhaps, but with fascinating life cycles and enrichment of school biology beyond the liver fluke, wild watercress, and schistosomiasis.

16. Parasites similar to Anisakis are:
Cod or seal worm (Pseudoterranova decipiens, Phocanema decipiens)
Contracecum spp.
Hysterothylacium (Thynnascaris) spp.  

 
 

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