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Parasitic round worms are frequently found in the guts and in the flesh of fish. Although only a small proportion of fish sold to the public is affected in this way, the worms are unsightly and consumers naturally object to their presence. This note gives a brief account of the nature and occurrence of round worms in fish, and describes means of reducing infestation in fish used as food. The note should also help fish traders and environmental health officers to dispel some of the misunderstanding of the problem when answering complaints from concerned members of the public.

What are parasites?

Animal parasites live in or on other animals from which they obtain at least some of their vital requirements, particularly nourishment. In general each kind of parasite confines itself to one kind of animal or group of animals, known as the host. Some parasites need more than one host at different stages in their development, the adult parasite living off one animal and the young or larval forms living off other animals.

Some fish parasites live on the outside of fish, others within the body; most are removed during gutting and washing. The kinds most frequently met with during subsequent handling and distribution are worms.

Round worms, or nematodes, in larval form are found in the guts and in the flesh of many fish marketed in the United Kingdom; two kinds predominate, the 'cod worm' and the 'herring worm'.

The 'cod worm', which is often found in cod, is also found in many other species. Its scientific name is Phocanema decipiens; other outdated scientific names are Porrocaecum decipiens or Terranova decipiens. It grows up to 4 cm long in fish, and varies in colour from creamy white to dark brown. It is frequently found in the flesh of fish, particularly in the belly flaps, where it often remains for long periods curled up and encased in a sac-like membrane produced by the fish tissue.

The 'herring worm' is often found in herring, mackerel, whiting and blue whiting, but it also occurs in many other species. Its scientific name is Anisakis simplex. It grows up to 2 cm long in fish, is almost colourless, and is found tightly coiled and encased in the guts and flesh, sometimes in considerable numbers, particularly in the belly flaps. Anisakis can migrate from guts to flesh in fish left ungutted after capture, notably in herring, mackerel and blue whiting.

How do round worms get into fish?

The life history of a parasitic round worm is complex. The adult lives in the stomach of a marine mammal, Phocanema in the grey seal and Anisakis mainly in dolphins, porpoises and whales. Eggs of the parasite pass into the sea with the mammal's excreta, and when the eggs hatch the microscopic larvae must invade a new host in order to develop. The larval worms of Anisakis are eaten by a small shrimplike crustacean, a euphausiid; the first host of Phocanema is a small isopod crustacean that lives on the sea bed.

When crustaceans infested with Anisakis or Phocanema are eaten by a fish the larval worms are released into its stomach. They then bore through the stomach wall and eventually become encased in the guts or in the flesh of the host fish. The life cycle of the parasite is completed when an infested fish is eaten by a suitable marine mammal.

Large fish tend to be more heavily infested by round worms than small fish of the same species. This is because large fish eat more, and therefore ingest greater numbers of parasites, and also because the larval worms, although inactive, can survive for a long time in fish, and therefore their numbers accumulate as the fish grows older.

Are round worms dangerous?

There have been cases of human illness caused by the ingestion of live Phocanema or Anisakis larvae in countries where raw or lightly cured fish is commonly eaten. By 1980, there had been only one reported case of illness in the United Kingdom caused by larval round worms from fish; this is because in the UK fish products are normally cooked before consumption. Phocanema and Anisakis larvae are killed in 1 minute at a temperature of 60°C or over. In practice this means that cooking a fillet 3 cm thick for 10 minutes at 60°C will kill any worms present. The temperature of a cold smoking process, for example kippering, is not high enough to kill parasites, but in a commercial hot smoking process a high enough temperature is usually maintained for long enough to kill them. Freezing of fish at - 20°C for 60 hours kills all worms.

Anisakis larvae are resistant to salting; immersion in 80° brine, 21 per cent salt, for 10 days will kill all larvae, but in brine of lower strength they can survive for much longer. Anisakis is also resistant to marinating. When there is any doubt about whether Anisakis will survive a process it is safest to use frozen fish. The ability of Phocanema to withstand salting or marinating is not known, but it is probably similar to that of Anisakis.

Can infestation of fish be reduced?

The abundance of Phocanema and Anisakis varies in fish from different areas; Phocanema is usually more abundant in inshore fish, whereas Anisakis occurs in greater numbers in offshore fish, but many species of fish from all areas fished by British vessels are likely to be infested to some degree. Of the two kinds of worm, Anisakis is by far the more abundant and widespread.

The only way to reduce the numbers of parasites reaching the consumer is to inspect the fish and process them in such a way that most parasites are removed. The guts and gut cavity of many fish are often heavily infested; whiting for example often contain large numbers of Anisakis. For this reason it is always advisable to gut fish and clean out the gut cavity before offering them for sale.

Most Phocanema and almost all Anisakis in fish flesh are found in the belly flaps; it follows that trimming off and discarding flaps from fillets will remove most of the worms. The greater the area of flap discarded, the greater will be the proportion of worms removed.

Visual inspection of fillets will reveal worms embedded near the surface; these can be removed easily with a knife. Worms embedded deep in the flesh are not immediately obvious, but some can be detected by candling, that is shining a bright light through the fillet. In commercial practice candling is effective in detecting Phocanema in thin skinless fillets of white fish, particularly cod; the method does not work well on thick fillets with the skin on. Candling is less effective in detecting Anisakis.

Time can be saved by candling a sample of fillets from a batch of suspect fish to determine the level of infestation; it can then be decided whether the whole batch needs to be candled, and whether the batch is more suitable for one purpose than another.

Design and use of a candling table

The simplest kind of candling table is a box about 50 cm square with a ground glass or perspex top about 6 mm thick. The inside of the box is white, and is lit by two fluorescent tubes giving a white, not a coloured, light. Electrical wiring should be installed by an electrician who understands the wet conditions in which the box is to be used. The box should be ventilated but splashproof.

To use the box, the fillet is laid down on the illuminated top; worms show up as dark shadows in the flesh, and can be removed with forceps or a knife. Light from above the box should be restricted; the box is useless in bright sunlight for example. An experienced operator can handle up to 300 fillets an hour, but the eyes rapidly become fatigued and efficiency falls during long spells, with the consequent risk of greater numbers of worms passing undetected.

How serious is the problem of worms in fish?

No matter how carefully fish is inspected by processors, caterers and retailers, some worms will occasionally be discovered in fish by the consumer. In reply to complaints it should be pointed out that every reasonable precaution is taken to prevent worms being present in the edible part of a fish. A model purchase specification proposes a maximum of 3 worms in 3·2 kg of fillets of white fish in the United Kingdom, as judged by visual examination. An international standard, Codex Alimentarius, allows a maximum of 5 worms in 1 kg of fish of certain species; only worms of encapsulated diameter of 3 mm, or 1 cm in length, are considered to be of significance. It should therefore be emphasized that the presence of worms in fish offered for sale does not imply carelessness or bad practice on the part of the processor or retailer. It can also be explained that the presence of worms does not reduce the nutritional value of the fish, and that correct cooking or freezing will kill all parasites.

The information in this note has been prepared jointly by the Marine Laboratory of the Department of Agriculture and Fisheries for Scotland and the Torry Research Station of the Ministry of Agriculture, Fisheries and Food.

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