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2007-06-06 10:52:40
Round worm in fish
2007-05-29
Introduction
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 Its scientific name is Phocanema decipiens; other outdated scientific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.