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Spud's Down to Earth Advice on...ANISAKIS and PSEUDOTERRANOVA (Roundworms) |
Humans can become infected with Anisakis simplex or Pseudoterranova decipiens by eating raw, undercooked or insufficiently frozen infected marine fish containing infective larvae. The resulting illness is called anisakiasis.
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Life Cycle
 - 1. Eggs are excreted by adult female worms living in the stomachs of marine mammals such as whales and dolphins.
- 2. These develop into free-swimming larvae that are then eaten by crustaceans, which in turn get eaten by fish and squid.
- 3. These larvae migrate from the intestine to the tissues in the peritoneal cavity and grow up to 3 cm in length. Upon the host's death, larvae migrate to the muscle tissues, and are transferred from fish to fish when eaten by predators.
[N.B. It is the (L3- and L4-) larval stages in fish and squid that are infective to humans. After ingestion of infected fish, the larvae penetrate the gastric and intestinal mucosa, causing the symptoms of anisakiasis. Fish known to harbour these parasites (and therefore to be of some risk) include cod, haddock, mackerel, pacific salmon, herring, flounder, and monkfish.]
- 4. When fish or squid containing larvae are ingested by marine mammals, the larvae moult twice and develop into adult worms.
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Symptoms
Symptoms arise from within 4 - 6 hours to 2 weeks after ingestion of infective larvae and result from the activities of the juvenile worms (as anisakids do not mature to adult roundworms in humans). Symptoms include:
- violent abdominal pain
- nausea
- vomiting
- coughing up of the larvae (occasionally)
- fever and bloody stools may occur within 7 days of ingesting infective fish (in more severe cases)
If the larvae pass into the bowel, a severe response may also occur 1 to 2 weeks following infection, causing symptoms resembling Crohn's disease.
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Control
Anisakiasis can be prevented by avoiding the consumption of raw or undercooked fish. Sushi, ceviche, and sashimi should be consumed only when properly prepared from fish that has undergone inspection for the absence of infective larvae.
Candling or examining fish on a light table is used by commercial processors to reduce the number of nematodes in certain white-flesh fish that are known to be infected frequently.
Infective larvae can be destroyed/rendered uninfective by cooking, freezing at -20oC for at least 60 hours (subject to further research at the moment), curing or marinating to a pH below 4.
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Additional Information
- Anisakis simplex larvae are more harmful than those of Pseudoterranova decipiens because they often penetrate the mucosal lining of the stomach, whereas most Pseudoterranova decipiens larvae are passed in faeces or coughed up or vomited after irritating the stomach lining.
- Anisakis simplex is most often found in cases in Japan and Holland; Pseudoterranova decipiens is more often seen in North America. Japan has the greatest number of reported cases because of the large volume of raw fish consumed there.
- The illness is rare in the UK and fewer than 10 cases are diagnosed in the U.S. annually. However, it is suspected that many other cases go undetected. Its incidence could increase with the increasing popularity of sushi and sashimi bars.
- Severe cases of anisakiasis are extremely painful and require surgical intervention. Physical removal of the nematode(s) from the lesion is the only known method of reducing the pain and eliminating the cause (other than waiting for the worms to die).
- Some evidence exists that the nematode larvae migrate from the internal organs of the fish hosts to the flesh if they are not gutted promptly after being caught. This process therefore needs to undertaken as soon as possible after catching.
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