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DIPHYLLOBOTHRIUM LATUM

The cestode Diphyllobothrium latum (D. latum), the fish or broad tapeworm, is the largest human tapeworm and causes an illness termed diphyllobothriasis. Infection in humans arises from the consumption of raw or undercooked freshwater fish or fish that are anadromous (migrating from salt water to fresh water for breeding e.g. salmon). Such fish may become infected from sewage-contaminated sources such as rivers, ponds and lakes.

Life Cycle

The definitive (primary) hosts for D. latum are humans and other fish-eating mammals.

1. Following excretion of eggs in the faeces of infected animals there are two intermediate hosts in the life cycle - crustaceans and freshwater fish.
2. Copepods (plankton-eating crustaceans) eat the free swimming larvae that have developed from the eggs (originating from sewage contamination). The larvae migrate to the body cavity of the crustacean.
3. Copepods are then in turn eaten by a suitable second intermediate host, typically minnows and other small freshwater fish. The larvae are released from the crustacean and migrate into the fish flesh where they further develop into a larval stage that is infective for humans. These small second intermediate hosts can be eaten by larger predator species, e.g. trout, perch, pike, salmon. In this case, the infective larvae can migrate to the flesh of the larger predator fish and humans can acquire the disease by eating these later intermediate infected host fish raw or undercooked.
4. After ingestion of the infected fish, the infective larvae develop into immature adults and then into mature adult tapeworms which will attach to, and live in, the small intestine. Adult Diphyllobothrium worms may survive longer than 10 years. The adults can reach more than 10 m in length, with more than 3,000 proglottids (egg-bearing parts). Immature eggs are discharged from the proglottids (up to 1,000,000 eggs per day per worm) and are passed in the faeces. Eggs appear in the faeces 5 to 6 weeks after infection.

Symptoms

Diphyllobothriasis can be a long-lasting infection (for decades) although most infections are asymptomatic (i.e. do not produce symptoms in infected people).
Where symptoms do arise, they occur about 10 days after infection and include:

abdominal discomfort and cramps
diarrhoea
vomiting
weight loss/sensation of hunger
pernicious anaemia, and a deficiency of vitamin B₁₂ (for which D. latum has an unusual affinity)
massive infections may result in intestinal obstruction
migration of proglottids can cause gallstones or cholangitis (inflammation of the biliary tract)

Control

Diphyllobothriasis can be prevented in humans by not eating raw or undercooked freshwater fish.
Although the elimination of raw sewage from waters will undoubtedly help to reduce the incidence of diphyllobothriasis, it will not break the life cycle of this tapeworm, as humans are not the only definitive host.
Thorough cooking or freezing will kill the organism.

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