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Diphyllobothriasis (Fish Tapeworm Infection)

By Richard D. Pearson, MD, Emeritus Professor of Medicine, University of Virginia School of Medicine

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Diphyllobothriasis is infection with the intestinal tapeworm, Diphyllobothrium latum, a parasite of freshwater fish. Treatment is with praziquantel.

D. latum is the largest parasite of humans (up to 10 m in length). D. latum and the cestodes that cause sparganosis are the only human tapeworms with aquatic life cycles. In freshwater, eggs of D. latum from human feces hatch into free-swimming larvae, which are ingested by microcrustaceans. The microcrustaceans are ingested by fish, in which the larvae become infective.

Diphyllobothriasis occurs worldwide, especially where cool lakes are contaminated by sewage. Infections in the US and northern Europe occur in people who eat raw freshwater fish. Infection is less common with current sewage treatment.

Infection is usually asymptomatic, but mild GI symptoms may be noted. Fish tapeworms take up dietary vitamin B12, occasionally resulting in vitamin B12 deficiency and megaloblastic anemia.

Diagnosis of diphyllobothriasis is by identification of characteristic operculated eggs or broad proglottids (tapeworm segments) in stool. CBC is done to check for anemia.


  • Praziquantel

  • Sometimes niclosamide (outside of the US)

Treatment of diphyllobothriasis is with a single oral dose of praziquantel 5 to 10 mg/kg. Alternatively, a single 2-g dose of niclosamide (unavailable in the US) is given as 4 tablets (500 mg each) that are chewed one at a time and swallowed. For children, the dose is 50 mg/kg (maximum 2 g) once.

Vitamin B12 may be needed to correct megaloblastic anemia if present.

Thoroughly cooking freshwater fish or freezing it at recommended temperatures can kill fish tapeworms. For freezing, recommendations include the following:

  • Freezing at -20° C (-4° F) or below for 7 days (total time)

  • Freezing at -35° C (-31° F) or below until solid and storing at -35° C (-31° F) or below for 15 h

  • Freezing at -35° C (-31° F) or below until solid and storing at -20° C (-4° F ) or below for 24 h