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Diphyllobothriasis (Fish Tapeworm Infection)


Chelsea Marie

, PhD, University of Virginia;

William A. Petri, Jr

, MD, PhD, University of Virginia School of Medicine

Reviewed/Revised Dec 2021 | Modified Sep 2022
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Diphyllobothriasis is infection with intestinal tapeworms Overview of Tapeworm Infections Tapeworms (cestodes) are flat, parasitic worms. The four main intestinal cestode pathogens of humans are Taenia saginata ( beef tapeworm) Taenia solium ( pork tapeworm) Hymenolepis... read more of the family Diphyllobothriidae. They are acquired by eating raw or undercooked freshwater fish. Treatment is with praziquantel or niclosamide.

D. latum is the most common and among the largest parasites of humans (up to 10 m in length). D. latum and other Diphyllobothriidae species have 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. Several other Dibothriocephalus species and Adenocephalus pacificus can infect humans after ingestion of raw fish, but they are not as common.

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

Diagnosis of diphyllobothriasis is by identification of characteristic operculated eggs or broad proglottids in stool. Complete blood count is done to check for megaloblastic anemia.

Treatment of Diphyllobothriasis

  • Praziquantel

  • Alternatively, 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. A stool sample should be repeated one month after therapy is completed to verify cure.

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

Prevention is by thoroughly cooking freshwater fish (internal temperature of ≥ 63° C [≥ 145° F]) 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 hours

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

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