Recent research has led to the reclassification and renaming of the Diphyllobothriid tapeworms. 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.
Infection is usually asymptomatic, but mild gastrointestinal symptoms (eg, abdominal discomfort, diarrhea, weight loss) may be noted. Fish tapeworms take up dietary vitamin B12, occasionally resulting in vitamin B12 deficiency and megaloblastic anemia. Rarely, heavy infection leads to intestinal obstruction or gallbladder disease due to migration of proglottids.
Diagnosis of diphyllobothriasis is by identification of characteristic operculated eggs or broad proglottids (tapeworm segments) in stool. Complete blood count is done to check for megaloblastic anemia.
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 (internal temperature of ≥ 63° C [≥ 145° F]) or freezing it at recommended temperatures can kill fish tapeworms. For freezing, recommendations include the following: