D. latum is the most common and among the largest parasites that infect 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 United States and northern Europe occur in people who eat raw or undercooked freshwater fish. Infection is less common with current sewage treatment.
Symptoms and Signs of Diphyllobothriasis
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 (tapeworm segments).
Diagnosis of Diphyllobothriasis
Microscopic examination of stool for ova and proglottids
Complete blood count
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
Alternatively, niclosamide (outside of the United States)
1). Alternatively, a single dose of niclosamide (unavailable in the United States) is given. 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.
Treatment reference
1. Braseth AL, Elliott DE, Ince MN: Parasitic Infections of the Gastrointestinal Track and Liver. Gastroenterol Clin North Am 50(2):361-381. 2021. doi:10.1016/j.gtc.2021.02.011
Prevention of Diphyllobothriasis
Prevention of diphyllobothriasis 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
