Diphyllobothrium

Diphyllobothrium is a genus of tapeworms which can cause diphyllobothriasis in humans through consumption of raw or undercooked fish. The principal species causing diphyllobothriasis is D. latum, known as the broad or fish tapeworm, or broad fish tapeworm. D. latum is a pseudophyllid cestode that infects fish and mammals. D. latum is native to Scandinavia, western Russia, and the Baltics, though it is now also present in North America, especially the Pacific Northwest. In Far East Russia, D. klebanovskii, having Pacific salmon as its second intermediate host, was identified.

Other members of the genus Diphyllobothrium include D. dendriticum (the salmon tapeworm), which has a much larger range (the whole northern hemisphere), D. pacificum, D. cordatum, D. ursi, D. lanceolatum, D. dalliae, and D. yonagoensis, all of which infect humans only infrequently. In Japan, the most common species in human infection is D. nihonkaiense, which was only identified as a separate species from D. latum in 1986. More recently, a molecular study found D. nihonkaiense and D. klebanovskii to be a single species.

The adult worm is composed of three fairly distinct morphological segments: the scolex (head), the neck, and the lower body. Each side of the scolex has a slit-like groove, which is a bothrium for attachment to the intestine. The scolex attaches to the neck, or proliferative region. From the neck grow many proglottid segments which contain the reproductive organs of the worm. D. latum is the longest tapeworm in humans, averaging ten meters long. Unlike many other tapeworms, Diphyllobothrium eggs are typically unembryonated when passed in human feces.

In adults, proglottids are wider than they are long (hence the name broad tapeworm). As in all pseudophyllid cestodes, the genital pores open midventrally.

Diphyllobothrium has seen many changes since Linnaeus first described Taenia lata. Over 50 species are currently recognized, and at least 14 of these have been reported in human infections, especially in circumpolar and Pacific regions.

Despite this, the evolutionary relationships within the genus are still not well understood, mainly due to limited DNA data for many species. Some studies suggest that D. pacificum and D. stemmacephalum are among the earliest lineages. D. nihonkaiense appears to branch off early in a group that includes D. latum, D. dendriticum, and D. ditremum, as well as related bird parasites from the Cestodes genera Ligula and Digramma.

This suggests that Diphyllobothrium may not be a single, unified group—possibly forming a para- or polyphyletic cluster instead. Interestingly, molecular data also shows that Diplogonoporus balanopterae, a human-infecting species with doubled genitalia per segment, should be considered part of Diphyllobothrium as well.

Adult tapeworms may infect humans, canids, felines, bears, pinnipeds, and mustelids, though the accuracy of the records for some of the nonhuman species is disputed. Immature eggs are passed in feces of the mammal host (the definitive host, where the worms reproduce). After ingestion by a suitable freshwater crustacean such as a copepod (the first intermediate host), the coracidia develop into procercoid larvae. Following ingestion of the copepod by a suitable second intermediate host, typically a minnow or other small freshwater fish, the procercoid larvae are released from the crustacean and migrate into the fish's flesh where they develop into a plerocercoid larvae (sparganum). The plerocercoid larvae are the infective stage for the definitive host (including humans).