Diphyllobothrium is a genus of tapeworms which can cause diphyllobothriasis in humans through consumption of 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 pseudophyllidcestode 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, which has a much larger range, 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.
Morphology
The adult worm is composed of three fairly distinct morphological segments: the scolex, 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. Adults can shed up to a million eggs a day. In adults, proglottids are wider than they are long. As in all pseudophyllid cestodes, the genital pores open midventrally.
Life cycle
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. After ingestion by a suitable freshwater crustacean such as a copepod, 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. The plerocercoid larvae are the infective stage for the definitive host. Because humans do not generally eat undercooked minnows and similar small freshwater fish, these do not represent an important source of infection. Nevertheless, these small second intermediate hosts can be eaten by larger predator species, for example trout, perch, walleye, and pike. In this case, the sparganum can migrate to the musculature of the larger predator fish and mammals can acquire the disease by eating these later intermediate infected host fish raw or undercooked. After ingestion of the infected fish, the plerocercoids develop into immature adults and then into mature adult tapeworms which will reside in the small intestine. The adults attach to the intestinal mucosa by means of the two bilateral grooves of their scolex. The adults can reach more than 10 m in length in some species such as D. latum, with more than 3,000 proglottids. One or several of the tape-like proglottid segments regularly detach from the main body of the worm and release immature eggs in fresh water to start the cycle over again. Immature eggs are discharged from the proglottids and are passed in the feces. The incubation period in humans, after which eggs begin to appear in the feces is typically 4–6 weeks, but can vary from as short as 2 weeks to as long as 2 years. The tapeworm can live up to 20 years.
