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procercoid

Diphyllobothrium

Diphyllobothrium is a genus of tapeworm best known for causing the infection Diphyllobothriosis. The principal species causing diphyllobothriosis is Diphyllobothrium latum, known as the broad or fish tapeworm, or broad fish tapeworm. D. latum is the longest tapeworm in humans, averaging ten meters long. Adults can shed up to a million eggs a day. D. latum is a pseudophyllid cestode that infects fish and mammals. It is morphologically very similar to other members of the genus Diphyllobothrium, but can sometimes be distinguished by host. Even this is not always possible, as there have been twelve other species of this genus reported to infect humans, especially Diphyllobothrium dendriticum (the salmon tapeworm), which has a much larger range (the whole northern hemisphere). 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 Japan, the most common infecting species is D. nihonkaiense, which was only identified as a separate species from D. latum in 1989.

In adults, proglottids are usually relatively wide (hence the name broad tapeworm). As in all pseudophyllid cestodes, the genital pores open midventrally.

Several other Diphyllobothrium species have been reported to infect humans, but less frequently; they include D. pacificum, D. cordatum, D. ursi, D. lanceolatum, D. dalliae, and D. yonagoensis.

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 (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).

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, and walleyed 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 (bothria) of their scolex. The adults can reach more than 10 m (up to 30 m) in length in some species such as D. latum, with more than 3,000 proglottids. Immature eggs are discharged from the proglottids (up to 1,000,000 eggs per day per worm) and are passed in the feces. Eggs appear in the feces 5 to 6 weeks after infection. The tapeworm can live up to 20 years.

References

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