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 Diphyllobothrium latum, commonly 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.

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Morphology

thumb|Diphyllobothrium latum scolex

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.&nbsp;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.

Taxonomy

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.

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 (bothria) of their scolices. The adults can reach more than 10 m (up to 30&nbsp;ft) in length in some species such as D. latum, with more than 3,000 proglottids. One or several of the tape-like proglottid segments (hence the name tapeworm) regularly detach from the main body of the worm and release immature eggs in freshwater to start the cycle over again. Immature eggs are discharged from the proglottids (up to 1,000,000 eggs per day per worm) 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.

Disease

Diphyllobothriasis is considered a parasitic, zoonotic infection. Humans have been parasitized by these tapeworms for thousands of years. D.&nbsp;latum causes a wide spectrum of disease and severity. The tapeworm induces changes in the concentration of several immunomodulators in the host. It can also cause structural changes in the GI tract as it modulates neuroendocrine responses and enhances secretion and gut motility. Damage may also come from the body's immune response against the worm and its millions of eggs (around 1 million/day) mediated by mast cells, eosinophilic cell degranulations resulting to inflammatory cytokines. Diphyllobothriosis is considered as the most important fish-borne zoonosis with up to 20&nbsp;million individuals infected.

D. latum causes vitamin B<sub>12</sub> deficiency in humans leading to megaloblastic or pernicious anemia. The worm absorbs around 80% of dietary B<sub>12</sub> and prolonged infection can also cause abdominal pain, mechanical obstruction, and symptoms of iron deficiency anemia. Patients with prolonged infection of D.&nbsp;latum should be offered B<sub>12</sub> supplementation along with anti-parasitics such as niclosamide or praziquantel.

Diphyllobothriosis is mainly transmitted through the consumption of raw or undercooked fish, a common element in many traditional dishes worldwide. In Europe, it’s linked to foods like raw or marinated fish in Scandinavian and Baltic cuisines, Italian carpaccio, French preparations using lake fish, and Jewish gefilte fish. In Latin America, ceviche—fish marinated in lemon juice and salt—is a known risk. Japanese cuisine, especially sushi and sashimi made with fish like ayu and cherry trout, also contributes to exposure. The global rise in popularity of such dishes has led to an increase in fish-borne parasitic infections. Occupational exposure is another factor, particularly for fishermen who eat fresh roe or liver and cooks who taste raw fish during preparation. While salmon is the most common source, other fish such as whitefish, trout, and pike are also capable of transmitting the parasite.

See also

  • List of parasites (human)

References

Further reading

  • Bonsdorff, B von: Diphyllobothriasis in Man. Academic Press, London, 1977
  • Keas, B. E: Microscopy - Diphyllobothrium latum. Michigan State University, East Lansing, 1999