Cestode in the context of "Life history theory"

Taenia solium, the pork tapeworm, belongs to the cyclophyllid cestode family Taeniidae. It is found throughout the world and is most common in countries where pork is eaten. It is a tapeworm that uses humans (Homo sapiens) as its definitive host and pigs and boars (family Suidae) as the intermediate or secondary hosts. It is transmitted to pigs through human feces that contain the parasite eggs and contaminate their fodder. Pigs ingest the eggs, which develop into larvae, then into oncospheres, and ultimately into infective tapeworm cysts, called cysticerci. Humans acquire the cysts through consumption of uncooked or under-cooked pork and the cysts grow into adult worms in the small intestine.

There are two forms of human infection. One is "primary hosting", called taeniasis, and is due to eating under-cooked pork that contains the cysts, resulting in adult worms in the intestines. This form generally is without symptoms; the infected person does not know they have tapeworms. This form is easily treated with anthelmintic medications which eliminate the tapeworm. The other form, "secondary hosting", called cysticercosis, is due to eating food, or drinking water, contaminated with faeces from someone infected by the adult worms, thus ingesting the tapeworm eggs, instead of the cysts. The eggs go on to develop cysts primarily in the muscles, and usually with no symptoms. However, some people have obvious symptoms, the most harmful and chronic form of which is when the cysts form in the brain. Treatment of this form is more difficult but possible.

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.