The four major tapeworms that cause noninvasive infections in humans are the beef tapeworm Taenia saginata, the pork tapeworm Taenia solium, the fish tapeworm Diphyllobothrium latum, each of which can reach many meters in length, and the dwarf tapeworm Hymenolepis nana. Taenia and Hymenolepis species are broadly distributed, especially in the tropics; D latum is most prevalent in temperate regions. Other tapeworms that can cause noninvasive human disease include the rodent tapeworm Hymenolepis diminuta, the dog tapeworm Dipylidium caninum, and other Taenia and Diphyllobothrium species. Invasive tapeworm infections, including T solium (when infective eggs, rather than cysticerci are ingested) and Echinococcus species, will be discussed separately.
Infection is most common in cattle breeding areas. Humans are the definitive host. Gravid segments of T saginata are passed in human feces to soil, where they are ingested by grazing animals, especially cattle. The eggs then hatch to release embryos that encyst in muscle as cysticerci. Humans are infected by eating raw or undercooked infected beef. Most individuals infected with T saginata are asymptomatic, but abdominal pain and other gastrointestinal symptoms may be present. Eosinophilia is common. The most common presenting finding is the passage of proglottids in the stool (eFigure 35–23).
Life cycle of cestodes. Taeniasis is the infection of humans with the adult tapeworm of Taenia saginata, Taenia solium, or T asiatica. Humans are the only definitive hosts for these three species. Eggs or gravid proglottids are passed with feces ; the eggs can survive for days to months in the environment. Cattle (T saginata) and pigs (T solium and T asiatica) become infected by ingesting vegetation contaminated with eggs or gravid proglottids . In the animal’s intestine, the oncospheres hatch , invade the intestinal wall, and migrate to the striated muscles, where they develop into cysticerci. A cysticercus can survive for several years in the animal. Humans become infected by ingesting raw or undercooked infected meat . In the human intestine, the cysticercus develops over 2 months into an adult tapeworm, which can survive for years. The adult tapeworms attach to the small intestine by their scolex and reside in the small intestine . Length of adult worms is usually 5 M or less for T saginata (however, it may reach up to 25 M) and 2–7 M for T solium. The adults produce proglottids which mature, become gravid, detach from the tapeworm, and migrate to the anus or are passed in the stool (approximately 6 per day). T saginata adults usually have 1000 to 2000 proglottids, while T solium adults have an average of 1000 proglottids. The eggs contained in the gravid proglottids are released after the proglottids are passed with the feces. T saginata may produce up to 100,000, and T solium may produce 50,000 eggs per proglottid respectively. (From Global Health, Division of Parasitic Diseases and ...