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The cestode Taenia saginata (beef tapeworm) (along with T. solium [pork tapeworm], and T. asiatica [Asian tapeworm]) causes intestinal taeniasis in infected humans. Taenia solium (like T. saginata) has a worldwide distribution (T. asiatica, a close relative of T. saginata, is limited to Asia). (Centers for Disease Control Parasites and Health website) Genetically, T. saginata is far more similar to T. asiatica (to which it is also morphologically very similar) than it is to T. solium (Jeon et al. 2007; Eom et al. 2009) and it appears that the (relatively recent) divergence of T. saginata and T. asiatica was associated with a host shift (cattle vs. pigs).
Humans are the only definitive host (i.e. the host that harbors adult parasites) for Taenia saginata (as well as T. solium, and T. asiatica). An infection usually involves just a single tapeworm (Nakao et al. 2010). Eggs or gravid proglottids (bisexual reproductive segments) are passed with feces; the eggs can survive for days to months in the environment. Cattle 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 beef. In the human intestine, the cysticercus develops over several months into an adult tapeworm, which can survive for years. The adult tapeworm attaches to the small intestine by its scolex and resides in the small intestine. The length of an adult T. saginata is typically 5 m or less (but may reach as much as 25 m). The adults produce proglottids which mature, become gravid, detach from the tapeworm, and migrate to the anus or are passed in the stool (several per day). Taenia saginata adults usually have 1,000 to 2,000 proglottids. The eggs contained in the gravid proglottids are released after the proglottids are passed with the feces. Taenia saginata may produce up to 100,000 eggs per proglottid. (Centers for Disease Control Parasites and Health website)
Conlan et al. (2009) explored the role of interspecific competition among Taenia species in modulating T. solium infection of humans. In Southeast Asia, T. solium faces competition in both the definitive host (humans) and the intermediate host (pigs). In humans, adult worms of T. solium, T. saginata and T. asiatica compete through density-dependent crowding mechanisms. In pigs, metacestodes of T. solium, T. hydatigena and T. asiatica compete through density-dependent immune-mediated interactions. Humans are the definitive host for T. solium, T. saginata, and T. asiatica. Pigs are the known intermediate host for T. solium, T. asiatica and T. hydatigena. Canines are the definitive host for T. hydatigena and bovines are the intermediate host for T. saginata. Conlan et al. (2009) compared the biological characteristics of T. solium, T. saginata, T. asiatica, and T. hydatigena. (Conlan et al. 2009 and references therein)