Eggs of the tapeworm Hymenolepis nana are infective when passed with the stool, but cannot survive more than about 10 days outside the body.
When eggs are ingested by an arthropod intermediate host (a variety of fleas and beetles can serve as intermediate hosts), they develop into cysticercoids, which can infect humans or rodents upon ingestion and develop into adults in the small intestine.
When eggs are ingested (in contaminated food or water or from hands contaminated with feces), the oncospheres contained in the eggs are released. The oncospheres (also known as "hexacanth larvae") -- see picture at right -- penetrate the intestinal villi and develop into cysticercoid (encysted) larvae. Upon rupture of the villus, the cysticercoids return to the intestinal lumen, evaginate their scolices, attach to the intestinal mucosa and develop into adults that reside in the ileal part of the small intestine and produce gravid proglottids. Eggs, when released in the small intestine, are passed in the stool.
An alternate mode of infection occurs, known as internal autoinfection, where the eggs release their oncospheres, which then penetrate the villi continuing the infective cycle without leaving the host's body. The life span of adult worms is four to six weeks, but internal autoinfection can permit an infection to last for years.
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