The Dwarf Tapeworm (Hymenolepis nana) is the most commonly encountered tapeworm infecting humans and is encountered worldwide. In temperate regions, its incidence is higher in children and institutionalized groups. It is one of two tapeworm species that cause hymenolepiasis in humans (the other being the Rat Tapeworm, H. diminuta). Adult H. nana are unusually small for tapeworms, measuring just 15 to 40 mm in length.
Eggs of Hymenolepis nana are immediately infective when passed with the stool and cannot survive more than 10 days in the external environment. When eggs are ingested by an arthropod intermediate host (various species of beetles and fleas may serve as intermediate hosts), they develop into cysticercoids, which can infect humans or rodents upon ingestion and develop into adults in the small intestine. A morphologically identical variant, H. nana var. fraterna, infects rodents and uses arthropods as intermediate hosts. 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 (hexacanth larvae) penetrate the intestinal villus and develop into cysticercoid 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 portion of the small intestine and produce gravid proglottids. Eggs are passed in the stool when released from proglottids through the genital atrium or when proglottids disintegrate in the small intestine. An alternate mode of infection consists of internal autoinfection, in which the eggs release their hexacanth embryo, which penetrates the villus, continuing the infective cycle without passage through the external environment. The life span of adult worms is 4 to 6 weeks, but internal autoinfection allows the infection to persist for years.
From Centers for Disease Control Parasites and Health website