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Causal Agent:

Hymenolepiasis is primarily caused by the cestode (tapeworm) species, Hymenolepis nana (the dwarf tapeworm, adults measuring 15 to 40 mm in length).

Life Cycle:

Hymenolepis nana life cycle

Eggs of Hymenolepis nana are immediately infective when passed with the stool and cannot survive more than 10 days in the external environment The number 1. When eggs are ingested by an arthropod intermediate host The number 2 (various species of beetles and fleas may serve as intermediate hosts), they develop into cysticercoids, which can infect humans or rodents upon ingestion The number 3 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 The number 4 (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 The number 5. Upon rupture of the villus, the cysticercoids return to the intestinal lumen, evaginate their scoleces The number 6, attach to the intestinal mucosa and develop into adults that reside in the ileal portion of the small intestine producing gravid proglottids The number 7. Eggs are passed in the stool when released from proglottids through its genital atrium or when proglottids disintegrate in the small intestine The number 8. An alternate mode of infection consists of internal autoinfection, where the eggs release their hexacanth embryo, which penetrates the villus continuing the infective cycle without passage through the external environment The number 9. The life span of adult worms is 4 to 6 weeks, but internal autoinfection allows the infection to persist for years.

Life cycle image and information courtesy of DPDx.