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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 scoleces
,
attach to the intestinal mucosa and develop into adults that reside in the
ileal portion of the small intestine producing gravid proglottids
.
Eggs are passed in the stool when released from proglottids through its
genital atrium or when proglottids disintegrate in the small intestine
.
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 life span of adult worms is 4 to 6 weeks, but internal autoinfection
allows the infection to persist for years.
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