Development
Infection is acquired most commonly from eggs in the feces of another infected individual, which are transferred in food, by contamination. Eggs hatch in the duodenum, releasing oncospheres, which penetrate the mucosa and come to lie in lymph channels of the villi. Oncospheres develops into a cysticercoid which has a tail and a well formed scolex. It is made of longitudinal fibers and is spade shaped with the rest of the worm still inside the cyst. In five to six days cysticercoids emerge into the lumen of the small intestine, where they attach and mature.
The direct life cycle is doubtless a recent modification of the ancestral two-host life cycle, found in other species of hymenolepidids, because cysticercoids of H. nana can still develop normally within larval fleas and beetles. One reason for facultative nature of the life cycle is that H. nana cysticercoids can develop at higher temperatures than can those of the other hymenolepidids. Direct contaminative infection by eggs is probably the most common route in human cases, but accidental ingestion of an infected grain beetle or flea cannot be ruled out. The direct infectiousness of the eggs frees the parasite from its former dependence upon an insect intermediate host, making rapid infection and person-to-person spread possible. The short life span and rapid course of development also facilitate the spread and ready availability of this worm.
Read more about this topic: Hymenolepis Nana
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