Transmission of Hymenolepis spp.
Humans and other animals become infected when they intentionally or unintentionally eat material contaminated by insects. H. nana is the only cestode that parasitizes humans without requiring an intermediate host. It is possible for the worm's entire life cycle to be completed in the bowel, so infection can persist for years. H diminuta is a cestode of rodents infrequently seen in humans.
H. nana adults reside in the ileal portion of the small intestine. From them, proglottids are produced, which release eggs that are immediately infective. Passed in the stool, these eggs cannot survive for more that 10 days outside the host. After ingestion of eggs in contaminated food, water, or anything contaminated by feces, the oncosphere is released from within the egg. This hatches in the small intestine, and the hexacanth burrows into the tissues of the small intestine and a cystercoid forms. When it matures upon rupture of the villus, the cysticercoid returns to the intestinal lumen. Here, it envaginates its scolex, which attaches to the intestinal mucosa to develop into an adult. An alternate mode of infection consists of internal autoinfection, where the eggs release their oncospheres directly into the intestine without passage through tthe external environment. The life span of an adult worm is 4-6 weeks, but internal autoinfection allows it to perist for years with hight host population density. In addition, when eggs are ingested by insects, they develop into cystercoids, which can infect humans or rodents upon ingestion.
H. diminuta is a parasite of rodents and requires an arthropod as an intermediate host. Eggs ingested by the arthropod develop into a cystercoid larvae. Rodents become infected by ingesting the arthropods, and humans can be accidentally infected through the same mechanism.
Eggs of these worms are ingested by insects and mature into a life form refered to as a "cysticercoid" in the insect. The Tribolium confusum(confused beetle) is a common "pest" found in stored grain products, including flour and breads stored in your kitchen. Adult beetles of both the flour beetle, Tribolium spp., and the grain beetle, Tenebrio spp., are about 4 mm in length and serve as the intermediate host for several Hymenolepis spp.