The Rat Tapeworm (Hymenolepis diminuta) is infrequently seen in humans, but frequently in rodents. It is one of two tapeworm species that cause hymenolepiasis in humans (the other being the Dwarf Tapeworm, H. nana). Adult H. diminuta measure 20 to 60 cm in length.
Eggs of Hymenolepis diminuta are passed out in the feces of the infected definitive host (rodents, humans). The mature eggs are ingested by an intermediate host (various arthropod adults or larvae) and oncospheres are released from the eggs; these penetrate the intestinal wall of the host and develop into cysticercoid larvae. Tribolium beetles are common intermediate hosts for H. diminuta. The cysticercoid larvae persist through the arthropod's morphogenesis to adulthood. Hymenolepis diminuta infection is acquired by the mammalian host after ingestion of an intermediate host carrying the cysticercoid larvae. Humans can be accidentally infected through the ingestion of insects in pre-cooked cereals or other food items, as well as directly from the environment (e.g., via oral exploration of the environment by children). After ingestion, the tissue of the infected arthropod is digested, releasing the cysticercoid larvae in the stomach and small intestine. Eversion of the scoleces occurs shortly after the cysticercoid larvae are released. Using the four suckers on the scolex (the anterior part of the warm specialized for attachment to the gut wall of the host), the parasite attaches to the small intestine wall. Maturation of the parasites occurs within 20 days and the adult worms reach an average of around 30 cm in length. Eggs are released in the small intestine from gravid proglottids (serially repeated bisexual reproductive segments) that disintegrate after breaking off from the adult worms. The eggs are expelled to the environment in the mammalian host's feces, beginning the cycle again.
From Centers for Disease Control Parasites and Health website