Hymenolepis microstoma, also known as the rodent tapeworm, is an intestinal dwelling parasite. Adult worms live in the bile duct and small intestines of mice and rats, and larvae metamorphose in the haemocoel of beetles. It belongs to the genus Hymenolepis; tapeworms that cause hymenolepiasis. H. microstoma is prevalent in rodents worldwide, but rarely infects humans. Ecology Hymenolepis microstoma is an obligate parasite. Adults live in the bile duct and small intestine of rodents such as mice (Mus musculus), and larvae infect grain beetles such as Tribolium spp., in which they metamorphose from larvae into juvenile worms. Worms vary from 4 to 30 cm in length, depending on the age and number of worms within the host. Adults have completely lost their mouth and intestine. Instead they use their skin (tegument) to absorb nutrients directly from the host gut. Hymenolepis species and other tapeworms often exhibit a 'crowding effect' in which the total biomass of the worms stays more or less constant, regardless of the intensity of infection. Thus low intensity infections result in larger worms and high intensity infections produce smaller worms. Under laboratory conditions, H. microstoma adult infections in mice are typically limited to approximately 12 worms. Life cycle The cycle begins as arthropods become intermediate hosts by ingesting the parasite eggs. Oncospheral larvae are released from the eggs and use hooks and secreted enzymes to penetrate the gut of the beetles and enter the haemocoel. In the haemocoel the larvae undergo complete cellular reorganization (i.e. metamorphosis), transforming into cysticercoid larvae in approximately 7–10 days. The larvae can remain in the cysticercoid stage in the beetle as long as the lifespan of the adult beetle (up to 3 years), although any age-related decrease in viability has not been studied. When ingested, the eggs develop into cysticercoids. Rodents can become infected when they eat arthropods, such as flour beetles (Tribolium ssp). Humans, especially children, can ingest the arthropods as well and therefore become infected via the same mechanism. Rodents, especially rats, are definitive hosts and natural reservoirs of H. microstoma. As the definitive host (rats) eats an infected arthropod, cysticercoids present in the body cavity transform into the adult worm. Juvenile worms establish in the bile duct of mice after approximately 3 days movement within the upper gastrointestinal tract. Once established in the bile duct, the worms then mature sexually and begin producing eggs within approximately 1 week. Eggs