Rodent Tapeworm, Hymenolepis microstoma
The rodent tapeworm (Hymenolepis microstoma) is a parasitic worm that is classified within the Platyhelminthes phylum. This species affects rodents across the world, causing hymenolepiasis, but it does not often affect humans. Most of the available information regarding tapeworms is derived from the studies conducted on this worm and the other members of its genus, Hymenolepis. These worms have been present in laboratories since the 1950’s and can either be raised and kept in a culture or be maintained within a rodent host, which allows for live study and education.
The rodent tapeworm can reach an average length between 1.5 and 11.8 inches, depending upon the worm’s age and the number of worms within a single host. This species does not have a mouth or digestive system, so it must gather nutrients by absorbing them directly into the skin, also known as the worm’s tegument. It displays a “crowding effect” within a host, where worms will gather and maintain a constant biomass that is not dependent upon the strength of the infection. However, worms that reside in a higher density tend to grow smaller than those that reside in a lower density. Tests conducted on mice have shown that the average number of worms within a laboratory mouse is about twelve.
As is typical to tapeworm species, the rodent tapeworm is able to reproduce body segments throughout its life. These segments are created in the neck area when totipotent stem cells divide to form new segments. These segments mature over time to contain reproductive and other organs. The rodent tapeworm is hermaphroditic, so the segments that contain organs hold both male and female reproductive organs that will eventually produce eggs.
The lifecycle of the rodent tapeworm begins when an arthropod consumes the eggs of the species. The digestive fluids of the intermediate host, which are often beetles, will break down the egg and release oncospheral larvae. The larvae will use the hooks located on their front side to attach themselves to the gut of the host, where they will undergo metamorphosis and develop into cysticercoid larvae. This process takes between seven and ten days, but the larvae can remain encysted for as long as three years, which is the average lifespan of a beetle.
Rodents contract the worm when they eat infected arthropods, including the flour beetle, which is also a common method of human children contracting the worm. Once the encysted worms are safely within the body cavity of the definitive host, most often a rat, the cysts will develop into adult tapeworms. Young worms that are digested will travel through the gastrointestinal tract for about three days, reaching the bile duct after this period. After one week of developing, the worms reach sexual maturity and begin laying eggs. These eggs are released in the feces and continue the cycle of the species.
The rodent tapeworm does not often infect humans, and most human hosts do not experience symptoms of infection. However, some hosts have reported symptoms like abdominal pain, itching, irritability, and eosinophilia. Some cases have been treated using praziquantel, but more research is needed to confirm a preferred treatment in humans.
Image Caption: Scolex of the rodent tapeworm Hymenolepis microstoma. Credit: Magdalena ZZ/Wikipedia (CC BY-SA 3.0)