Tapeworm Infestations

Wild, pet, and laboratory rodents can be infested with 3 separate species of adult tapeworm, including Rodento- lepis (formerly Hymenolepis) nana (dwarf tapeworm), Hymenolepis diminuta, and Rodentolepis microstoma.

Epizootiology

A variety of species of laboratory animals are susceptible to infection with the dwarf tapeworm, including mice, rats, and hamsters. Its wide host range also includes humans. Husbandry conditions have essentially elimi­nated H. diminuta and R. microstoma and have greatly reduced the prevalence of R. nana in laboratory mouse populations. These tapeworms all utilize arthropods (flour beetles, fleas, moths, etc.) as intermediate hosts, but R. nana can also have a direct life cycle in which onchospheres penetrate the mucosa and develop into the cysticercoid stage, subsequently emerging into the lumen as adults. The entire life cycle can occur in the intestine within 20-30 days. Thus, superinfections can occur in the absence of an intermediate host. Immunity develops to these worms, with reduction of parasite numbers over time in immunocompetent mice. In addi­tion, R. microstoma can have a direct life cycle in immu­nodeficient nude and NOD-scid, NOD-scid-IL-2R gamma null mice. Clinical signs associated with heavy infesta­tions include poor weight gains and diarrhea.

Mice may serve as the intermediate host for the cat tapeworm, Taenia taeniaformis. The larval strobilocercus form, Cysticercus fasciolaris, consists of a scolex and segments within a cyst and thus resembles an adult tapeworm (Fig. 1.92). The liver is the most frequent location for strobilocerci. The source of the parasite is usually via feed contaminated with cat feces. While cysticercosis should be nonexistent in laboratory mice, the authors have seen infected laboratory mice on mul­tiple occasions.

Pathology

Rodentolepis nana adults are threadlike worms in the small intestine. Microscopic findings include the pres­ence of cysticeri within the lamina propria and adults with prominent serrated edges in the lumen. Occasion­ally, cysticeri can be found in the mesenteric lymph nodes. Hymenolepis diminuta adults are much larger, and intermediate forms do not appear in the mucosa. Rodentolepis microstoma adults are the size of H. diminuta and often exist within the biliary or pancreatic ducts, inciting inflammatory and atrophic changes in the pan­creas and cholangitis. The strobilocerci of T. taeniaefor- mis are most often embedded in the liver

Diagnosis

Adult tapeworms can be identified grossly. Rodentolepis nana is typically threadlike (1 mm wide), while the other species are much larger (4 mm wide). The R. nana scolex possesses hooks, and the ova have polar filaments, while those of H. diminuta do not (figures of R. nana and H. diminuta are shown in Hamster Chapter 3).