Perspectives on intestinal tapeworm infections: An evaluation of direct and indirect life-cycles with a special emphasis on species of Hymenolepis

Numerous experimental studies have been conducted on the rodent tapeworm, Hymenolepis microstoma. In contrast, less is known about the life-cycle and immunobiology of the zoonotic dwarf tapeworm, Hymenolepis nana. However, H. nana appears to be unique in that; (i) it can complete its entire life-cycle within a single mammalian host, and (ii) cysticercoids that develop in beetle intermediate hosts are tailed, while those that develop in the intestinal tissue of mammals are tailless. This is in contrast to all other Hymenolepis spp., which only appear to develop tailed cysticercoids in beetles or experimentally infected immunodeficient rodents. Even though H. microstoma and H. nana are phylogenetically much closer to each other than to Hymenolepis diminuta, when mice with severe combined immunodeficiency (SCID) were inoculated with H. microstoma eggs, hatched oncospheres invaded the intestinal tissue and developed into infective tailed cysticercoids in approximately 11 days. Therefore, H. nana appears to be truly unique in its ability to develop tailed cysticercoids in beetles and tailless cysticercoids in mammals. These unique evolutionary characteristics are discussed in relation to other cyclophyllidean cestodes, including Taenia solium and Echinococcus spp.