Causal Agent:
Cestodes in the genus Mesocestoides; M. lineatus and M. variabilis have been reported to infect humans.

Life Cycle:

Mesocestoides spp. require a three-host life cycle to complete their development.  The definitive hosts are primarily carnivores, including canids, felids and mustelids. Gravid, motile proglottids are shed in feces .  Within the proglottids, hundreds of oncospheres are contained within the parauterine organ .  The first intermediate host is presumed to be an arthropod , and becomes infected after eating proglottids or oncospheres.  Several arthropods have been looked at as potential first intermediate hosts for Mesocestoides spp, including ants and oribatid mites; however, none of these species have been demonstrated in nature to serve as an intermediate host.  In the first intermediate host, it is believed the oncosphere develops into a second-stage larva (cysticercoid or procercoid).  When the first-intermediate host is eaten by a second intermediate host, including small mammals, birds, reptiles and amphibians , the second-stage larva develops into an infective, third-stage larva (tetrathyridium).  Domestic and wild canids, which usually serve as the definitive host, may also serve as dead-end intermediate hosts upon ingestion of infected first intermediate hosts.  The definitive host ultimately becomes infected after eating meat contaminated with tetrathyridia .  Upon ingestion, the cestode settles in the small intestine where it matures.  Gravid proglottids can be seen in the stool as early as two weeks.  Humans are not usual definitive hosts, but can serve as such after eating undercooked meat containing tetrathyridia .

Geographic Distribution:
North America, Europe, Asia and Africa; M. lineatus is an Old World species; M. variabilis is North American.