The severe combined immunodeficient mouse as a definitive host for Sarcocystis muris

Peroral and intraperitoneal inoculation of severe combined immunodeficient (SCID) mice with cystozoites of three coccidia of the genus Sarcocystis (Pr otozoa, Apicomplexa; S. dispersa, Sarcocystis sp., and S. muris) revealed t hat after peroral administration, only S. muris could develop in the immuno deficient mouse host. The cystozoites of S. muris transformed into gamonts and, after fertilization, performed sporulation with the production of infe ctious sporocysts in the small intestine of the SCID mice. Impaired immunit y is probably responsible for the unusual behavior of S. muris (which is no rmally the heteroxenous mouse-cat parasite) in the SCID mice. We hypothesiz e that the phylogenetic distance between the intermediate and final hosts i s the reason why cystozoites of the two other Sarcocystis species tested (S . dispersa with a mouse-owl cycle and Sarcocystis sp. with a murine rodent- snake cycle) could not develop when inoculated into SCID mice.