Klossiella muris is a sporadic apicomplexan protozoan pathogen of mice. In this case, the mouse was a pet. Contemporary infections in laboratory mice are rare. Sporocysts are shed in the urine and thus transmitted to naive animals by ingestion. Ensuing hematogenous spread facilitates localization to the glomerular capillaries where schizogony occurs. The renal tubular epithelial cells are the site of gametogony and sporogony. Infiltrates of lymphocytes and plasma cells can sometimes accompany infection, which is usually asymptomatic. Renal klossiellosis may also be seen in horses (Klossiella equi), guinea pigs (Klossiella cobayae), and other species