Cerebral Microsporidiosis Caused by Encephalitozoon cuniculi Infection in a Young Squirrel Monkey

This is a case report of cerebral microsporidiosis found in a young squirrel monkey (Saimiri sciureus) in a colony located in Japan, which probably died of yersiniosis due to Yersinia pseudotuberculosis infection. The microsporidia, Encephalitozoon cuniculi, was detected in the brain of the yersiniosis-diseased monkey and was further characterized as a genetically unique type of strain III. The agent was microbiologically and genetically undetectable in other organs tested. Gram-positive organisms, which were confirmed immunohistochemically as Encephalitozoon spp. including mature spores, were histologically detected in pseudocysts formed inside neurons and in neuropils. Reactions in the surrounding tissue were not observed for most parasitized lesions. Neurons in the brain of younger hosts might provide a site for latent and active infection by E. cuniculi.