Spontaneous Frenkelia infection in a laboratory-reared rat.

Numerous, lobulated, thin-walled protozoan cysts were found in the brain and cervical spinal cord of a male Fischer rat with granulomatous encephalitis. The cysts were compartmented, 470-1000 mum in diameter, and packed with crescent-shaped organisms. Light- and electron-microscopic features of the organism were characteristic of the protozoan Frenkelia, a parasite usually found in the brain of wild rodents. Many cysts were replaced by granulomas, whereas others elicited no inflammation. Perivascular and meningeal mononuclear cell infiltrates and gliosis were common.