Sarcocystis-associated encephalitis in chickens.

Sarcocystis-associated encephalitis was diagnosed in a backyard chicken flock that had nervous manifestations. The main histopathologic lesion was necrotizing encephalitis characterized by a large focal area of necrosis infiltrated and surrounded by mononuclear cells, heterophils, and multinucleated giant cells. Schizonts and merozoites were observed in the lesion. Immunohistochemical staining of the brain lesion revealed positive reaction to Sarcocystis antiserum. The ultrastructural characteristics of the parasite were typical of Sarcocystis, including the presence of a nucleus, a conoid, numerous micronemes, and lack of rhoptries. Medication with amprolium and sulfamethazine or with chlortetracycline was not effective in controlling the mortality. Trapping of opossums on the farm and relocating the chickens to clean, new premises seemed to reduce mortality from this infection.