Sarcocystis neurona encephalitis in a dog.

A 1.5-year-old male Feist dog was presented to a veterinarian for reluctance to stand on the hind legs. Treatment included dexamethasone and resulted in a favorable initial response, but posterior paresis returned and progressed to recumbency, hyperesthesia, and attempts to bite the owner. The dog was euthanized. The brain was negative for rabies by fluorescent antibody analysis. Multiple foci of encephalitis were found in the cerebrum and particularly in the cerebellum. Protozoa morphologically consistent with Sarcocystis sp. were identified at sites of intense inflammation and malacia. Additionally, multiple schizonts were identified in areas without inflammation. Immunohistochemistry using both polyclonal and monoclonal antibodies specific for Sarcocystis neurona was strongly positive. No reaction to polyclonal antisera for Toxoplasma gondii or Neospora caninum was found. Polymerase chain reaction confirmed that the protozoa were S. neurona. Additional aberrant hosts for S. neurona other than horses have been identified, but S. neurona encephalitis has not been documented previously in the dog.