This report discusses the unique case of a 1.5-year-old, 20-pound male Feist crossbred dog that was diagnosed with a naturally occurring infection of protozoal encephalitis attributed to Sarcocystis neurona. The dog was initially presented to the veterinarian due to reluctance to stand on its hindlimbs. Survey radiographs were taken, and the veterinarian suspected a herniated lumbar intervertebral disk. The dog was treated with dexamethasone and responded well for approximately 3 days. However, the hindlimbs weakened gradually over the following week, and by day 10, the dog could not lift itself. When the dog was reexamined on day 11, it had hyperesthesia and attempted to bite when placed on its right side. The dexamethasone shots were repeated, but the dog did not respond. The dog was euthanized on day 15, and the necropsy showed good body condition and no gross lesions. A rabies fluorescent antibody examination of brain tissue was negative. Histologic lesions were confined to the brain and included widespread, asymmetric encephalitis mostly in the gray matter; inflammation was present and most pronounced in the cerebellum.
Other areas of the brain showed collections of macrophages, neutrophils, lymphocytes, and occasional eosinophils, and there were numerous protozoal organisms mostly associated with the areas of extreme inflammation. Schizonts were seen at the periphery of the inflammatory areas. The protozoal organisms stained intensely with polyclonal and monoclonal S neurona antibodies. Polymerase chain reaction (PCR) was performed, and a subsequent search of GenBank showed the sequence to be 100% identical to several published S neurona ITS-1 sequences. S neurona was definitively diagnosed based on the limitation of the infection to the central nervous system, the presence of an equine protozoal myeloencephalitis-like disorder with necrosis, morphologically consistent schizonts and merozoites, strong positive immunoreactivity to S neurona antibodies, and the PCR analysis.