Increased presence of T lymphocytes in central nervous system of EPM affected horses.

Equine protozoal myeloencephalitis (EPM), caused by Sarcocystis neurona infection in the central nervous system (CNS), affects up to 1% of all horses during their lifetimes. Neither the protective immune response nor the immunopathology associated with the disease is well understood. To begin to clarify the pathogenesis of the disease, immunohistochemical staining for B and T lymphocytes was performed on spinal cord sections obtained from 17 horses, all of which were all positive for S. neurona based on immunohistochemical staining. Fifteen of the 17 horses included in the study were killed due to neurologic dysfunction; 2 of the 17 horses were killed because of fractures. All 17 horses had histologic changes consistent with S. neurona infection. A significantly greater number of T cells were seen in sections from S. neurona-infected versus control horses. Because this was a small descriptive study, we were not able to determine the mechanisms of enhanced T-cell recruitment in the sections from the S. neurona-infected horses.