Equine Protozoal Myeloencephalitis (EPM) is a serious neurologic disease of horses in the United States. The primary etiologic agent is Sarcocystis neurona (S. neurona). Currently, there is limited knowledge regarding the protective or pathologic immune response to infection to the intracellular protozoa S. neurona. The objective of these studies was to determine the effects of S. neurona infection on the immune response of horses that had EPM due to natural infection (experiment 1) and experimental infection (experiment 2). In experiment 1, twenty-two horses with naturally occurring cases of EPM, which were confirmed positive based on detection of antibodies in the serum and/or CSF and clinical signs, and 20 clinically normal horses were included to determine whether S. neurona altered the immune responses, as measured by immune cell subsets (CD4, CD8, B-cell, monocytes, and neutrophils) and leukocyte proliferation (antigen specific and non-specific mitogens). Our results demonstrated that naturally infected horses had significantly higher percentages of CD4 and neutrophils (PMN) in peripheral blood mononuclear cells (PBMCs) than clinically normal horses. Leukocytes from naturally infected EPM horses had a significantly lower proliferation response, as measured by thymidine incorporation, to a non-antigen specific mitogen phorbol 12-myristate 13-acetate (PMA) / ionomycin (I) than did clinically normal horses (p=0.04). The implications of these findings will be discussed.

In experiment 2, 13 horses were randomly divided into two groups. Baseline neurologic examinations were performed and all horses were confirmed negative for S. neurona antibodies in the CSF and serum. Then, one group with 8 clinically normal seronegative horses was inoculated intravenously with approximately 6000 S. neurona infected autologous leukocytes daily for 14 days. All the challenged horses showed neurologic signs consistent with EPM. PBMCs were isolated from the control and infected horses to determine how S. neurona alters the immune responses based on changes in immune cell subsets and immune function. There were no significant differences in the percentage of CD4 cells in peripheral blood lymphocytes or IFN-γ production by CD4 and/or CD8 cells. PMA/I stimulated proliferation responses in PBMCs appeared suppressed compared to that of uninfected controls. Additional studies are necessary to determine the role of CD4 and CD8 cells in disease and protection to S. neurona in horses, as well as to determine the mechanism associated with suppressed in vitro proliferation responses. This project was funded by Patricia Stuart Equine grants and paramutual racing funds from Virginia Tech.