Objective—To evaluate the effect of intermittent oral administration of ponazuril on immunoconversion against Sarcocystis neurona in horses inoculated intragastrically with S neurona sporocysts.
Animals—20 healthy horses that were seronegative for S neurona–specific IgG.
Procedures—5 control horses were neither inoculated with sporocysts nor treated. Other horses (5 horses/group) each received 612,500 S neurona sporocysts via nasogastric tube (day 0) and were not treated or were administered ponazuril (20 mg/kg, PO) every 7 days (beginning on day 5) or every 14 days (beginning on day 12) for 12 weeks. Blood and CSF samples were collected on day – 1 and then every 14 days after challenge for western blot assessment of immunoconversion. Clinical signs of equine protozoal myeloencephalitis (EPM) were monitored, and tissues were examined histologically after euthanasia.
Results—Sera from all challenged horses yielded positive western blot results within 56 days. Immunoconversion in CSF was detected in only 2 of 5 horses that were treated weekly; all other challenged horses immunoconverted within 84 days. Weekly administration of ponazuril significantly reduced the antibody response against the S neurona 17-kd antigen in CSF. Neurologic signs consistent with EPM did not develop in any group; likewise, histologic examination of CNS tissue did not reveal protozoa or consistent degenerative or inflammatory changes.
Conclusions and Clinical Relevance—Administration of ponazuril every 7 days, but not every 14 days, significantly decreased intrathecal anti–S neurona antibody responses in horses inoculated with S neurona sporocysts. Protocols involving intermittent administration of ponazuril may have application in prevention of EPM.