A 10-day toxicity study of toltrazuril 5% suspension in the horse

Equine protozoal myeloencephalitis (EPM) is a serious disorder of the nervous system of horses caused by Sarcocystis neurona. Recently, toltrazuril has begun to be used for treatment of EPM. The purpose of this study was to evaluate the potential toxicity of toltrazuril in horses when administered at a dose of 50 mg/kg for 10 days. Five horses were given 50 mg/kg of toltrazuril once daily for 10 days by nasogastric tube. Complete blood cell counts, serum chemistry values, and coagulation panels were evaluated before and after treatment; then a full postmortem examination was completed on day 11. Results of the study show that only mild clinical signs (i.e., anorexia, weight loss, and colic in one of the five horses) were observed after treatment. Minimal changes were noted in the serum chemistry analysis (i.e., increased bilirubin and serum protein, mildly increased hematocrit and hemoglobin concentration, and mildly increased albumin). Renal medullary congestion was noted on gross postmortem examination; however, no microscopic changes due to compound administration were noted in the kidneys or other tissue. Results of this study suggest that administration of toltrazuril at 50 mg/kg (5x to 10x anticipated clinical dosage) for 10 days resulted in only mild clinical abnormalities.