Merck Animal Health, known as MSD Animal Health outside of the United States and Canada, a division of Merck & Co., Inc., Rahway, N.J., USA (NYSE:MRK), today announced the results of a new study on PROTAZIL, its pelleted treatment for equine protozoal myeloencephalitis (EPM), which is a neurological disease in horses. The study shows the active ingredient, diclazuril, reaches effective levels, known as the minimum inhibitory concentration (MIC), againstSarcocystis neurona, the primary protozoa to cause EPM, in the cerebrospinal fluid (CSF) within just 12 hours of one oral dose.With a progressive neurological disease like EPM, time is of the essence, says Phil van Harreveld, DVM, MS, DACVS-LA, senior equine professional services veterinarian for Merck Animal Health. Prior to this study, we didnt have clear data on how quickly diclazuril reaches a concentration level in the cerebrospinal fluid known to inhibit this parasite. We can now definitively state that PROTAZIL rapidly reaches therapeutic levels in the CSF after just one dose, offering veterinarians and horse owners reassurance in their treatment of this devastating disease.EPM is a debilitating neurological disease caused by protozoal infection of the central nervous system, leading to symptoms such as incoordination, muscle weakness, and gait abnormalities in affected horses. Up to 90% of the U.S. horse population has been exposed toS. neurona;fortunately, not all infected horses will develop clinical disease. For those that progress to disease, 60 to 70% of horses show clinical improvement with early treatment.1Key study findingsThe North Carolina State University study involved six healthy adult horses that were neurologically normal and negative for EPM. Diclazuril levels in both plasma (blood) and CSF were measured at several time points following a single oral dose administered at the labeled dose of 1 mg/kg top-dressed on sweet feed. Highlights include:Diclazuril concentrations in the CSF surpassed the MIC forS. neurona(1 ng/mL) in all horses by 12 hours post-administration.The CSF concentrations peaked at 24 hours, while plasma concentrations peaked later, at 48 hours. A horse with active EPM carries the parasite in its nervous system, which is why its helpful to know CSF concentrations and not just blood levels of the drug.Not only did diclazuril reach the MIC quickly, but it also remained well above the MIC for eight days, suggesting a sustained therapeutic effect.References1Reed SM, et al. Equine Protozoal Myeloencephalitis: An Updated Consensus Statement with a Focus on Parasite Biology, Diagnosis, Treatment and Prevention.J Vet Intern Med2016;30:491-502.Our most significant finding from the research is the speed at which diclazuril reached the MIC for S. neurona in the horses CSF, says Kate Hepworth-Warren, DVM, DACVIM-LAIM, lead researcher and assistant professor of equine internal medicine at the NC State College of Veterinary Medicine. This is especially critical in acute cases where the clock is ticking. Initiating diclazuril therapy early can give the horse the best possible head start against aggressive disease progression.This research was presented at the 2025 International Veterinary Emergency & Critical Care Society Meeting.Horse owners are encouraged to speak with their veterinarian to determine whether PROTAZIL is the right treatment for their horse.Learn more about EPM here.Please see Prescribing Information for PROTAZILHERE.IMPORTANT SAFETY INFORMATION:PROTAZILis contraindicated in horses with known hypersensitivity to diclazuril. The safety of PROTAZILin horses used for breeding purposes, during pregnancy, or in lactating mares, and use with concomitant therapies in horses has not been evaluated. Do not use in horses intended for human consumption. Not for human use. For complete safety information, refer to the product label.