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8 Covetrus | ph: 855.724.3461 | fx: 888.329.3861 | northamerica.covetrus.com The risk of equine protozoal myeloencephalitis (EPM) is six times higher in the fall. Opossums and other wildlife that serve as hosts to the EPM-causing parasites, Sarcocystis neurona and Neospora hughesi, are more active when the weather begins to cool. Autumn is also a time when months of heavy exercise, transport, and other stressful events for horses can take a toll, which can increase their risk of EPM. Although the incidence of EPM clinical disease is low, up to 90 percent of the U.S. horse population has been exposed to S. neurona, depending on geographic location. 1 This creates a high seroprevalence in healthy horses, which can make diagnosis tricky, yet all the more important. Though EPM can be diagnosed anytime of year, it is more likely in the fall months. Before prescribing treatment, make sure a thorough diagnostic workup is completed. Keeping an eye on EPM this fall By Bryant Craig, DVM, Merck Animal Health Equine Veterinary Professional Services Diagnose before treating While early treatment is critical to stopping the disease from causing further nerve damage, if the horse does not have EPM, an EPM treatment product will not be effective. Many diseases cause symptoms similar to EPM but will not respond to EPM treatment. If that horse has another disease, not only have we wasted money on unnecessary treatment, but also time that could have been better used pursuing the true cause of the horse's problem. A thorough diagnostic workup is critical to make sure we are treating the horse appropriately and improving his chance of recovering. The best initial course of action is a combination of reviewing the health history of that horse and conducting a thorough neurologic and physical exam. The organism will generally cause asymmetric deficits. Experts say it is rare to have equal deficits on both sides of the horse, so asymmetry is a tell-tale marker of a suspect EPM case. EPM is a progressive, multi-focal disease, often with muscle atrophy. While other clinical signs may be present, these are generally the more common signs in EPM cases and point to the need for antibody testing to further differentiate EPM from other neurological diseases. The recommendation for antibody testing is to collect spinal fluid and blood samples and compare the antibody titers in each to determine if there is evidence of a central nervous system infection.