The incidence of EPM is unknown. The clinical signs of this disease can be subtle and progress slowly or not at all, which means that many horses that are mildly affected may never be diagnosed. Overall, the most information about this disease has been collected from horses that have died and been necropsied, which is not a good representation of the average case of EPM. Horses throughout the Western Hemisphere are affected by EPM, with cases identified in Canada, Panama, Brazil, and Mexico(see reference list). Several cases of EPM have been identified in Britain and France in horses imported from the American continents. Seroprevalence of horses in Pennsylvania, Kentucky, and Ohio is approximately 40-50%(see reference list). At the Livestock Disease Diagnostic Center in Lexington, KY, EPM accounts for about 11% of horses (1988-1991) that were euthanatized or expired due to neurologic disease. At the New York State College of Veterinary Medicine between 1977 and 1987, EPM accounted for 25% of neurologic cases seen (Boy et al., 1990). This disease is most commonly observed with a sporadic distribution, but outbreaks have been observed on two farms in Kentucky (Fenger et al., 1997, b) and one in Ohio.
There may be some breed predilection for EPM. Standardbreds and Thoroughbreds appear to be over represented in necropsy studies. There has only been one pony identified in the literature with EPM (Dubey and Miller, 1986), which suggests that ponies are resistant to EPM. Epidemiology studies that evaluate which breeds or groups of horses are most susceptible should be carefully read and evaluated by the discriminating reader. Many of these studies fail to address institutional or study bias, or inherent sources of error in the studies. For example, most studies have agreed that ponies are unlikely to die of EPM, but all of the horses in the EPM experimental infection trial were ponies. Institutional bias could mean that the University reporting the study never had ponies brought in for post-mortem examinations.
The seasonal incidence of death because of EPM may be slightly higher in the late spring and summer months in Kentucky, but wider epidemiologic study reveals no specific seasonal incidence. The reason for a seasonal incidence in Kentucky is not clear. The disease is slowly progressive, and may require months to years to progress to death. It is most likely that horses euthanatized because of EPM are more likely to be closely monitored when the weather becomes temperate. Therefore, the seasonal incidence would reflect how heavily the horses are monitored, rather than the true incidence of the disease. Young horses may be more susceptible to EPM, or at least more likely to be euthanatized because of severe clinical signs from EPM.