A majority of the information known is gathered from fnaturally occurring infections. Regional epidemiological studies of EPM have yielded dissimilar results. Although regional differences probably occur, the most reliable general information for North America was provided by an EPM workshop in 1988. It was based on 364 histologically confirmed cases from 10 different North American centers. In descending order of frequency, Thoroughbreds, Standardbreds, and Quarter Horses, were most often affected, although many other breeds and ponies were represented. Usually only one animal in a herd or location is affected, although recently there have been 3 reports of farms with higher incidence of EPM from Ohio and Kentucky. The age of affected horses ranged from two months to nineteen years but over 60% were four years old or less. No geographic or seasonal predilection could be established.

Preliminary seroprevalance data have been collected using immunoblot testing to detect antibodies to S. neurona-specific proteins in equine serum samples. Initial testing in Kentucky and Ohio detected an average exposure rate above 20% among clinically normal horses. Recent investigations conducted at the University of Kentucky laboratory on serum samples collected from horses in Ohio, Kentucky and Pennsylvania have demonstrated an exposure rate above 40% among clinically normal horses. Findings from our study regarding Ohio horses indicated an increase in seroprevalence with age and a decrease in seroprevalence with an increase in number of freezing days. Exposure rates on individual farms ranged from approximately 0% to 100%. In 1978, Cornell University reported that 25% of all their equine neurologic disease accessions were due to EPM. The number of cases diagnosed at the University of Kentucky, Lexington, KY, has increased from about 8% of all neurologic accessions to 15% over the last few years.

A recent report from The Ohio State University Veterinary Medical Center revealed a 25% incidence of EPM in all horses presented for spinal ataxia in 1992, increasing to 52.9% in 1995 based on Western Blot analysis of serum and CSF. It is difficult to determine if the increase is actual or due to heightened awareness caused by the availability of immunoblot testing of serum and cerebrospinal fluid (CSF). Horses that actually reach the post mortem room represent a small portion of the total number of horses affected annually. Samples processed for immunoblot testing at the Gluck Equine Research Center suggest that several thousand cases occur in the US each year.