There had been limited data collected on the prevalence of S. neurona-specific antibodies in horses. The purpose of our first study was to determine the prevalence of serum antibodies to S. neurona in horses in Ohio.
Although data have been published on the prevalence of serum antibodies to Sarcocystis neurona-specific antigens in horses, some reports indicated that the seroprevalence was approximately 20%, based on horse farms in Ohio and central Kentucky. A seroprevalence rate of greater than 50% was reported for greater than 1,000 horses randomly selected from serum submitted for EIA testing in Ohio. This seroprevalence rate is considerably higher than that documented in previous studies, however, indications are that horses from a county in Pennsylvania revealed a seroprevalence rate of greater than 40%. Reports from the State of Oregon indicated seroprevalence rates between 20 and >60% from different regions of that state (Linda Blythe; Personal Communication).
Age was associated with seroprevalence of antibodies to S. neurona in horses in Ohio. The youngest seropositive horse in our study was 5 months of age. This is not surprising since several reports have indicated that foals as young as 2 months of age have had EPM. There was a linear relationship between age and seroprevalence indicating that as horses age, the risk for exposure increases. There was not an effect of gender on the seroprevalence of antibodies to S. neurona which is consistent with EPM.
Breed did not affect seroprevalence of antibodies to S. neurona, however, there was considerable variation between the breeds. The highest seroprevalence was in the Draft Breeds (>65%). There appeared to be an effect of location on seroprevalence of antibodies to S. neurona. Northeast Ohio had a seroprevalence rate of >45%, while the southwest area had a seroprevalence rate of >61%. While studies have not been performed to examine the effect of climate or soil conditions on S. neurona sporocysts, this has been examined with other Sarcocystis spp There appeared to be an effect of freezing days which correlated to different areas of the State of Ohio. Seroprevalence increases from the north group of counties to central to south. Although this would only affect sarcocysts in the intermediate host and scavenger transmission, this may explain our difference in seroprevalence from north to central to south Ohio.
In conclusion, the seroprevalence rate of antibodies to S. neurona in horses in Ohio was >50%. This was considerably higher than previously reported or anticipated. A high seroprevalence such as that found in our study suggests that the organism is ubiquitous in the environment and therefore, control of this disease is going to be very difficult. This further demonstrates the importance of examining cerebrospinal fluid for S. neurona-specific antibodies in attempting to diagnose EPM.