Seroprevalence of antibodies to Sarcocystis neurona in horses residing in northern Colorado

Objective: To determine the seroprevalence of antibodies to Sarcocystis neurona in hor ses residing in northern Colorado during 1995 and 1996. Design: Prevalence survey. Sample Population: Aliquots of serum were collected from 608 equids from samples submitted to the Veterinary Diagnostic Laboratory at Colorado State University for testi ng for antibodies to equine infectious anemia (EIA). Procedure: Sera were analyzed for the presence of antibodies to S. neurona using Weste rn blot analyses. Information regarding age, gender, breed, county of origi n and quarter of the year when the sample was collected was recorded for ea ch animal from the EIA form. Data were analyzed using chi-square analysis a nd multiple logistic regression, Results: Seroprevalence was 33.6%. Gender and county of origin were not associated w ith seroprevalence. Variables that were associated with seropositivity incl uded age, breed, and quarter of the year. Seroprevalence increased with age . The highest seroprevalence, 66.6%, was found in the group that represente d ponies and non-horse equids. Stock breeds had a seroprevalence of 32.6% a nd hot-blooded breeds had a seroprevalence of 27.9%. Seroprevalence was low est during the coldest months (20.1%). Clinical Implication: Data from this sample population indicate that exposure of horses to S. neu rona in northern Colorado is less than that reported for eastern regions of the United States, The overall seroprevalence is similar to that reported in horses from eastern Oregon. Although additional data are necessary, this finding suggests that areas of lower seroprevalence in the Rocky Mountain states corresponds to areas of lower opossum density. The results of this s urvey further support the conclusion that although a negative serum antibod y test for S. neurona in a horse with neurologic signs may help rule out eq uine protozoal myeloencephalitis (EPM) as a diagnosis, a positive serum tes t result alone, especially in a clinically normal horse, does not lead to a definitive diagnosis of EPM.