Equine toxoplasmosis: a survey for antibodies to Toxoplasma gondii in horses.

: A nationwide sample of horses was tested to determine the prevalence of toxoplasma antibodies in equine animals. Of 1,294 serum samples from horses tested by the microtitration indirect hemagglutin test, 20% were positive. Among the popular breeds of horses, the following antibody prevalences were found: Arabian, 19%; Paint, 22%; Quarter Horses, 13%; Thoroughbred, 24%; and Standardbred, 17%. The antibody prevalence increased from 2% in horses 1 year of age to 18% in 2-year-olds, and rose to 38% in horses 12 years of age. Significant differences were not evident by broad geographic locations across the United States. However, there were differences in prevalence according to premises where the horses were kept. On some ranches, there was no evidence of infection, whereas up to 67% of the horses on other premises were seropositive for toxoplasmosis.