Objective - To estimate risk of exposure and age at first exposure to Sarcocystis neurona and Neospora hughesi and time to maternal antibody decay in foals. Animals - 484 Thoroughbred and Warmblood foals from 4 farms in California. Procedure - Serum was collected before and after colostrum ingestion and at 3-month intervals thereafter. Samples were tested by use of the indirect fluorescent antibody test; cutoff titers were ≥ 40 and ≥ 160 for S neurona and N hughesi, respectively. Results - Risk of exposure to S neurona and N hughesi during the study were 8.2% and 3.1%, respectively. Annual rate of exposure was 3.1% for S neurona and 1.7% for N hughesi. There was a significant difference in the risk of exposure to S neurona among farms but not in the risk of exposure to N hughesi. Median age at first exposure was 1.2 years for S neurona and 0.8 years for N hughesi. Highest prevalence of antibodies against S neurona and N hughesi was 6% and 2.1%, respectively, at a aman age of 1.7 and 1.4 years, respectively. Median time to maternal antibody decay was 96 days for S neurona and 91 days for N hughesi. There were no clinical cases of equine protozoal myeloenchaphlitis (EPM). Conclusions and Clinical Relevance - Exposure to S neurona and N hughesi was low in foals between birth and 2.5 years of age. Maternally acquired antibodies may cause false-positive results for 3 or 4 months after birth, and EPM was a rare clinical disease in horses ≤ 2.5 years of age.
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