Risk of postnatal exposure to Sarcocystis neurona and Neospora hughesi in horses

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 titres 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 mean 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.