| Equine protozoal myeloencephalitis (EPM) is an important neurological disease of horses in North America. The study objectives were to compare the accuracy of a serum indirect fluorescent antibody test (IFAT) with the Western blot (WB) and a modified Western blot (mWB) and to evaluate the accuracy of the IFAT in serum and cerebrospinal fluid (CSF) of naturally-, experimentally-, and vaccinated horses for the diagnosis of EPM caused by S. neurona . Using the IFAT, the risk of transplacental transmission and the risk and age at first exposure to S. neurona and N. hughesi in a cohort of mares (n = 337) and foals (n = 484) in 4 California farms was assessed. Using receiver-operating-characteristic (ROC) analysis, the area under the curve (AUC) of the IFAT was greater than the AUCs of the WB and the mWB (p = 0.025 and p = 0.044, respectively). The AUCs for the IFAT were 0.97 (serum) and 0.99 (CSF). Sensitivity and specificity were 83.3% and 96.9% (serum, cut-off 80), and 100% and 99% (CSF, cut-off 5), respectively. Titer-specific likelihood ratios ranged from 0.03 to187.8 for titers between <10 and 640. There was no serologic or histologic evidence of transplacental transmission of either parasite. The risk of post-natal exposure to S. neurona and N. hughesi was 8.2% and 3.1% respectively, over the study period (2.5 yrs). There was a significant difference in the risk of exposure to S. neurona among farms (P = 0.005) but not in the risk of exposure to N. hughesi (P = 0.83). The median age at first exposure was 1.2 yr for S. neurona and 0.8 yr for N. hughesi . The annual incidence rate of EPM among mares and foals were 0.2% and 0%, respectively. In conclusion, the IFAT was reliable and accurate using serum and CSF, and has potential for use in the diagnosis of EPM caused by S. neurona. There was no detectable risk of transplacental transmission of S. neurona and N. hughesi, and the risk of exposure to either parasite was low between birth and 2.5 yrs of age. EPM was a rare clinical disease of mares and foals. |
