Diagnosis of equine protozoal myeloencephalitis (EPM) involves a technique called Western blotting (WB), which identifies antibodies against Sarcocystis neurona, the parasite responsible for the disease. A positive result on WB does not guarantee current infection, but rather exposure, since antibodies and not organisms are being identified.

Currently, young foals with neurological disease are regularly tested for EPM using WB. There is concern that some foals might test false positive because mares with S. neurona antibodies in their blood might transfer them to the foals through colostrum. For this reason, researchers from the Virginia-Maryland Regional College of Veterinary Medicine designed a study to determine whether S. neurona antibodies can be detected in healthy foals which nursed colostrum from EPM antibody-positive mares.

Fifteen foals two to seven days old had serum and cerebrospinal fluid (CSF) collected for WB analysis for S. neurona antibodies. Each mare was serum tested as well. Thirteen of the mares and their foals were found to be antibody-positive in the serum samples. Twelve of the 13 foals were also positive in their CSF, which is the fluid of choice for EPM testing. Five foals were re-tested between 13-41 and 62-90 days of age. S. neurona antibodies decreased over time, but three of these five foals were still antibody-positive in the CSF at two to three months of age. The authors concluded that S. neurona antibodies can be detected in the cerebrospinal fluid of healthy foals born to antibody-positive mares, and therefore WB testing in foals less than three months old is not reliable and not recommended.

Cook, A.G.; Maxwell, V.B.; Donaldson, L.L.; et al. Journal of the American Veterinary Medical Association, 220 (2), 208-211, 2002