While cerebrospinal fluid (CSF) testing for equine protozoal myloencephalitis (EPM) is widely regarded as the only definitive way to diagnose the disease, the difficult, expensive, and potentially dangerous test might not be necessary, according to new research from Colorado State University.

This study compared different testing schemes for EPM including assessment of the value of indirect fluorescent antibody testing (IFAT) of the combination of blood serum and CSF tests in relation to that of blood serum testing alone. In a previous study, using naturally exposed, experimentally infected, and vaccinated horses the researchers demonstrated that the IFAT has good accuracy in serum, and CSF correctly determined the infectious status of the horse more than 80% of the time.

The difficulty of diagnosing EPM is that a horse exposed to Sarcocystis neurona (the parasite that most frequently causes EPM) will retain antibodies whether the horse is actually displaying symptoms of EPM or not. The western blot test, which is currently the most accessible and affordable test for owners, can only return a positive or negative response. This has resulted in a number of false positives.

Indirect fluorescent antibody testing (IFAT) of either CSF or blood serum offers a higher specificity than the western blot. These tests can show not only the presence of antibodies to S neurona, but also quantify the concentration of antibodies, giving a far more specific result.

"Because we can quantify the antibodies in serum with the IFAT, and because when there is such a high concentration of antibodies in serum, it is likely that the horse will have antibodies in CSF, even if the parasite is not in their brain or spinal cord, there will be less need to use CSF to make a final diagnosis." says Paulo C. Duarte, DVM, MPVM, PhD, assistant professor at Colorado State."Hence, we can potentially reduce the risk associated with CSF collection and the costs for horse owners."
 

The researchers suggest that only horses that are already undergoing a spinal tap for other diagnostic procedures should be CSF tested for EPM. For those not requiring CSF testing for other issues, a blood serum IFAT might reveal as much information without the associated costs and risks.

Researchers for this study, published in the May 5 edition of the American Journal of Veterinary Research, included Duarte, Eric D. Ebel, DVM, MS; Josie Traub-Dargatz, DVM, MS; W. David Wilson, BVMS, MS; Patricia A. Conrad, DVM, PhD; Ian A. Gardner, BVSc, MPVM, PhD.