Many thanks to EPM expert Dr. Dan Howe, Professor at the Gluck Equine Research Center, for his review of this announcement. This announcement supersedes our original announcement that went out on January 26, 2017. Please accept our apologies for any confusion.
Equine protozoal myeloencephalitis (EPM) is a common neurologic disease of horses; it has been reported in most of the states of the USA, southern Canada, Mexico, and several countries in Central and South America. Most cases of EPM are caused by a protozoan parasite, Sarcocystis neurona.
Horses are infected by ingestion of S. neurona sporocysts in contaminated feed or water from an infected opossum.
Testing for S. neurona-specific antibodies is the basis for presumptive antemortem diagnosis of EPM. The UKVDL is now offering a test that was developed by faculty/staff at UC-Davis to detect antibodies against S. neurona, using the method of indirect fluorescent antibody test (IFAT) to aid in the diagnosis of EPM in samples from clinical horses. The test result will include a titer as well.
It has been estimated that over 50% of horses in some regions of the United States have been exposed to S. neurona. Therefore, test results should be interpreted based on the clinical signs and other laboratory test results. The most recent ACVIM consensus statement on EPM recommends testing paired serum and CSF to assess intrathecal antibody production. Measurement of antibodies in the CSF alone is informative, but can be confounded by passive transfer across the blood-brain barrier or blood contamination. Measurement of serum antibodies alone has minimal diagnostic value. There have been necropsy-confirmed EPM cases that were sero-negative, but with positive CSF titers. If a negative test on serum is interpreted as negative diagnosis for EPM, the clinician may not treat or may delay treatment leading to a poor outcome.
When IFAT is run on both serum and CSF, a serum/CSF ratio of antibody/titer of <64 is highly supportive of the diagnosis of EPM. An IFAT CSF titer of>:5 is considered supportive of the diagnosis of EPM.
Note: This ratio and titer cutoff is based on UC-Davis work and may be different when run at UKVDL.
This information was provided by Dr. Craig Carter, Director & Professor, Epidemiology, UDVDL.