EPM Vaccine and the Western Blot Test

I am very excited to hear that there is a vaccine for EPM (equine protozoal myeloencephalitis)! Three and a half years ago, I had to put down an outstanding colt who was only 14 months old. He suffered from both CVM (cervical vertebral myelopathy) and EPM. The vet did not seem to feel that the EPM was severe and probably was a new infection. Since the colt came from New Mexico in the desert area at six months of age, it is not likely that he contracted it there, but more likely that he contracted it in his new home in Southern California.

To my question: if I vaccinate my horse against EPM, does it negate our most widely used test, the Western blot? Will a horse test positive for EPM if he is vaccinated assuming he was negative prior to vaccination?


At this point, the Western blot test cannot distinguish between natural exposure to Sarcocystis neurona--the parasite that causes EPM--and immunization with the new EPM vaccine. Consequently, horses which are given the vaccine will test positive, and this appears to include testing of the cerebrospinal fluid (CSF).

This has been a major concern to both researchers and equine veterinarians, since it will negate the negative predictive value of the test. In other words, the test will not be useful for ruling out EPM in horses which have been vaccinated. However, if we assume that the new vaccine provides at least some protection against EPM (difficult to determine at this point), the benefits of vaccinating probably far outweigh the value of getting an accurate result from the Western blot test. You should discuss your situation with your veterinarian.

About the Author

Daniel K. Howe, PhD

Daniel K. Howe, PhD, is an Associate Professor at the Gluck Equine Research Center, University of Kentucky.

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