Are there other diseases that mask as EPM?

Q:  My friend’s horse may have EPM as he tested positive on several of the markers for the protozoa that causes EPM. Are there other diseases that mask as EPM as it just seems harder to catch on the West coast?

A:   In general, equine protozoal myeloencephalitis (EPM) is difficult to diagnose because a positive test tells us that a horse has been exposed to the disease, but exposure does not guarantee that a horse will develop the disease and exhibit symptoms. Many horses mount a significant immune response to the protozoa and keep the disease at bay. EPM is considered a disease of opportunity; it waits in the body until conditions allow it to flourish. Often horses that have been stressed or horses that are older and have a compromised immune response develop the disease years after the original exposure.

Typically, veterinarians consider both test results and physical symptoms when diagnosing EPM. Other diseases do not “mask” EPM but rather EPM mimics other neurological diseases, which makes a definitive diagnosis more problematic. Overall, the incidence of EPM on the west coast is lower because of the smaller possum population, so in some cases what appears to be EPM is in fact some other neurological problem.