Probably the most over-diagnosed equine disease is EPM, or equine protozoal myeloencephalitis. Dr. Amy Johnson indicates in the horse.com that the reason is that there are multiple diseases that can resemble EPM including Wobbler syndrome, equine herpes virus, West Nile virus, eastern and western encephalitis, trauma, and even rabies. Symptoms include nervous system disease and maybe an unusual lameness. EPM is difficult to diagnose because although there are multiple blood tests for the disease, none of them are 100 percent and there are a lot of false positives. There is also testing of cerebrospinal fluid, and the most accurate test compares blood results to spinal fluid results. However, there can also be false positives with spinal fluid testing, and which is more expensive and not without some risk. Because of this, many veterinarians perform a neurological exam and if they rule out all other neurological diseases, they assume it is EPM and treat. Then there are vets that do not do any testing and just start treating for EPM without attempting to rule out other diseases. I understand why this is done, but the problem is that EPM treatment is expensive at around $800 for the first month and some vets recommend treating for two months. This is really expensive if the horse does not even have EPM and has another disease as that money is wasted. Secondly, if you spend one to two months treating for EPM and another disease is involved, you have allowed the real disease to proceed for two months while treating incorrectly.
As far as prevention, try to keep opossums out of your barn as they are the definitive host of the infectious organism. Keep your horse’s stress level as low as possible as stress can decrease immunity and allow EPM to develop.