From what I understand basically any horse can test positive whether they actually HAVE EPM or if they're just tested positive. Most test come back positive, and that's why they normally tell you not to worry about it unless the horse actually shows symptoms. Many times, the only accurate way to really tell if they have EPM would be the spinal fluid test. I'm pretty sure that's the most accurate way.
I wouldn't be too worried if you're vet isn't, as many horses as I've seen with positive test (Which basically don't have it.) and with as many horses.. which is just a few that I've seen whom actually HAVE EPM.. you'd most definitely see some sort of symptoms, I'd say, if your horse actually did have EPM.
Ya that was my understanding that most horses test positive on the blood test. Nope not a single symptom. I've had horses and known horses over the years that had mid and even low grade epm but they all showed symptoms. Have heard that the blood test almost always shows positive regardless if the horse actually has it
I also heard from someone talking about it at the barn that some states (I think it was Missouri) always test positive. It was something the person researched and the site said basically Missouri has a TON of Opossums in it.. so basically EVERY horse tested will come up positive just for being exposed. You should do some research online about the area you're in and see if it's a Opossum filled.. place, I guess? Not sure, just a suggestion.