We recently had to put down a boarded horse that the owner had been given. We knew nothing of its history. It had a recent bout of colic he appeared to recover from then developed what the Vet called EPM which I was unfamiliar with. It was sad to watch as he appeared to walk drunkenly. After he stopped eating the owner had no good option.
I have since discussed this with my sister who is a VET and has put down a few horses recently with EPM. There are two things that are starting to be connected with this issue. EPM seems more prevalent in horses that originated in the deep southern states and now live up north. Also feeding older round bales that varments have inhabited.(feces being a host for Protozoa). This is what my sister is seeing and is in no wat scientifically verified. Does anyone else out there have thoughts on this subject.

Don't know if it is similar, but EPSM is thoroughly covered in the Vet Clinic of this web site by Dr Beth Valentine. Go to RuralHeritage.com and select the Vet Clinic.

I had an Arab gelding get EPM. I am originally from the south and we moved to NY and brought him with us. He had been healthy his whole life up to the time he came down with EPM. His symptoms started with him having a scratched eye (and was in a new boarding facility). A week after that he had symptoms of mild colic and the next day he was thrashing in his stall. He was emergency trailered to the boarding facilities emergency vets for colic surgery where he could not be stabilized and put down on the table. In consulting with my vet, in retrospect, he had started going blind causing the eye scratch, and then had a seizure in his stall which the emergency vet assumed was torsion colic. Only through blood work/spinal samples (if I remember correctly) did we figure out it was EPM. I hope this helps.

My Shire X survived a bout with it. You're correct - it comes from eating or drinking in a contaminated area. I was told by my vet that opposums are the vector species. Don)'to know it they're the only vermin that cause it.

He lost a frightening amount of weight & I almost had him put down. Fortunately, there is a new, very reasonably priced drug on the market for treating it. After he finished the meds, I moved him to a retirement field board farm that had acres of good grass for him. We supplemented the grass with about 19 pounds of concentrated feed a day (he was a large horse). The key was a vitamin E supplement from Kentucky Performance to help repair the damaged nerve material. I was able to taper him off the vitamin E with no lose
of nerve function. But I understand that some
EPM horses have to stay on it forever.

He went from being so sick that the vet could have
pulled him off his feet doing the nerve function test back to basically full health.
No one could believe that he'd had EPM. The new drug is a real silver bullet, and a fraction of the cost of whatever they used to use.

Forgot to mention , the only symptom he had to point the vet towards doing the blood test was that he choked several times. He'd never done that before. One vet thought it was just an A fib and left it at that. I didn't feel that would have caused everything else we were seeing and had another vet out. She tacked on the EPM test on a hunch.

Sorry for your loss. Sounds like you tried everything. EPM seems to manifest itself in so many different ways it's difficult to diagnose. Hopefully Cornell Univ. can come up with a vaccine. Tha southern connection may be meaningless but it is curious.