here's another link ...
click
and here's one by H&R about the prevention, or at least reducing exposure ...
click
A few years back, a horse at the boarding barn I'm at had EPM. It is treatable, but not curable to the best of my knowledge. It is sad watching it progress into the later stages, the horse I knew with it had trouble walking for months before they lost him. But he did live to be 33 years old, and since his owner managed it best she could he was a happy pony even in the last days of his life. The best thing you can do is recognize it early on and start treatment.