We went through the EPM ordeal just a year ago. I did not have him tested though. Had heard it is very expensive and can't tell if they have EPM or just exposed to it as many horses are exposed to it. I don't know for sure on those ideas if they are factual. But my vet recommended trying the Marquis. If in two weeks we saw an improvement in the symptoms we were seeing, then we would continue. And they reevaluated at the end of each month whether to continue the expensive drug for another month or not. We ended up seeing great improvement at 2 weeks so continued with the Marquis. Each month we still wanted to see more improvement and ended up continuing the Marquis for 3 months. We have a little muscle atrophy in the right hind and some weakness there too that seems permanent. But a year later he seems to still be doing well. There are a bunch of products out there with claims to treat or prevent. No proof if they work. I am giving him Vit E & folic acid supplements for the immune system.
If you have tangible symptoms that you can monitor, I would suggest giving the Marquis a month trial and see. I totally get though that it is very expensive. But with the C's horse you already have to deal with muscle wasting and if the EPM is contributing more, you have your hands full.
2005 (although I do not currently have an IR or PPID horse, I stay in touch and continue to learn from this wonderful group)