MangoRoX87,
I agree with what Cindy is saying, but there is a little more to the FDA drugs. Diclazuril has been approved by the FDA for two years as 'Protazil.' This drug is not yet on the market, but you can get diclazuril compounded for about $150 per month. It has gone through the same clinical trials as Marquis and Navagator.
As for Karbo....any substance used to treat EPM must do two things. first, it must pass the blood brain barrier to treat the central nervous system (CNS). Second, it must be able to enter individual cells to kill the protozoa that are within other cells in the CNS. If the treatment that you are using does not do these two things, it won't kill the protozoa.
Karbo is not an FDA approved drug. It has no formal clinical studies to prove it's effectiveness. If your horse has a nutritional deficiency, the Karbo may improve the deficiency just enough to make it look like there is improvement. BUT, it won't kill the protozoa. Do not rely on Karbo to treat for EPM.
Please, have your vet consult with a teaching hospital. Your horses health is at stake. There is another site where you can find more information on FDA drugs and protocols for free:
www.EPMhorse.org
Go to the treatment page. The FDA 'Freedom of Information' sheets for each of the four FDA approved drugs are at the bottom. It talks about costs, side effects, and efficacy of each drug. Several clinical studies are available for free on different pages.
And, like Cindy, I would highly suggest an IFAT test through UC Davis. This test will give you better information about the chances of your horse having EPM.
EPMhorse