My first pass treatment is Rebalance, which I obtained from Abba Vet Supply. Rebalance is an Antiprotozoal Oral Suspension for the treatment of EPM. Each ml of ReBalance Antiprotozoal Oral Suspension contains 250 mg sulfadiazine and 12.5 mg pyrimethamine. Give 4 ml per 110 lbs. body weight orally once per day. The usual treatment regimen ranges from 90 to 270 days. Sulfadiazine and Pyrimethamine.
They have been wonderful to work with and currently the product is on sale. As of this writing, if you buy 3 bottles ($75) you get one free. A bottle lasts 20-30ish days if you're treating a standard horse (dosage is by weight). Harv is 1100 lbs and a bottle lasts him about 23 days.
A folic acid and vitamin e supplementation is also recommended -- about $20/month. I'm finding that Harv is not a good horse to dose with oral suspension (viscous liquid, not a paste) -- he holds it in for 10 minutes and spits it out when I'm not looking -- so I may switch to Oroquin, a new treatment that is less proven but great anecdotal success. It is a ten day paste treatment with 3 months followup powder supp.
Abba Vet has been super responsive and nice -- they ship fast too. I use them for most of my meds.
If Harv continues to "resist" the oral dosage (he is not easy about oral meds if he can't eat them with grain), I will switch to Oroquin, a product that is "less proven" but about the same cost and a much easier treatment regimen. I don't know that much about it so I won't risk a mis-statement.