My horse was diagnosed with EPM last year. We tried Baycox for one month–no change. Since then, he has been on a daily dosage of sulfadiazine/pyrimethamine combination (SDZ/PYR) for approximately nine months. Would it be advisable to give him Marquis and SDZ/PYR together? One veterinarian says yes, another no. Also, isn’t staying on the SDZ/PYR indefinitely compromising his gastrointestinal health? The horse has had two spinal taps and his numbers went from about 25 down to 10. Will his numbers ever reach “0?” He presently shows no symptoms. Sorry, I know I’ve asked quite a few questions here.
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It’s hard to make a one-size-fits-all comment on dose duration for SDZ/PYR, as some veterinarians base recommendations on duration and severity of disease as well as other treatment history and response.

It is safe to say that most veterinarians recommend a minimum of 60 to 90 days on SDZ/PYR, and many horses are treated for up to six months or longer (nine months in your case). It is recommended that the SDZ/PYR combination be given on an empty stomach (most veterinarians recommend giving no food for two hours before through two hours after treatment). The food interferes with the absorption and availability of the drugs