Equine Protozoal Myeloencephalitis: Treatment, Prognosis, and Prevention

Treatment for equine protozoal myeloencephalitis (EPM) was introduced in 1974 based on protocols used for treatment of toxoplasmic encephalitis of humans. The original regimen of a sulfonamide (sometimes with trimethoprim) plus pyrimethamine has continued, with minor alterations, to this day. Since 2000, the folate inhibitors sulfadiazine/pyrimethamine, the triazinetrione ponazuril, and the nitrothiazole nitazoxanide have all been licensed by the FDA for the treatment of EPM. A triazinedione drug, diclazuril, is pending approval. Dosage regimens for anti-EPM drugs have been determined by considerations of cost, convenience, in vitro potency against Sarcocystis neurona, pharmacokinetic/pharmacodynamic considerations, safety, and the results of clinical efficacy studies. Because of differences in techniques and experimental design among studies performed for each of these drugs, objective comparison of different treatments is not yet possible. On the basis of published results of clinical efficacy studies, it is reasonable to expect that about 60% of horses with moderate to severe EPM will improve after treatment with any of the approved medications, with 10% to 20% recovering completely.