Equine protozoal myeloencephalitis (EPM) is a central nervous system disease of horses in the Americas, and is not commonly seen in other parts of the world. Horses with EPM most commonly present with gait abnormalities, but can also may present with signs of brain disease.¹ The disease ranges in severity from mild lameness to abrupt recumbency, and the clinical signs are usually progressive. The causative agent, Sarcocystis neurona, has been isolated from affected horses since the 1970s, and research suggests that approximately 50% of horses in the United States have been exposed. In the late 1990s, the protozoan Neospora hughesi was also shown to also cause EPM in horses but, compared to S. neurona, it is a relatively rare cause of this disease.²

Pyrimethamine + Levamisole for EPM in Horses

Pyrimethamine is an antiprotozoal that is used in veterinary medicine to treat Hepatozoon americanum infections, and toxoplasmosis (Toxoplasma gondii) in small animals, often in combination with sulfonamides. In horses, it is used to treat EPM (sometimes called equine toxoplasmosis).²

Pyrimethamine administered to adult horses at 1 mg/kg IV or PO for 1 dose, and 1 mg/kg PO once daily for 10 days resulted in oral bioavailability ≈55%. Food, including alfalfa hay, may reduce pyrimethamine absorption by up to 50%. Steady-state plasma concentrations were reached within 5 days, with CSF levels 25%-50% of plasma values.^4,5

Levamisole is an antiparasitic and immune stimulant that is typically used to eliminate nematodes in livestock such as pigs, cattle and sheep. It can be found in a variety of deworming medications and has been used in horses off-label as an adjunctive medication in the treatment of equine protozoal myeloencephalitis.

Where to buy Pyrimethamine + Levamisole 

Pyrimethamine + Levamisole is available in the U.S. through several pharmaceutical manufacturers and through veterinary custom compounding companies. 

This product carries several potential drug interactions. Please consult your veterinarian prior to beginning any treatment regimen.

FOR RX ONLY: A valid prescription from a licensed veterinarian is required for dispensing this medication.

¹MacKay, R. Equine Protozoal Myeloencephalitis. In: J. Veterinary Clinics of North America: Equine Practice, Volume 13, Issue 1, April 1997, P. 79-96.

²aaep.org.

⁴Clarke C.R., et. al. Pharmacokinetics of intravenously and orally-administered pyrimethamine in horses. Am J Vet Res. 1992;53(12):2292-2295.

⁵Clarke C.R., et. al. Pharmacokinetics, penetration into cerebrospinal-fluid, and hematologic effects after multiple oral administrations of pyrimethamine to horses. Am J Vet Res. 1992;53(12):2296-2299.