Equine Protozoal
Myeloencephalitis (EPM)

Disease Overview

Equine Protozoal Myeloencephalitis (EPM) is a progressive disease of the central nervous system (brain and spinal cord) in horses. It is most commonly caused by the protozoal organism Sarcocystis neurona. Horses become infected when they ingest sporocysts in feed or water contaminated by opossum feces. Because the parasite can attack any part of the central nervous system, it can cause a variety of clinical signs, but signs of spinal cord involvement, such as weakness and loss of coordination, are most common. While EPM can be effectively treated with antiprotozoal drugs, it is challenging to diagnose and lasting damage to the central nervous system is possible.

Merck Animal Health Solutions

Parasite Life Cycle

  • The opossum (definitive host for S. neurona) sheds infective sporocysts in feces.
  • Horse ingests sporocyst, which matures to become a merozite.
  • Merozites migrate to central nervous system, damaging tissues and causing disease.

Clinical Signs

  • Asymmetric incoordination and weakness of limbs
  • Regional muscle atrophy
  • Abnormal gait
  • Lameness
  • Loss of sensation along the face, neck or body
  • Obtundation
  • Seizures
  • Facial paralysis
  • Head tilt
  • Difficulty swallowing or vocalizing
  • Other signs of cranial nerve dysfunction

Risk Factors

  • Presence of opossums in environment
  • Contaminated grain, water, pasture or hay
  • Stress associated with illness, transport or strenuous exercise
  • Young age (1-5 years old)
  • Eating off the ground
  • Compromised immune system


SM Reed et al. “Equine Protozoal Myeloencephalitis: An Updated Consensus Statement with a Focus on Parasite Biology, Diagnosis, Treatment and Prevention.” J Vet Intern Med 2016;30:491-502.

Rob MacKay, “Equine Protozoal Myeloencephalitis,” American Association of Equine Practitioners, 2020, https://aaep.org/horsehealth/equine-protozoal-myeloencephalitis.

Robert J. MacKay, “Equine Protozoal Myeloencephalitis,” Merck Veterinary Manual, December 2020, www.merckvetmanual.com/nervous-system/equine-protozoal-myeloencephalitis/equine-protozoal-myeloencephalitis.

Important Safety Information

Use of Protazil® (1.56% diclazuril) Antiprotozoal Pellets is contraindicated in horses with known hypersensitivity to diclazuril. Safe use in horses used for breeding purposes, during pregnancy, or in lactating mares has not been evaluated. The safety of Protazil® (1.56% diclazuril) Antiprotozoal Pellets with concomitant therapies in horses has not been evaluated. For use in horses only. Do not use in horses intended for human consumption. Not for human use. Keep out of reach of children.