Equine Protozoal Myeloencephalitis

Equine Protozoal Myeloencephalitis (EPM) is an inflammatory disorder of the brain and spinal cord caused by the protozoan parasite Sarcocystis neurona and Sarcocystis rileyi.

Equine protozoal myeloencephalitis (EPM) is one of the most common neurologic diseases of horses in the United States. It is caused by either of 2 parasites, Sarcocystis neurona and Neospora hughesi. The parasite is ingested by the horse through infected food or water, or through direct contact with sporocytes in infected animal feces. Opossums, striped skunks, raccoons, armadillos and cats are intermediate hosts that spread the spores, but horses do not transmit the infection to other animals. The parasite damages the brain, brainstem and spinal cord of the horse. EPM can affect a horse of any age, breed, or sex.

The complete life cycle of N. hughesi is unknown, so all mode(s) of transmission of this parasite to horses remain poorly understood. Canids are a definitive host for the related species Neospora caninum, but it has not been established that dogs or wild canids are a definitive host for N. hughesi. Vertical transmission of N. caninum is very efficient in cattle, and several recent studies indicate that N. hughesi can be transmitted transplacentally in horses.1

Only a small percentage of horses become sick after ingesting the parasite. Signs include dragging a toe, incoordination, dropped eyelid, malposition of a limb, muscle atrophy, wobbling, head tilt, and occasional lameness. All neurologic disease in horses is not EPM and a complete work-up by your veterinarian is needed in many cases to arrive at a specific diagnosis of the problem.

Blood tests detect antibodies to the Sarcocystis neurona but do not indicate if the horse will develop the disease. If antibodies are found in the cerebral spinal fluid along with neurolical signs are positive indicators of the disease.

Horses are treated for EPM until they have negative test results for antibodies to S. neurona. Many horse develop complications and adverse reactions to medications: fever, loss of appetite, depression, incoordination, mild anemia and abortions.

Young horse

The disease is progressively debilitating to the horse and requires extensive treatment. A vaccine for EPM is available but has unknown efficacy. Response to treatment is an important indicator of survival. Treatment with ponazuril minimizes, but does not eliminate, infection and clinical signs of EPM in horses.1-2 Use good hygiene when it comes to storing a horse's food containers, water buckets and tubs.


  1. Equine Protozoal Myeloencephalitis: An Updated Consensus Statement with a Focus on Parasite Biology, Diagnosis, Treatment, and Prevention S.M. Reed,corresponding author 1 , 2 M. Furr, 3 D.K. Howe, 2 A.L. Johnson, 4 R.J. MacKay, 5 J.K. Morrow, 6 N. Pusterla, 7 and S. Witonsky 8 J Vet Intern Medv.30(2); Mar-Apr 2016PMC4913613

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