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Learn More About Equine Protozoal Myeloencephalitis

Equine protozoal myeloencephalitis (EPM) is caused by a protozoan parasite, Sarcocystis neurona. The horse ingests infective sporocysts from water or feed (hay, pasture or grain) that has been contaminated by fecal material from an infected animal. Opossums are considered the major source, but other animals including skunks, raccoons, rodents, etc., are also being investigated.


The symptoms seen in the horse occur when the organism migrates to the spinal cord, brain stem or brain and causes neurological problems. Symptoms include cranial nerve damage which may cause a drooped lip, ear or tongue; difficulty swallowing and chewing; or blindness. Muscle atrophy, frequently on one side only, may be observed. Loss of coordination and/or asymmetric gaits can also be symptoms. Horses may lose the ability to sense where their limbs are positioned. The symptoms will depend on where the lesions are located in the central nervous system.


There is currently no test which can be performed on water or feed (hay, pasture, grain) to determine if or which feed is a source of the organism. There may be a delay of several weeks or months between the horse being infected and the onset of symptoms. Horses may be exposed to the organism and show antibodies in blood serum, but never show symptoms. A definitive diagnosis by a veterinarian may require a spinal tap to provide appropriate fluid for an antibody test. New tests are currently under development.


  • Keep feeders clean and store all feed and hay properly, away from fecal material from birds and animals.
  • Vitamin E may be beneficial for horses at risk of EPM. A total of 400 to 600 IU per day of vitamin E is a minimum recommendation. Horses under stress should be receiving 1,000 IU per day. At the direction of a veterinarian, higher doses of vitamin E in the range of 6,000 to 10,000 IU per day have been administered.


Accurate diagnosis is difficult and may be confused with other problems. Treatment is uncertain as well as expensive. There is no vaccine or medically accepted preventive measure.