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Equine Protozoal Myeloencephalitis in Horses


Head horse close-up Picavet/Photographer's Choice RF/Getty Images


Equine Protozoal Myeloencephalitis is also known by the initials EPM.


The feces of opossums may contain sporocysts - cysts that contain spores that can reproduce asexually. Horses can ingest these sporocysts with feed, grass or water contaminated with opossum feces. The protozoa can leave lesions on the spinal cord and brain stem. It is this neurological damage that can cause the various symptoms of EPM.


One of the difficulties with diagnosing EPM is that it can look like many other neurological diseases. Symptoms vary between horses. Some symptoms may include:
  • loss of coordination
  • muscle atrophy
  • difficultly swallowing
  • sore back
  • stumbling
  • roaring
  • locking of the stifle joint
  • weakness
  • drooping eyelid
  • head tilt

Careful examination, blood or spinal fluid tests must be done to rule out diseases like West Nile Virus, rabies or viral encephalitis.


If a horse is mildly affected you may only notice stumbling or slight lameness. If left untreated the horse may be unable to stand or swallow (can be confused with Wobblers Syndrome) and death can occur. Horses of any age, sex or breed can develop EPM. Younger horses and horses who are transported frequently seem to be at greater risk. Risk is thought to be greater in the autumn months than at other times of the year.


Opossums carry the organism that causes this disease so it is important to make your stable area unattractive to these animals. Opossums will eat almost anything including dead animals (road kill), dog and cat food or horse feed. It's important that all food stores be secure and any animal carcasses buried promptly. Clean up any spilled feed promptly. If opossums live on your property they should be humanely trapped and removed. Fencing has been designed to prevent entrance of these animals and should be considered if opossums are a nuisance.


Because EPM can look like many other neurological disorders a thorough veterinary examination is necessary. With quick diagnoses and proper medication most horses will recover from EPM, however some permanent damage may exist. Your vet will examine gait and movement, will take blood and spinal fluid samples. Treatment includes anti-protozoal, anti-microbial and anti-inflammatory drugs administered by your veterinarian. Treatment may be lengthy and expensive and often unsuccessful.

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