Equine Protozoal Myeloencephalitis in Horses

Head horse close-up
EPM affects the neurological system of a horse causing balance and mobility problems. Picavet/Photographer's Choice RF/Getty Images

Equine Protozoal Myeloencephalitis is a devastating disease that can leave horses severely disabled and may result in death. Diagnosis is often difficult because the onset is very similar to other diseases. Late in 2015, a horse with a confirmed case of Chagas Disease was first thought to have EPM and was treated for the disease. But, Chagas and EPM are caused by different protozoa, and treatment must be specific to the problem.

Unlike Chagas however which affects humans and some other mammals, EPM only affects horses.


Equine Protozoal Myeloencephalitis is also known by the initials EPM.


The vector or carrier of the protozoa for EPM is the opossum. 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. And that is why it's wise to dissuade opossums from living in horse pastures or hay storage.


One of the difficulties with diagnosing EPM is that it can look like many other neurological diseases. Symptoms vary between horses so not all horses will have all the symptoms. Some symptoms may include:

  • loss of coordination
  • muscle atrophy
  • difficulty 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. Once the diagnosis has been confirmed the most effective course of treatment can begin.


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 (which 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, perhaps because opossums are looking for homes in and around stables as cooler weather approaches.


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.  Styles like diamond mesh wire link fence can make it difficult for opossums to climb, keeping them out of your pastures.


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. These will be examined for the protozoa. Treatment includes antiprotozoal, antimicrobial and anti-inflammatory drugs administered by your veterinarian. Treatment may be lengthy and expensive and can unsuccessful if the protozoa have left the spinal cord and brain stem badly damaged.