Animal Care

Equine Protozoal Myeloencephalitis Disease in Horses

Equine protozoal myeloencephalitis (EPM) 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, especially Chagas. Chagas and EPM are caused by different protozoa and require different treatments. Unlike Chagas, which affects humans and some other mammals, EPM only affects horses.

Equine Protozoal Myeloencephalitis

Equine protozoal myeloencephalitis is a disease that affects the brain and/or spinal cord of horses. It is caused by protozoan organisms. Horses are commonly exposed to protozoan organisms without a problem, but in rare instances, they can develop clinical diseases from them.

Symptoms of EPM

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 of these symptoms

Signs of EPM

  • Loss of coordination
  • Muscle atrophy
  • Difficulty swallowing
  • Sore back
  • Stumbling
  • Roaring
  • Locking of the stifle joint
  • Weakness
  • Drooping eyelid
  • Head tilt

If a horse is mildly affected you may only notice stumbling or slight lameness. If left untreated, the horse may eventually 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. The 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.

Causes of EPM

A few types of wild animals and all opossums can be the carrier of the protozoa for EPM. While other animals may carry the protozoa in their body, only opossums can transmit EPM. The feces of opossums contain sporocysts and that is how they transmit the disease. These sporocysts are cysts that contain spores of the protozoa and they reproduce asexually. If there is feed, grass, or water that has been contaminated with the opossum’s feces, the horses will ingest the sporocysts. Once the sporocysts are transferred to the horse, they can cause neurological damage, including lesions on the spinal cord and brain stem. It is this neurological damage that can cause the various symptoms of EPM. Horses with EPM cannot pass it onto other horses.

Diagnostic Process

To determine if a horse is suffering from EPM, it must be examined by an equine veterinarian. The veterinarian will perform blood or spinal fluid tests to rule out diseases like West Nile virus, rabies, or viral encephalitis, especially since EPM can look like many other neurological disorders. The veterinarian can confirm an EPM diagnosis based on antibody levels in the horse’s blood or fluid tests. Additionally, the equine veterinarian will examine the horse’s gait and movement to help confirm the diagnosis.


Once the diagnosis has been confirmed, the most effective course of treatment can begin. With quick diagnoses and proper medication, most horses will recover from EPM but some permanent damage may exist. The FDA-approved treatments for EPM are ponazuril for 28 days, diclazuril for 28 days, and a combination of sulfadiazine and pyrimethamine for at least 90 days. The treatment course may be lengthy and expensive and can be unsuccessful if the protozoa have left the spinal cord and brain stem badly damaged.

How to Prevent

Opossums carry the organism that causes EPM. It is important to make your stable area unattractive to these animals. Opossums will eat almost anything including dead animals (roadkill), dog and cat food, or horse feed. It’s essential 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 the entrance of these animals and should be considered if opossums are a nuisance. Styles like diamond mesh wire link fences can make it difficult for opossums to climb, keeping them out of your pastures.


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