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 (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. However, Chagas and EPM are caused by different protozoa and require different treatment. Unlike Chagas, which affects humans and some other mammals, EPM only affects horses.

How EPM is Transmitted and What It Does to a Horse

The vector or carrier of the protozoa for EPM is a number of types of wildlife and all opossums. It was originally thought that only opossums could transmit EPM, however, it has been proven that other animals can carry it as well. To complete its life cycle, the parasite needs two hosts: a definitive and an intermediate. While the intermediate host can be a variety of animal species like raccoons, cats, skunks, and others, the definitive host is always an opossum and it passes the disease through its feces. 

The feces of opossums contain sporocysts. These sporocysts are cysts which contain spores that can reproduce asexually. Horses can ingest these sporocysts when they are mixed in with feed, grass, or water contaminated with the animal feces. Once the protozoa are transferred to the horse it 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.

The Signs and 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 the symptoms. Some of the signs and symptoms to look for are:

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

How EPM Impacts a Horse

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.

Diagnosing EPM

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.

Standard Treatment Protocols for EPM

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, however, some permanent damage may exist. 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.

Preventing EPM

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 (road kill), 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 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. Horses with EPM cannot pass it onto other horses.