Equine Protozoal Myeloencephalitis (EPM) in Horses

Causes, Treatment, and Prevention

Veterinarian with horse
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Equine protozoal myeloencephalitis (EPM) is a devastating neurological disease that can leave horses severely disabled and may result in death. It is caused by infection of the horse's central nervous system with a protozoan, which is a single-celled organism. The most common protozoan to cause EPM is Sarcocystis neurona. Less commonly, it can be caused by infection with Neospora hughesi.

Because EPM can affect any part of the horse's central nervous system, which includes the brain and the spinal cord, symptoms can vary, but common signs of this disorder include gait abnormalities, involuntary muscle movements, lethargy, muscle atrophy, and difficulty swallowing.

Any horse of any age can develop EPM, but it is more common in young horses that race or participate in other athletic activities.

What Is Equine Protozoal Myeloencephalitis?

Equine protozoal myeloencephalitis (EPM) is a neurological disease that affects the central nervous system of horses. The term "myeloencephalitis" means inflammation of the spinal cord and brain. The disease is caused by a protozoan, usually Sarcocystis neurona. This protozoan's primary host is the opossum. The feces of an infected opossum can contain sporocysts, which are the immature, egg-like form of the protozoan. If a horse ingests grass, water, or other substances containing the sporocysts, the horse may develop EPM.

For reasons that are not well understood, however, only a very tiny percentage of horses exposed to sporocysts from Sarcocystis neurona actually develop EPM. In fact, although as many as 90 percent of the horses in some areas of North America test positive for exposure to the sporocysts, less than 1 percent actually develop the disease.

Symptoms of EPM in Horses

One of the difficulties with diagnosing EPM is that it can look like many other neurological diseases, particularly in the beginning. It is a progressive disease, meaning it tends to worsen over time. However, some horses progress through the illness faster than others, and it is not uncommon for a horse to develop worsening symptoms, then level off for awhile, sometimes even for months, before once again beginning to deteriorate.

Symptoms can vary depending on whether the horse's brain or spinal cord is more heavily affected. But the following are common symptoms to watch for.


  • Loss of coordination
  • Muscle atrophy
  • Difficulty swallowing
  • Gait abnormalities
  • Stumbling
  • Behavioral changes
  • Standing with feet splayed or leaning against a wall for support
  • Weakness
  • Drooping eyelid
  • Head tilt

The symptoms of EPM can begin quite suddenly or slowly progress over time. As a general rule, the symptoms are asymmetrical, meaning they affect one side of the horse's body more than the other. So you might notice that your horse moves the legs on its left side in an uncoordinated fashion, but still moves them normally on the right. Commonly, the horse will seem to have poor balance and may stumble or move in an abnormal way. Its muscles might atrophy, leading to weakness and further inability to walk normally.

If the horse's spinal cord is most affected, which is the commonest presentation of EPM, you'll notice more difficulties with movement, balance, and coordination. If the horse's brain is more affected, symptoms can include paralysis in the facial muscles, making it difficult for the horse to swallow or move its tongue, as well as lethargy, behavioral changes, and drooping eyelid.

Causes of Equine Protozoal Myeloencephalitis

Some protozoa, including Sarcocystis neurona, are capable of causing disease in animals. Typically, these types of protozoa require a host animal in which to carry out the reproductive part of their lifecycle. For S. neurona, that host is the opossum. An opossum ingests the protozoans, which reproduce in the animal's digestive system. The opossum doesn't become ill itself, but does, however, pass infectious sporocysts, or eggs, in its feces.

When a horse ingests the sporocysts from the grass, water, or ground, they enter its digestive system. For reasons that are not well understood, in a very small percentage of horses the sporocysts mature into a form called merozoites, which then migrate through the horse's bloodstream into its central nervous system. There, they infect the brain and/or spinal cord, causing the disease called equine protozoal myeloencephalitis.

It is important to note that EPM is not contagious from horse to horse, nor is the horse contagious to other animals.

Diagnosing EPM in Horses

Diagnosing EPM can be tricky, as it can come on slowly and resemble other neurological diseases initially, such as viral encephalitis, West Nile disease, meningitis, or even rabies. A definitive diagnosis can only be obtained by examining the brain tissue of an infected horse after its death. However, equine veterinarians diagnose their EPM patients indirectly by observing the characteristic signs and symptoms, ruling out other causes of similar symptoms, such as trauma or other infectious diseases, and performing blood tests or tests on the horse's spinal fluid to look for antibodies to S. neurona, which indicate that the horse has been exposed to the organism.


Once the diagnosis has been confirmed, the most effective course of treatment can begin. There are currently three different FDA-approved treatments for EPM.

  1. Ponazuril (an anti-protozoal drug) by mouth for 28 days
  2. Diclazuril (also an anti-protozoal) by mouth for 28 days
  3. A combination of sulfadiazine (an antibiotic) and pyrimethamine (an anti-parasitic drug) by mouth for at least 90 days

Along with one of the above treatments, many equine vets also prescribe anti-inflammatories such as steroids, vitamin E supplementation, or immune-system-modifying drugs.

Prognosis for Horses With EPM

With treatment, up to 60 percent of horses with EPM will improve, but less than 25 percent recover completely. Relapses are also common for as long as two years after the initial treatment with anti-protozoal medications. Still, your horse has a good chance of regaining at least some of its former health with prompt intervention. If left untreated, the prognosis for horses with EPM is very poor, as the condition can deteriorate to seizures and death.

How to Prevent Equine Protozoal Myeloencephalitis

There is no vaccine for EPM. Prevention consists of minimizing the presence of opossums on your property as much as possible. Wire-mesh fencing can help keep these marsupials out of your stable area, although because opossums are good climbers, you may have to resort to electrified fencing if they are especially persistent.

Opossums are generally looking for food and are not picky eaters. They will raid trashcans, eat dog food that's left outside, consume your horse's grains, and even eat carcasses of dead animals that they find on the road or within your property. It's essential that all food stores be secure and any animal carcasses buried promptly. Clean up any spilled feed right away, and make sure trashcans have tightly fitted lids.

If opossums make their home on your property, they should be humanely trapped and removed. Other options are motion-activated lights or sprinklers, which can frighten the animals away.

If you suspect your pet is sick, call your vet immediately. For health-related questions, always consult your veterinarian, as they have examined your pet, know the pet's health history, and can make the best recommendations for your pet.
Article Sources
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  1. Equine Protozoal Myeloencephalitis (EPM). UC Davis School of Veterinary Medicine.

  2. Equine Protozoal Myeloencephalitis. American Association of Equine Practitioners.

  3. Overview Of Equine Protozoal Myeloencephalitis. Merck Veterinary Manual.