Equine Protozoal Myeloencephalitis (EPM) in Horses

Courtnee Morton, DVM
By Courtnee Morton, DVM on Sep. 20, 2022
Brown horse look out from stable window

In This Article


What is EPM in Horses?

Equine Protozoal Myeloencephalitis, or EPM, is one of the most common causes of equine neurologic disease in the United States. It is a non-contagious disease that causes neurological lameness and other potentially vague symptoms in horses. It is a complicated disease process that veterinarians still do not completely understand. EPM was first discovered in the 1970s and can affect a horse of any age or breed, although it’s most commonly seen in younger horses. It has not been observed in donkeys or mules.

Causes of EPM in Horses

Horses acquire EPM by consuming opossum feces that have infective sporocysts of the protozoa Sarcocystis neurona or Neospora hughesi. After ingestion, they are absorbed into the bloodstream, and then can cross the blood-brain-barrier into the central nervous system where they begin to wreak havoc. It is unclear whether it is the protozoa itself that causes the disease, or the immune system’s reaction to their presence that causes inflammation and secondary symptoms.

N. hughesi can also be transferred from mare to offspring; this may lead to abortion of the infected fetus or may allow for the birth of a normal foal that carries the organism. Toxoplasma gondii is another organism that may be involved in the disease process, but its role is unknown.

Symptoms of EPM in Horses

Symptoms of EPM can be quite subtle at first and the disease shares signs with many other conditions. It is estimated that over half of all horses in the United States have been exposed to EPM, but very few develop disease. Clinical signs may have a slow or sudden onset; they can also stabilize for some time before a more significant relapse. A horse may develop one, or many of the possible symptoms; however, the big three symptomatic “As” of EPM include:

  • Atrophy: loss of muscle. This may be seen in the rear, across the topline or neck, or even be visible in the facial muscles.

  • Ataxia: describes a “drunken” or uncoordinated gait due to decreased muscle control, usually in the hind end.

  • Asymmetry: muscle atrophy and ataxia/lameness in affected horses may present more significantly on one side of the body.

  • Other potential clinical signs include:

    • Head tilt

    • Loss of bodily awareness

    • Muscle atrophy

    • Difficulty eating/swallowing

    • Lethargy

    • Behavioral changes

    • Weakness or recumbency

How Veterinarians Diagnose EPM in Horses

Diagnosing EPM can be tricky because it is the “great pretender” in terms of symptoms. Unfortunately, the only time EPM can be diagnosed definitively is post-mortem (after death). There are a few tests for EPM that can be run if a horse starts to show clinical signs, but other diseases must also be ruled out.

Your vet will complete thorough physical, lameness, and neurological exams. If they suspect EPM may be causing your horse’s issues, they will discuss pulling blood or performing a spinal tap to collect fluid to submit for lab testing. Routine bloodwork may be examined for potential infectious causes and other neurologic diseases. The spinal tap can usually be performed in the field or clinic with minimal complications if the horse is calm and not severely ataxic/wobbly with mild sedation and restraint using ultrasound guidance. EPM-specific tests give the most accurate information when both a serum and fluid sample are compared. These tests include:

  • Western blot: looks for antibodies specific to S. neurona; because so many horses in the US have been exposed, this is not helpful with potential diagnosis except in areas where the organism is rare. However, it is good for ruling out disease and exposure.

  • ELISA: tests for antigens produced by some, but not all, S. neurona protozoa only; this limits its full diagnostic capabilities.

  • IFAT: allows for quantitative evaluation of antibodies, so it can give a rough probability of EPM as the cause of a horse’s clinical signs rather than simple exposure to the protozoa.

Treatment of EPM in Horses

There are currently three FDA-approved drugs for the treatment of EPM:

  • Ponazuril (Marquis®): a paste medication administered orally daily for 1-3 months. A loading dose is started on day one of treatment to help the ponazuril reach a therapeutic level in the system. This is currently the “gold standard” in the EPM treatment world, and usually costs roughly $1,000-$1,500 per month.

  • Diclazuril (Protazil®): an alfalfa-based top feed medication. This can be administered daily usually for 1-3 months for treatment, and typically costs $800-$1,000 per month. Protazil is also used as a form of prevention in some cases.

  • Sulfadiazine/pyrimethamine (ReBalance®): a liquid medication usually prescribed for 3-9 months at a cost of $250-$400 per month. Prolonged treatment can sometimes lead to anemia, so a change to alfalfa or green pasture may be warranted or supplementation such as RedCell may be required. This medication shouldn’t be given within an hour of feeding hay.

Duration of treatment is usually based on the improvement (or lack thereof) of clinical signs in the horse. There are some compounded versions of these medications available but can only be legally prescribed in certain situations. ponazuril and diclazuril may be absorbed better if given with vegetable oil.

An anti-inflammatory (Banamine® or phenylbutazone) or steroid (dexamethasone or prednisone) may be given for the first few days if your horse is exhibiting severe clinical signs to help prevent secondary inflammatory side effects at the beginning of treatment. Vitamin E. may also be recommended in conjunction with one of the above medications for its antioxidant and neuro-supportive properties.

Recovery and Management of EPM in Horses

With swift diagnosis and treatment, 60-80% of horses show significant signs of improvement; up to 25% recover completely from EPM. However, 10-20% of horses that show symptoms may have a relapse within the first three years after treatment. Left untreated, EPM can lead to permanent and significant cognitive and physical deficits. EPM is a progressive disease that can be fatal as it causes increased deterioration of the central nervous system.

Horses with compromised immune systems while battling EPM may be at risk for developing other infections. With significant weakness and loss of bodily awareness, your horse should be kept in a safe area with sound footing to help prevent any limb injury. During treatment, stress should be minimized for your horse; if they have anything more significant than a very mild ataxia, riding and exercise should be avoided both for your safety and the safety of your mount.

Preventing EPM in Horses

While preventing opossums from wandering onto your property is rarely practical or successful, there are some preventative measures you can implement to lessen the chances of your horse ingesting infected feces, including:

  • Sealing all food and supplement bins to keep opossums out and lessen potential attraction to the area

  • Avoid feeding horses on the ground

  • Offer fresh water: horses often prefer this over drinking out of a pond, and this is harder for opossums to contaminate

  • Dispose of any opossum carcasses far away from horse pastures

EPM in Horses FAQs

What are the first signs of EPM in horses?

The first signs of EPM may be singular, or very vague. They can range from gait abnormalities, difficulty eating, muscle loss, head tilt, or behavior changes.

Can horses get EPM from eating hay?

Horses can ingest infective sporocysts from opossum feces in grain, water, hay, or even in the pasture, so keeping potential food sources in critter-free areas is ideal.

Is EPM fatal?

EPM is a progressive disease in the horse; if left untreated, or if treatment is significantly delayed, EPM can be fatal with deterioration of the central nervous system due to increased inflammation.

Featured Image: iStock.com/mgstudyo

Courtnee Morton, DVM


Courtnee Morton, DVM


Dr. Courtnee Morton is a 2017 Ross University School of Veterinary Medicine graduate. Since graduation, she completed an equine internship...

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