Equine Protozoal Myeloencephalitis, commonly known as EPM, remains one of the most challenging and misunderstood neurological diseases in the horse world. Often referred to as "the great mimicker," its ability to imitate dozens of other musculoskeletal and neurological issues makes it a source of significant concern for horse owners, trainers, and veterinarians alike. As of 2026, while our diagnostic tools and treatment protocols have become more refined, the disease continues to affect horses of all breeds and disciplines across the Americas.

At its core, EPM is a disease of the central nervous system (CNS). It occurs when specific protozoal parasites invade the brain, brainstem, or spinal cord, causing inflammation and tissue damage. Because these lesions can occur anywhere within the CNS, the clinical signs are incredibly variable. One horse might show a slight hitch in its gallop, while another may struggle to stand up or swallow.

The Biological Culprits: Where Does EPM Come From?

The vast majority of EPM cases (around 90% or more) are caused by the protozoan Sarcocystis neurona. A smaller percentage is attributed to Neospora hughesi. Understanding the life cycle of these parasites is the first step in understanding why horses get sick.

Sarcocystis neurona requires two types of hosts to complete its life cycle: a definitive host and an intermediate host. In North and South America, the definitive host is the opossum. The parasite reproduces sexually within the opossum's intestinal tract, resulting in the production of infectious sporocysts. These sporocysts are then excreted in the opossum's feces.

In the natural cycle, intermediate hosts—such as skunks, raccoons, armadillos, or cats—ingest these sporocysts. The parasite then forms cysts (sarcocysts) in the muscle tissue of these animals. When an opossum scavenges on the carcass of an infected intermediate host, the cycle continues.

Horses enter this picture as "aberrant" or dead-end hosts. A horse becomes infected by consuming feed or water contaminated with opossum feces containing the sporocysts. Unlike intermediate hosts, the horse does not typically develop muscle cysts; instead, the parasite migrates into the central nervous system. Crucially, EPM is not contagious between horses. You cannot "catch" EPM from a stablemate, and a horse cannot pass the parasite back to an opossum.

Recognizing the Symptoms: The Signature of Asymmetry

Because the parasite creates focal (localized) or multifocal lesions in the brain or spinal cord, the symptoms are rarely uniform. However, the hallmark of EPM is asymmetry. If a horse is wobbling or weak, and that weakness is significantly worse on the left side than the right, EPM moves to the top of the list of suspicions.

Clinical signs usually fall into three categories: coordination, strength, and muscle mass.

1. Ataxia and Gait Abnormalities

Ataxia, or the lack of voluntary coordination of muscle movements, is the most common sign. This often presents as stumbling, tripping, or a "floating" hind end. Owners might notice the horse hitting its own legs together (interfering) or having a strange, wide-based stance when standing still. In more subtle cases, the horse may simply seem "off" or show a decline in performance that doesn't respond to traditional lameness treatments.

2. Focal Muscle Atrophy

EPM can cause rapid and localized muscle wasting. You might notice one side of the rump (the gluteal muscles) looks sunken compared to the other, or the muscles of the face or shoulder may suddenly lose their definition. This happens because the parasite damages the nerves that stimulate those specific muscles.

3. Cranial Nerve Deficits

If the parasite settles in the brainstem, it can affect the cranial nerves. This leads to symptoms such as drooping ears, a tilted head, difficulty swallowing (dysphagia), or even a protruding tongue. Some horses might exhibit changes in behavior, becoming unusually dull or irritable.

The Diagnostic Maze: Why a Blood Test Isn't Enough

One of the most frustrating aspects of EPM is that a positive blood test does not necessarily mean the horse has the disease. Studies show that in some regions, upwards of 60% to 80% of healthy horses have been exposed to S. neurona and have developed antibodies. These horses have successfully fought off the exposure and are not clinically ill.

To move toward a definitive diagnosis, a veterinarian must combine several pieces of evidence:

  • Neurological Examination: A thorough exam involving walking the horse in tight circles, backing it up, walking over obstacles, and performing a "tail pull" test to check for hind-end weakness.
  • Serum Testing: Measuring the level of antibodies in the blood. While a negative test is very useful for ruling EPM out, a positive test only indicates exposure.
  • Cerebrospinal Fluid (CSF) Analysis: This involves a spinal tap to collect fluid from either the neck or the lumbosacral space. Comparing the ratio of antibodies in the blood to the antibodies in the CSF is currently the gold standard. If the ratio suggests that the antibodies are being produced inside the central nervous system rather than just leaking in from the blood, the diagnosis of active EPM becomes much more likely.
  • Ruling Out Other Issues: EPM can look like Equine Herpesvirus (EHV-1), West Nile Virus, Wobbler Syndrome (Cervical Vertebral Stenotic Myelopathy), or even Vitamin E deficiency. Radiographs or other viral tests may be needed to exclude these possibilities.

Modern Treatment Protocols in 2026

If the diagnosis points to EPM, time is of the essence. The longer the parasite is allowed to cause inflammation in the CNS, the more likely the horse is to suffer permanent nerve damage. Fortunately, there are several FDA-approved treatments that are highly effective at inhibiting the parasite.

Antiprotozoal Medications

Most treatments today focus on one of three active ingredients:

  1. Ponazuril: An oral paste (commonly known as Marquis) that is usually administered daily for 28 days. It is designed to cross the blood-brain barrier and target the parasite specifically.
  2. Diclazuril: Often administered as a top-dress pellet (Protazil). It is highly convenient for horses that are difficult to paste and has a similar mechanism of action to ponazuril.
  3. Sulfadiazine and Pyrimethamine: A combination liquid (Rebalance) that inhibits the parasite's ability to use folic acid. This treatment often requires a longer duration, sometimes 90 to 120 days, but remains an effective and often more budget-friendly option.

Supportive Therapies

Killing the parasite is only half the battle. The body’s inflammatory response to the dying protozoa can sometimes make neurological symptoms temporarily worse. To manage this, veterinarians often prescribe:

  • Non-Steroidal Anti-Inflammatory Drugs (NSAIDs): Like flunixin meglumine or phenylbutazone, to reduce swelling in the CNS.
  • Vitamin E: As a powerful antioxidant, high doses of natural Vitamin E (RRR-alpha-tocopherol) help protect nerve cells from further oxidative damage and assist in the repair process.
  • Immunomodulators: In some cases, drugs that stimulate the horse’s own immune system may be used to help the body fight the infection more effectively.

Prognosis: What to Expect During Recovery

The road to recovery from EPM is rarely a straight line. Roughly 60% to 70% of horses show significant improvement with treatment, but only about 15% to 25% return to their previous level of athletic performance. Success depends heavily on how early the disease was caught and the severity of the initial symptoms.

Improvement is often measured in "grades" of ataxia. A horse that is a Grade 3 (obvious incoordination at a walk) might improve to a Grade 1 (coordination issues only visible during complex movements). It is important to note that a relapse can occur in about 10% to 20% of cases, particularly during times of high stress, such as heavy training or long-distance transport.

Risk Factors and the Stress Connection

Why does one horse get EPM while its pasture mate, eating the same hay and exposed to the same opossums, remains perfectly healthy? The answer likely lies in the immune system. Stress is a major trigger for the progression from "exposed" to "clinically ill."

High-risk factors include:

  • Age: Young horses (1–5 years) and seniors (over 13 years) are more frequently diagnosed.
  • Environment: Presence of wooded areas and water sources that attract opossums.
  • Stressors: Heavy exercise, recent surgery, transport over long distances, or weaning.
  • Seasonality: While EPM occurs year-round, cases often spike in the fall, possibly due to changes in opossum activity and the stress of changing weather.

Prevention: Management is Your Best Defense

Since there is currently no effective vaccine for EPM, prevention focuses entirely on reducing the horse's exposure to opossum feces and maintaining a robust immune system.

1. Secure Your Feed

Opossums are opportunistic scavengers. Keep all grain in metal bins with tight-fitting lids. Never leave cat or dog food out in the barn, as this is a primary attractant for wildlife.

2. Manage Hay and Bedding

Store hay in a closed loft or an area that can be sealed off. If you see signs of wildlife in your hay mow, discard any flakes that appear contaminated. Opossums often seek out the warmth of hay or shavings for nesting.

3. Clean Water Sources

Scrub water troughs regularly. If your horses drink from a pond or stream, be aware that these are high-traffic areas for wildlife. Providing a clean, elevated water source can reduce the risk of contamination.

4. Wildlife Control

While you don't need to eliminate every opossum in the county, you should make your barn as unappealing as possible. Remove fallen fruit from trees, keep trash cans sealed, and clear away brush piles where opossums might den.

5. Strategic Supplementation

For horses in high-stress environments or those in regions with high EPM prevalence, maintaining adequate Vitamin E levels can provide a layer of protection for the nervous system. Consult with a nutritionist to ensure the diet is balanced for optimal immune function.

Conclusion

Understanding what EPM is in horses is the first step toward managing the fear that often accompanies the diagnosis. It is a complex, frustrating disease, but it is not a death sentence. Through a combination of vigilant observation, sophisticated diagnostic testing, and modern antiprotozoal medications, many horses can overcome the infection and lead comfortable, productive lives.

The most important takeaway for any horse owner in 2026 is to trust your instincts. If your horse feels "different" under saddle, if they are suddenly struggling with lead changes, or if you notice a subtle asymmetry in their hindquarters, don't wait. Early intervention is the single most effective tool we have in the fight against EPM. By acting quickly and working closely with your veterinary team, you give your horse the best possible chance at a full recovery.