You know your horse inside and out. So, you notice when your gelding has a little trouble swallowing his treats or seems to be losing muscle instead of gaining it. You pay attention when your mare has an odd tilt to her head or her gait seems uncoordinated. And naturally you wonder what’s going on. While there could be many causes, one significant possibility
is EPM.

Formally known as equine protozoal myeloencephalitis, EPM is one of the most common equine neurologic disorders in North America. There is no vaccine for it, and prevention revolves around managing a cat-size marsupial—the opossum. Luckily, there are effective treatments, so infected horses do stand a chance, even if they’re standing on wobbly legs.

To put you in the know about this scary but manageable disease, we talked to two well-known experts on the subject: Martin Furr, DVM, MA Ed, Ph.D., Dipl. ACVIM, assistant dean of administration, Oklahoma State University College of Veterinary, and Amy Johnson, DVM, Dipl. ACVIM, professor of large animal medicine and neurology at the University of Pennsylvania’s New Bolton Center.

What Makes EPM Scary?

EPM can impact nearly any aspect of a horse’s neuromuscular system. Clinical signs can be subtle or severe, says Furr, ranging from vaguely unusual behavior to abnormal gait, difficulty standing, general weakness or lack of coordination to difficulty swallowing, facial paralysis, muscle atrophy or seizures. 

This wide range of signs can make it hard for a horse owner to spot EPM. So, when should you call your veterinarian? “You know how your horse feels. If he seems a little bit off and you can’t figure out where or if there’s a change in his behavior or his way of going, you should ask your veterinarian come take a look,” says Furr.

If the horse shows severe signs—like stumbling, head tilting or significantly altered behavior—that’s an indication the disease could be moving fast, he adds, making a vet visit even more urgent.

How Do Horses Get EPM?

Before we delve into diagnosis and treatment, though, let’s look at where the disease comes from and how horses get infected. EPM begins with one of two protozoa (single-celled parasites): Sarcocystis neurona or Neospora hughesi. Johnson notes that S. neurona accounts for most infections, while N. hughesi is substantially less common and seems to appear more on the West Coast. 

The S. neurona protozoa live in a variety of animals, including racoons, skunks and cats. Opossums pick it up by eating the carcass of one of these intermediate hosts, then shed the protozoa in their feces. Horses can ingest the parasite by eating grass, hay or other feed contaminated by the protozoa in its infective stage, explains Furr.

Most of the time, the parasite passes through and out of the horse’s system without causing trouble. But in less than 1% to 1.5% of infected horses, Furr says, the protozoa make their way to the horse’s central nervous system. The result can be inflammation of the horse’s spinal cord and brain, leading to neurologic signs.

One piece of good news: S. neurona isn’t contagious, so one affected horse won’t make his stablemates sick. “Although all horses at a barn might be exposed to the protozoa, outbreaks of EPM are very rare, and typically only a single horse is affected,” Johnson says.

What Are the EPM Risk Factors?

An enduring mystery of EPM—and a focal point of ongoing research—is why so few horses who are exposed to the protozoa develop the disease. One common theory is that it comes down to differences in how well an individual horse’s immune system can keep the protozoa away from the central nervous system, says Johnson.

In fact, horses who already have a compromised immune system—such as those with pituitary pars intermedia dysfunction (PPID, formerly Cushing’s disease) or who are older—may be at greater risk for developing clinical signs, explains Furr. 

High-stress situations, such as long-distance shipping, are another potential predisposing factor, he adds. However, lower levels of stress, like moving to a new barn or going to a competition, aren’t enough to change a horse’s risk level. “It has to be substantial [stress] and for a day or two,” he says.

Geography can also impact EPM risk. Opossums can be found all over North and South America, so EPM can likewise be found throughout the two continents. In some areas, says Johnson, as many as 60% to 80% of horses test positive for antibodies to the organism—meaning they’ve been exposed, even if they never showed clinical signs. Housing horses in particularly opossum-friendly environments, such as near creeks and low brushy areas, may increase the EPM risk, Furr notes.

On the other hand, he adds, there tend to be fewer cases in the frigid region along the Canadian border and in the hot, dry desert southwest because the protozoa may die before reaching the infective stage. 

However, Johnson says since horses and contaminated feed can cross state lines, the parasites and disease can still pop up nearly anywhere. In fact, while EPM is a disease of the Americas, “we’ve seen cases where a horse was in the U.S. and years after going to another continent, they would show clinical signs of EPM,” she adds.

How Is EPM Diagnosed?

The only way to definitively confirm a horse has EPM is during a postmortem, when a pathologist can see the organism under a microscope. The protozoa aren’t visible in living horses, so Furr relies on what he calls a diagnostic triad to identify a likely case of EPM. The first, fundamental diagnostic pillar, he says, “is that the horse is showing neurologic signs or abnormalities consistent with EPM.” 

The second pillar of the triad is ruling out health problems that could have similar clinical signs. That means eliminating orthopedic issues, wobbler syndrome and other disorders that might cause neurological signs, says Furr.

If a horse has clinical signs consistent with EPM and the vet has eliminated other issues, then diagnosis moves on to the third pillar—blood tests and a spinal tap, says Furr. 

These tests look for antibodies that indicate the horse has been exposed to the parasite—known as seroprevalence. However, Furr notes it’s important to remember that if a horse has antibodies but doesn’t show clinical signs, he most likely doesn’t have the disease.

If a horse does show clinical signs and tests positive for antibodies, then the vet will compare the ratio of antibodies in the blood to antibodies in the spinal fluid. If that ratio is less than 100, “we believe they probably have the disease,” says Furr, “and we are 93% to 94% accurate.”

Treating Horses With EPM 

If a horse is diagnosed with EPM, treatment will be based on one of the three antiprotozoal drugs currently approved by the U.S. Food and Drug Administration to fight EPM: ponazuril (an oral paste sold under the brand name Marquis^®), diclazuril (pellets sold as Protazil^®) and a liquid combination of sulfadiazine and pyrimethamine (sold as ReBalance^®). Each works by killing the parasite or inhibiting its reproduction.

Furr notes there are alternative remedies being sold, but he doesn’t feel they’ve been thoroughly evaluated. “My very strong recommendation is to use one of the approved drugs,” he says. “All three have been rigorously tested and shown to be effective.”

Beyond drugs, recovery for an EPM horse means rest and possibly physical rehabilitation. “It looks different from horse to horse,” says Johnson. 

For instance, a horse who is mildly affected may simply need to be taken out of ridden work during medical treatment. But a horse with severe clinical signs will likely require more intensive care. That’s because nerve cells (called neurons) may have been damaged or killed by the protozoa—and they won’t grow back, says Johnson. However, she adds, the nervous system can create workarounds—building new pathways to transmit information and compensate for the damage.

“To do that, the horse has to exercise and practice,” Johnson says, similar to a person recovering from a spinal injury. “Physical therapy is so important,” she adds. “Twenty years ago, a horse would be locked in a stall in an attempt to protect him, but he was losing muscle tone and not learning how to accommodate. It’s the worst thing you can do.”

Johnson suggests that horse owners work with their veterinarians to develop what she calls “a safe but aggressive neurological rehabilitation program.” That might start with hand walking in a straight line, then adding gentle hills, very slowly asking for longer walks, then faster work and eventually putting the tack on—with the vet approving each progressive step of recovery.

The timeline for recovery depends on disease severity and clinical signs. In general, Johnson notes, “you may see improvement within a couple weeks of starting treatment, but neurologic recovery could happen over as long as six months.”

Long-Term Prognosis for Horses With EPM

Even with drugs and rehab, not every horse with EPM will fully recover. In fact, early research on the drugs showed effectiveness in the low 60% range. However, Furr says, “my clinical experience in well documented cases is that the success rate is a fair bit higher than that. Certainly there are horses that do not respond, but a fairly small number.”

Johnson believes the disparity between research results and clinical results may be due to misdiagnoses. It’s likely that the initial drug approval studies in the 1980s and 1990s enrolled horses who didn’t have EPM. With today’s more accurate diagnostics, she feels that 80% to 90% of EPM horses get better with treatment. 

“We’ve always said we should do a new efficacy study, but [there’s] no funding because the drugs are already approved, and it would require high numbers [of horses],” she adds.

Even when a horse appears to have recovered, relapse is a possibility, agree Johnson and Furr. To help prevent that, both veterinarians have reconsidered how long the horse should be treated with medication. 

For example, Furr notes that one of his own earlier studies showed 8% of horses relapsed within 90 days of stopping treatment. Those horses were treated for one month. “When I use the drugs now, I usually treat for two months,” he says.

Johnson takes a different approach. “Once a horse has completed the treatment course, I recommend putting the horse on a preventive course, at a lower dose, for as long as the owner is willing, especially if the horse has a high-stress life,” she says. She likens this strategy to neurologic cases of toxoplasmosis, a human disease similar to EPM. “Those people may stay on treatment for life because [the infection] is so hard to clear,” she says. 

A Treatable Disease

As with many health issues, the earlier EPM is diagnosed and treated, the better the outcome is likely to be. While there are no guarantees, many horses can return to a high level of performance, says Furr. “We’ve learned a great deal about this disease and how to treat it,” he continues. “EPM is a treatable disease. It’s not a death sentence.”

EPM Research Continues

Here are a few EPM research studies underway at places including the University of California, Davis, the University of Kentucky and Virginia Tech University:

• Investigating why some horses are not affected by exposure to the EPM protozoa, with a particular emphasis on immune system response
• Analyzing current FDA-approved treatments to establish a gold standard
• Exploring new drugs and alternative pharmaceuticals, including bump-kinase inhibitors, which act on a protein the protozoa need to survive
• Taking the first steps toward developing an EPM vaccine
• Looking at ways to optimize treatment success, such as adding oil to improve drug absorption
• Studying geographical risk factors
• Investigating the potential role of genetics in susceptibility to EPM and recovery from the disease
• Further improving diagnostics, including looking for potential disease biomarkers (traits in blood, tissues or body fluids that may indicate infection)

This article originally appeared in the fall 2025 issue of Practical Horseman.