The majority of horses that live in the contiguous 48 states of the U.S. have been exposed to the protozoa that causes EPM. * Photo from Pexels

Five Things About EPM

No matter what part of the country you live in, if you’ve been around horses for a while, you’ve likely heard of Equine Protozoal Myeloencephalitis, or EPM. This disease has a bad reputation in the horse world due to the neurologic symptoms it can cause, which can sometimes be career-ending in upper-level performance horses or life-ending if not caught and treated early. Surprisingly, though, while the majority of horses that live in the contiguous 48 states of the U.S. have been exposed to the protozoa that causes EPM, only a small percentage will ever show signs of illness.

Quarter Horse News spoke with Sarah Reuss, VMD, DACVIM, an Equine Technical Manager with Boehringer Ingelheim Animal Health and the current American Association of Equine Practitioners Vice President, to get more information on what EPM is, how to detect and treat it, and what a diagnosis means for your horse’s future.

Opossums Are the Culprit

EPM’s primary cause is a protozoa (Sarcosystis neurona) that is spread to horses through opossum feces. This might occur if an opossum poops in a horse’s pile of hay, climbs into its water trough to get a drink or poops out in the pasture. Once the horse ingests the feces, the protozoa spreads through its body.

Opossum * Photo from Pexels

For the majority of horses, the immune system can fight the protozoa off, preventing it from ever reaching the nervous system and causing symptoms. A small number, however, often due to a weak immune system, are unable to fight off the disease, allowing it to get into the brain and spinal cord. These horses will start showing clinical signs soon afterward.

“EPM is one of those diseases that has a huge amount of horse owner awareness, which lends to a perception that it’s very common, but thankfully it’s not,” Reuss said. “If we take a blood sample and look at how many horses have antibodies against EPM, meaning they have been exposed, there are a lot of areas of this country that over 90% of horses will be “positive,” but thankfully, less than 1% of them actually develop true EPM, or infection of the nervous system.”

Stress Can Be a Factor

Research is currently ongoing regarding what makes certain horses susceptible to EPM. A common theory in performance horses is that stress plays a big role, as the pressure of hauling to new places and competing can weaken the immune system.

“We seem to often recognize it and diagnose it a fair amount in performance horses, and I think that’s probably for two reasons,” Reuss said. “One, those horses are under stress — they’re traveling, they’re competing, they have all of those risk factors. Two, they are usually handled closely by very diligent owners, trainers, grooms, etc. every day. They probably notice some of the more subtle things, versus a horse that lives out on the back 40 where if they trip a few times, maybe nobody sees it.”

There also might be some horses that are just predisposed to developing symptoms of EPM even in low-stress environments based on the make-up of their immune systems, Reuss added.

Symptoms Imitate Other Diseases

Mild gait abnormalities could be a sign of EPM. * QHN File Photo

EPM is often called “the Great Pretender” since its symptoms can mimic those of many other diseases. If it goes untreated, though, the protozoa will keep causing more and more damage to the horse’s neurologic system, which can ultimately end with the horse’s death. For that reason, it’s important to notify your veterinarian about any sudden physical or behavioral changes.

“It can start really subtly,” Reuss said. “It could be a mild gait abnormality, trouble picking up a lead or trouble in downward transitions. Other horses can go from normal to barely able to stand 24 to 48 hours later. Severity of signs, as well as progression of signs, varies a lot from horse to horse.”

EPM horses can present with asymmetry, where the horse’s left and right sides appear completely different from each other; atrophy, where the horse experiences rapid muscle loss, especially on one side of the body; and ataxia, or incoordination. Neurologic symptoms can also include trouble chewing and swallowing, head tilt, abnormal eye movement and behavior change.

Time Is of the Essence

A neurologic exam is the first place a veterinarian will begin when checking for EPM, followed by a blood test, which will tell you if your horse has been exposed to EPM protozoa. Some might go ahead and treat if the blood test comes back positive, but for a 100% diagnosis, your veterinarian will need to collect spinal fluid to determine if the protozoa have reached the central nervous system. If your horse is positive for EPM, the veterinarian will then put it on one of three FDA-approved drugs for treatment.

“All have roughly a 60% published success rate,” Reuss said. “If you’re expecting your horse to go back to an elite level of competition, some may do so, but others may not fully recover to the point that they can resume their previous level of athleticism. They may recover to a point where they’re still safe to ride and do lower-level things.

“About a third of horses, though, just don’t respond,” Reuss continued. “Either we’ve caught them too late — the disease went on for a period of time before we ever recognized it and got that horse treated — or something about their immune system, even in the face of appropriate treatment, just can’t kill off the parasite.”

About 20% of horses after initial treatment will have some form of relapse, whether due to reinfection from being in the same environment they were originally infected in, or due to recrudescence, where the protozoa were not all killed during treatment.

Management is the Best Prevention

There is currently no vaccine to prevent EPM, though studies are in the works looking at ways to prevent infection. For now, the best defenses against the disease are to maximize your horse’s immune system and to manage your property so as to discourage opossums from cohabitating with your horses.

“Anything you can do to decrease stress, keeping up on your horse’s vaccines so they’re not getting other systemic illnesses, and good nutrition are all things you can do to maximize their immunity,” Reuss said.

Keeping up on your horse’s regular vaccinations is one preventative measure. * QHN File Photo

Make sure there are multiple water sources on your property so that opossums aren’t climbing into your horses’ water troughs. If you have fruit trees nearby, pick up dropped fruit that might tempt critters looking for snacks. In the same vein, make sure horse feed and barn cat food is picked up so it’s not drawing pests to your barn or paddocks.

Final Note

EPM may not affect every horse, but it can be devastating for the horses that contract it. The sooner it’s treated, the better the prognosis for your horse, so don’t delay in notifying your veterinarian if you notice your horse acting strangely.

“If it’s been weeks or months since symptoms appeared, it’s really hard to treat because it’s gone untreated for so long, causing more damage to the nervous system,” Reuss said. “If you’re seeing any abnormal neurologic behavior, it’s always better to err on the side of caution and get the vet to look at the horse sooner rather than later.”