HE’S CUTE, BUT…The opossum is the primary transmitter of the disease EPM to horses. ISTOCKPHOTO.COM

This dreaded neurological disease has robbed many horses of their careers. Here’s the latest on diagnosis and treatment.

By Katie Navarra

Reprinted from the June 2016 issue of USDF Connection.

Last summer, in less than a week’s time, Kelly Starliper’s beloved Thoroughbred gelding went from “not quite right” to dangerously uncoordinated and nearly unable to stand.

The 18-year-old dressage rider from Pottstown, PA, then a rising high-school senior, had been riding and showing 17-year-old Jonah at Training Level when she sensed that something was wrong.

“She would say to me, ‘He just doesn’t feel right,’” said Kelly’s mother, Jill. But Jonah showed no discernible lameness.

At a July schooling show, Jonah had a few uncharacteristic stumbles during his test. He was tired, the Starlipers thought. Monday, the day after the show, Kelly took Jonah for a light hack, and he still wasn’t quite right. A day later, the horse was lame, with a swollen stifle.

Over the next 24 hours, the Starlipers monitored Jonah closely. “Our trainer even said his in-coordination looked neurologic, but it was so mild she couldn’t be sure,” Kelly said.

BETTER DAYS: Jonah and owner/rider Kelly Starliper in 2013. Photo Courtesy of Kelly Starliper

By Thursday, it was obvious that something was very wrong. By the time the veterinarian arrived at the barn, Jonah was staggering and tilting his head to one side. The vet suggested EPM as a probable cause, but only blood work and a spinal tap of the horse’s cerebrospinal fluid can confirm a diagnosis. Given the severity of Jonah’s symptoms, the Starlipers decided to take him to the University of Pennsylvania’s New Bolton Center in Kennett Square for diagnostics and treatment.

 “It progressed so slowly at first, and then all of a sudden he was completely unsteady,” Kelly recalls.

“When I fiirst met Jonah, he was bright but a little stressed by his condition,” says Michelle Abraham Linton, BSc, BVMS, DACVIM (LAIM), the veterinarian on call when Jonah arrived at New Bolton. Linton gave Jonah a grade of 3 on the Mayhew scale, which rates a horse’s degree of ataxia (impaired balance and voluntary muscle control) from 0 (normal—no ataxia) to 5 (unable to stand). Jonah’s neurologic deficits included obvious ataxia while walking and turning, when loin pressure was applied, and when his neck was extended. Some of the horse’s cranial nerves were also affected: His muzzle was pulled to one side, his tongue was weak, and he had fine tremors in his head and tongue.

“He had significant abnormalities to his gait,” Linton says, “and due to his neurologic deficits involving a number of nerves in different locations, EPM was high on my list.”

Jonah spent six days at New Bolton, where the suspicion of EPM was confirmed. Although he responded to treatment, it was months before he was deemed safe enough for Kelly to ride, and nearly a year after his diagnosis, his future in dressage remains unclear. His is a cautionary tale that EPM is still a clear and present danger to horses. In this article, we’ll look at this dreaded disease and report the latest on diagnosis and treatment.

Blame the Opossum

EPM is a protozoal disease that causes inflammation of the brain, the spinal cord, or both. The inflammation leads to asymmetrical ataxia, aka muscular in-coordination.

The disease was discovered in the 1960s, but it wasn’t until the 1970s that researchers identified the organism that causes EPM. That parasite, Sarcocystis neurona, remains the primary cause of EPM; but today a second parasite, Neospora hughesi, is emerging as another.

Both parasites rely on host animals to survive. The horse is not a host animal. So how do horses get infected?

VICIOUS CIRCLE: Illustration depicts the EPM transmission cycle, from intermediate hosts (cats, skunks, and others), to the definitive host (opossum), to the equine. Illustration Courtesy of Merial Inc.

“The life cycle of EPM is complicated and involves multiple host species, including an intermediate host, a definitive host, and an aberrant host,” says Megan M. Green, DVM, manager of Equine and Large Animal Veterinary Services at Merial Inc., manufacturer of the EPM treatment Marquis.

Raccoons, armadillos, cats, sea otters, harbor seals, and skunks all can serve as intermediate hosts. In a horse-farm setting, the most likely intermediate host is a cat, Green says.

The intermediate host ingests parasite-containing structures called sporocysts. The sporocysts travel to the host’s muscle tissue, where they form cysts containing the parasites, called sarcocysts.

 But horses don’t contract EPM from exposure to infected cats or other intermediate hosts. For a horse to get EPM, there needs to be one more step, and that’s for the so-called definitive host to get infected. That’s where the opossum comes in.

The infective form of S. neurona—the form that makes horses sick—develops in opossums that dine on dead cats, skunks, or other intermediate hosts that were carrying the parasites. The possum ingests the sarcocysts along with the intermediate host’s muscle tissue, and the sarcocysts reproduce in the possum’s gut, forming sporocysts. The sporocysts are passed in the opossum’s feces and are the infective form of the organism. When a horse ingests infected possum scat—say, while grazing in a field a possum has traversed— it risks contracting EPM.

“When the parasite crosses the blood/brain barrier,” explains Green, “that’s when a horse has the potential for developing neurologic defects due to direct neuronal damage and inflammation that is caused by the protozoa.”

Green emphasizes that “Just because a horse has been exposed to opossum scat does not mean it will develop EPM. It all depends on the status of the immune system and stress level of the individual horse.”

A 2013 University of California, Davis study found that EPM is more widespread than originally thought. After obtaining more than 3,100 blood samples taken from horses in 49 states, researchers determined that horses from 42 states were affected by parasites that cause EPM. Horses from 24 states tested positive for the antibodies against both S. neurona and N. hughesi. Horses from 17 states tested positive for antibodies against S. neurona only, while horses from one state tested positive for antibodies against N. hughesi only


EPM remains one of the more feared equine diseases because it’s difficult to diagnose, treat, and prevent. No vaccine exists.

According to Green, “EPM is a master of disguise, and often the symptoms mimic other diseases.” Mild depression and subtle gait abnormalities, both of which can be caused by EPM, are similar to symptoms of injury or lameness. Neuronal damage caused by EPM can cause upper-airway malfunction that makes it difficult for the horse to eat and swallow, causes the horse to “roar,” or changes the sound of his whinny.

 “The first horse I had diagnosed with EPM had the pitch of her whinny actually change,” says Adriane Alvord, an FEI-level trainer and competitor and a USDF bronze, silver, and gold medalist in Boyce, VA. “I used to know her whinny to a T, but after the EPM started getting bad, I didn’t even recognize it. I learned later that can be one of the symptoms.”

Depending on the progression and severity of the neurologic damage, the symptoms become more severe, and the horse can potentially collapse or become unable to stand. Alvord experienced this decline firsthand in 2014.

During a schooling session with a client’s horse, Kazul, a then-11-year-old Swedish Warmblood gelding working at Third and Fourth Levels, Alvord asked the horse to trot. He stumbled and was unable to catch himself, falling to the ground.

“It happened so fast,” Alvord recalls. “The fall felt like it was in slow motion. When he went down, he went sideways, and I was able to pull myself out from under him before I had his full weight on me.”

Kazul’s attitude remained good, and diagnosis proved difficult. At first the veterinarian thought he needed “maintenance,” so he was given Adequan and had his hocks injected, Alvord says. The horse seemed better for a short time, but then the neurologic symptoms returned and grew progressively worse. Soon Kazul was stumbling during every ride, and the ataxia seemed to move from limb to limb. He began “flinging himself” into the flying changes and would often fall out of the change into the trot, she says.

“The reason for the wide variety of clinical manifestations is that the protozoa have the ability to randomly infect many sites within the central nervous system,” says New Bolton Center’s Linton.

EPM is unpredictable. A field of horses may all be exposed to the parasite, but some are able to develop antibodies against it and others are not. After Kazul fell ill, his owner had 20 other horses on her farm at home tested. Many tested positive for EPM but never showed signs of neurologic damage.

“What was interesting to see was that the horses with similar bloodlines all had it [tested positive], whereas others didn’t,” Alvord said.

In fact, research and diagnostic testing have shown that not all horses exposed will develop EPM, and the annual incidence of EPM is less than 1 percent, according to Stephen M. Reed, DVM, Dipl. ACVIM, an EPM expert at Rood & Riddle Equine Hospital in Lexington, KY.

 Is My Horse at Risk?

Opossums are found throughout the Western Hemisphere— so yes, your horse is at risk for contracting EPM. His age and his stress level are secondary risk factors.

“Stress-induced immune suppression may be associated with susceptibility to developing disease after exposure,” says Linton.

According to Green, horses are more susceptible to neurologic damage from the ages of one to fi ve, and then again after the age of 13. The stresses of weaning and the subsequent social regrouping may be contributing factors in young horses, Green says. Another may be that most horses begin their training—with its accompanying physical and mental stresses—during these years, she says.

By the time a horse turns 13 it may be undergoing yet another set of life changes, which can range from retirement to entering the breeding shed. “Additionally, as horses age their immune systems may not be as robust,” says Green, “and when it is challenged it may not be able to protect the horse from developing the disease” after exposure.

SUCCESS STORY: After battling EPM twice, Kazul (pictured with rider Adriane Alvord at the 2014 Col. Bengt Ljungquist Memorial Championships) has since advanced to the FEI levels. Photo by Pics of You

Treatment and Prognosis

Green and Linton agree that early detection and treatment of EPM are critical in determining a horse’s chances of recovery. The Thoroughbred, Jonah, “was fairly typical of what we see with EPM in terms of the onset and progression of his signs. His signs had a sudden and progressive onset,” says Linton.

Even with prompt treatment, the numbers are sobering. Research shows that only 60 to 70 percent of diagnosed horses improve with treatment, and only 15 to 25 percent recover completely. And 10 to 20 percent of horses diagnosed with EPM relapse within two years. The challenge in treating EPM is that multiple sites in the central nervous system may be affected, and the protozoa cause physical damage as they migrate. Portions of the brain, multiple segments of the spinal cord, or both may be destroyed.

“This damage and therefore the clinical signs can be permanent, even once we have eliminated the protozoa,” Linton says.

Currently, EPM treatment consists of antiprotozoal medication. Th e FDA-approved drugs on the market are ponazuril (brand name Marquis), diclazuril (Protazil), and sulfadiazine/ pyrimethamine (ReBalance). Marquis is an oral paste, Protazil is an alfalfa-based top-dress pellet, and ReBalance is an oral suspension. New Bolton Center’s Linton treated Jonah with a combination of Marquis and ReBalance.

Horses’ responses to the medications vary. Some show gradual improvement after the first 24 hours of treatment, while it takes others a few days. In fact, “Sometimes we’ll see signs worsen as soon as treatment is initiated because the medications kill the protozoa, and that can set off a significant inflammatory response,” Linton says. Veterinarians often give anti-inflammatory drugs at the same time to counteract that response, she says.

Keeping EPM at Bay

Without a vaccine, it’s impossible to prevent EPM entirely. The best practice is to minimize the chances of a host possum’s contaminating a horse’s environment. Our experts recommend taking the following steps:

Don’t let your barn become a wildlife buffet. Avoid feeding horses off the ground, especially outside, where possums and other critters can easily get into feed tubs or buckets. Store feed in chew-proof containers. “Keep feed rooms closed, and minimize grain spillage by sweeping areas where horses drop grain out of feed tubs,” Green advises.

Keep water troughs and tubs clean. Wildlife like these convenient sources of water and can easily contaminate the water. Dump, clean, and refill with fresh water frequently.

Schedule regular wellness checks. Your veterinarian may be able to pick up on small clues early on. A slightly droopy ear, for instance, could be an early sign of paralysis, according to Green. Uneven muscle definition under the forelock may indicate muscle atrophy. And, of course, if you notice stumbling or any other “not quite right” symptoms, call your vet promptly.

Long-Term Outlook

Time is the only true predictor of how a horse will respond to treatment for EPM.

Nearly a year after he fell ill, Jonah isn’t quite 100 percent, but with lots of TLC and a slow rehabilitation program, he is back to light work. His rehab began with hand-walking, and eventually he graduated to lungeing and then to short under-saddle sessions, building up from walking to trotting and finally cantering. Unable to negotiate the slope of his large pasture at first, Jonah was turned out in a small, level paddock until he regained sufficient strength and coordination to rejoin his buddies.

 “Even now, when he gets fatigued he drops his back,” says Kelly Starliper. “We are focused on what we can do to build up the other muscles in his back.”

As for Kazul, after a relapse and a second round of EPM treatment in 2015, he recovered and has been able to progress in his training, advancing to Prix St. Georges. “He scored a 63 percent at his first show at Prix St. Georges,” says Alvord. “A year prior to that, I never thought he would be able to show Fourth Level again.”

 Kazul wasn’t Alvord’s first encounter with EPM: She’s had four other horses diagnosed with the dreaded disease. The first never made it back to the show ring. The others eventually competed again, but never to the level she had hoped they’d achieve before they got sick, she says.

Although it may seem to some that the incidence of EPM is on the rise, veterinarians can’t confirm that is necessarily the case.

“What I can say for certain is that, in the last few years, we have developed better tests that can more effectively diagnose EPM. So perhaps we are just doing a better job of being able to diagnose the disease,” says Linton.

 Katie Navarra is a writer and photographer based in Clifton Park, NY. She regularly contributes to equine and general agriculture publications. When she’s not writing, she is riding her horse, Bella.

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