A Conversation with Dr. Rob Franklin, DVM, Board-Certified Equine Internal Medicine Specialist and Co-Formulator of 6666 Equine Supplements
Key Takeaways
- EPM is the most prevalent cause of neurologic disease in horses in the Americas, yet it is far less common than most performance horse owners believe¹
- An estimated 50 to 90% of horses in the United States have been exposed to the organism. Fewer than 1% develop the actual disease¹
- A positive blood test does not diagnose EPM. It means exposure only. This is one of the most consequential misunderstandings in the performance horse world
- Performance horses on the road are at elevated risk because travel, competition, and intense training stress the immune system — and a stressed immune system appears to be the link between exposure and disease
- The most reliable signs are asymmetric weakness, stumbling, abnormal foot placement, and focal muscle atrophy that can appear overnight
- About two-thirds of horses treated with FDA-approved medications improve, but recovery depends heavily on how early treatment begins²
- Horses cannot spread EPM to other horses. It is not contagious.
- Opossum access to your feed is a primary prevention target
Quick Answer
EPM (equine protozoal myeloencephalitis) is a neurological disease caused by the protozoan parasite Sarcocystis neurona, which horses ingest from feed or water contaminated with opossum feces. Exposure is extremely common. Active disease is not. The horses most at risk are those with compromised or stressed immune systems, which includes performance horses hauled heavily, trained intensely, and competing on tight schedules. Diagnosis requires a full neurological exam and ruling out other causes, not just a blood test. Treatment with FDA-approved medications results in improvement in roughly two-thirds of affected horses, though recovery is not always complete and depends significantly on how early treatment began.
The Trending Disease
In the performance horse world, EPM has a reputation that outpaces its actual prevalence. When a barrel horse is reluctant to turn left, when a reiner loses impulsion behind, when a cutting horse starts drifting, EPM comes up in conversation fast.
Sometimes that instinct is right. Often it is not.
"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," Dr. Franklin says. "Thankfully, it's not."
Understanding what EPM actually is, how it actually spreads, and what it actually looks like is the difference between catching it early when it counts and chasing a misdiagnosis down an expensive, time-consuming road.
What Is EPM?
"EPM is a protozoa that causes an infection within the nervous system of the horse," Dr. Franklin explains.
More specifically, it is caused by a protozoan parasite called Sarcocystis neurona, written as S. neurona, that invades the central nervous system and can affect any part of it, from the brain all the way down to the end of the spinal cord. It is considered the most prevalent cause of neurologic disease in horses across North, Central, and South America.¹
It can look mild. It can look severe. It can progress slowly or drop a horse in 24 to 48 hours. That range is part of what makes it both feared and frequently misidentified.
How Does a Horse Actually Get EPM?
The opossum is at the center of this story.
The opossum is the definitive host for S. neurona. It feeds on dead animals that carry a certain cyst in their muscle tissue as part of the parasite's life cycle. That cyst opens in the opossum's digestive tract, replicates, and gets passed out in the opossum's feces as infective sporocysts.
"It's the possum eating another intermediate host and then passing the cyst in the manure of the possum, and then that somehow entering the food environment for the horse," Dr. Franklin explains. "You can obviously think about where you've seen possums in your barn and how close to feed troughs and feed buckets and grain rooms and all those areas where it's pretty easy for some poop to end up getting in front of a horse."
One fact that surprises many people and matters for biosecurity:
"Horses don't actually spread it to other horses. They're not contagious and they shouldn't be viewed with any sort of biosecurity hazard when you're dealing with a horse that's infected in a barn."
Horses are a dead-end host. The disease starts and stops with them. The possum is the problem.
How Common Is EPM? The Numbers That Matter
This is where the conversation shifts for most performance horse owners.
Exposure is extremely common. Active disease is not.
An estimated 50 to 90% of horses in the United States have been exposed to S. neurona. Fewer than 1% develop clinical EPM.¹ That gap between exposure and disease is significant, and it is not random.
"What we know through studies is that a lot of horses get exposed to EPM, but very few actually get the disease," Dr. Franklin says. "And it seems like horses that have a stressed immune system are probably the link between the ones that get it and don't get it."
This is where the performance horse world pays attention. Horses that are hauling constantly, competing on tight schedules, and training intensely are exactly the horses whose immune systems are under sustained pressure. Cortisol rises with stress. And cortisol suppresses the immune response.
"We seem to often recognize it and diagnose it a fair amount in performance horses, and that's probably because those horses are under stress. They're traveling, they're competing, they have all of those risk factors."
EPM is not more common in performance horses because they are out in the world more. It is more common because their immune systems are more frequently taxed.
The Biggest Myth in the Performance Horse World: A Positive Blood Test Means EPM
This is the misconception Dr. Franklin returns to more than any other.
"The whole idea that a positive blood test proves that a horse has EPM is not true."
A blood test for EPM detects antibodies to S. neurona. Antibodies mean the horse has been exposed to the organism. They do not mean the horse has active disease. They do not mean the horse is currently being harmed by S. neurona. And in a world where 50 to 90% of horses have been exposed, a positive blood test is not a diagnosis. It is a data point.
Dr. Franklin uses a straightforward analogy:
"Much like if I ran a blood test on you and said, do you have any antibodies to the flu? Well, probably you do, whether that's through vaccination or previously having the flu at some point. You've got some antibodies circulating that says you've seen the flu before. That doesn't mean that you have the flu right now."
A negative blood test is actually quite useful. If the blood test comes back negative, it provides a fairly high level of certainty that the horse has not been exposed and therefore probably does not have EPM.
A positive test can support a diagnosis when paired with other evidence. It does not stand alone.
"A positive blood test can help support a diagnosis, and a negative test can help rule out the chance that the horse has EPM. But a positive blood test does not mean the horse has the actual disease."
How Is EPM Actually Diagnosed?
Diagnosis requires more than a blood test. It requires a veterinarian.
"What we really need to know for a true diagnosis is: does a horse have neurological disease, and is the neurological disease consistent with where EPM tends to affect the nervous system? And have we ruled out other common causes of neurological disease?"
Young horses in training, particularly two and three-year-olds, can develop wobbler syndrome, a vertebral malformation in the neck that causes similar balance and coordination problems. It is especially common in thoroughbreds and warmbloods. Trauma and injuries are another common cause of neurological signs that must be ruled out before landing on EPM.
In some cases, a veterinarian will want to perform a spinal tap to collect cerebrospinal fluid and compare those results against the blood test.
"Comparing that with the blood to see if there's actually local antibodies being made around the spinal cord really helps support the diagnosis," Dr. Franklin says.
The process is methodical for a reason. Treatment for EPM is long, expensive, and not without risk. An accurate diagnosis before committing to a treatment course is worth the time.
What Does EPM Look Like? Signs to Watch For
Because EPM can affect any part of the central nervous system, the signs vary widely. This is part of why it gets blamed for so much. But the most consistent signs are specific.
Asymmetric weakness. One side is noticeably weaker or less coordinated than the other. This asymmetry is a hallmark of EPM and one of the most useful distinguishing features from other conditions.
Stumbling or poor footing. Difficulty maintaining coordination or proper foot placement. Ranges from subtle tripping to obvious stumbling at any gait.
Abnormal foot placement. The horse positions its feet in unusual ways or seems unaware of where its feet land.
Focal muscle atrophy. One of the most striking signs and one that can appear rapidly.
"You may look at the horse's hip or the jawline and see just a completely shoveled-out area where there's no muscle. And that can occur very quickly sometimes, whenever the EPM organism gets right into that nerve root and destroys that nerve. Then the muscle will fall away very quickly."
"Sometimes the hip muscles or facial muscles may just seemingly evaporate overnight. That's atrophy."
For performance horses, early signs often appear as a subtle loss of impulsion, a reluctance to take one lead, a horse that starts drifting or not tracking up evenly. These signs get blamed on soreness, lameness, saddle fit, training problems, and a dozen other things before EPM enters the picture. That is not always wrong. But EPM should be on the differential when those signs are also accompanied by any coordination changes, especially if they are asymmetric.
What is not a reliable clinical sign on its own is a horse that just seems off or is underperforming without other neurological indicators.
"That's not a true clinical sign. Typically we're looking at asymmetrical weakness, signs of atrophy, coordination issues. Those are the most consistent signs of EPM."
Why Performance Horses Are at Higher Risk
The stress connection is real and it is specific.
Cortisol is the body's primary stress hormone. It rises during hauling, competition, intense training, and exposure to new environments. Sustained elevated cortisol suppresses immune function, including the immune response that protects the horse from S. neurona progressing from exposure to active disease.
"We know that horses are constantly under stress from hauling, from lots of intense training. So trying to minimize stress, trying to keep their life as stable as possible."
Supporting the immune system through that sustained demand is not optional for a performance horse. It is part of the management program.
"We know that the immune system is largely regulated by the gut. So making sure that you've got a great forage-first diet that you're supplementing with pre and probiotics to regulate the microbiome in order to keep that gut stable and the immune system stable."
6666 Complete Gut Protection supports the hindgut microbiome with prebiotics, probiotics, and postbiotics alongside L-glutamine and oat beta glucan to maintain barrier integrity. Keeping the gut stable through the demands of training, hauling, and competition is one of the most practical things a performance horse owner can do to keep the immune system in a position to protect itself.
How to Lower the Risk: Opossum Control and Immune Support
Two fronts.
The first is environmental. The opossum is the source. Controlling opossum access to the feed environment is the most direct prevention available.
"Typically that's going to focus around managing your feed room. Trapping possums is obviously a good idea, but more importantly not leaving out those areas where they're going to be attracted to."
"If they've got the organism circulating through their digestive tract, they're going to put that infection possibly right into the grain meal that you may be feeding your horse. So keeping a secure grain room is going to be paramount."
Practical steps: do not leave pet food out around the clock, keep the grain room secured so wildlife cannot enter, and remove other attractors that draw possums to the barn.
The second is internal. A horse with a strong, stable immune system has a meaningfully better chance of being among the 99% of exposed horses that never develop clinical disease.
"Anything you can do to keep your horse's immune system as strong as possible is going to be great."
That means forage-first feeding, minimizing unnecessary stress stacking, consistent gut support, and thoughtful management of the hauling and competition schedule.
What Happens If Your Horse Gets EPM?
First, the encouraging news.
"The good thing is that there are several medications out there that are available. We always recommend to use the FDA-approved medications, just so that you know that what you're getting is truly lot-by-lot tested and there's no possibility that you got a bad batch of treatment."
Three FDA-approved treatments are currently available for EPM in horses: ponazuril, diclazuril, and a combination of sulfadiazine and pyrimethamine. Your veterinarian will guide the choice based on your horse's specific situation.
Treatment typically runs from one to six months. This is not a short course, and it requires commitment.
About two-thirds of treated horses show improvement, and many return to their previous level of performance.² But the nervous system is a different kind of tissue.
"We can go in there and kill the parasites, but we can't always allow the damage that the parasite has caused to recover fully. The nervous system is very finicky on its ability to repair itself. In some cases, it does quite well. In other cases, it simply can't make those connections."
Some horses do not respond even with extended treatment and different medication combinations. And some horses that improve will relapse.
The degree of recovery depends directly on how much neurologic damage occurred before treatment began. This is why early recognition matters. A horse with early, mild EPM has a meaningfully better prognosis than one that was misdiagnosed or left untreated while the damage accumulated.
Vitamin E and EPM: What You Should Know
One supportive therapy commonly recommended alongside EPM treatment is vitamin E. S. neurona infection causes oxidative damage to nervous tissue in addition to direct neurological injury. Natural vitamin E, particularly in a water-soluble form for better absorption, has been advocated by veterinarians treating EPM because of its antioxidant properties. Experts generally recommend 5,000 to 10,000 IU of natural vitamin E daily for horses with EPM or recovering from it. This is not a treatment for the infection itself but a supportive measure for the nervous tissue under attack. Always discuss with your veterinarian before changing supplementation during an active EPM case.
After Treatment: Watching for Relapse
Recovery from EPM is not a straight line and the work does not stop when the medication course ends.
"Know where you finish on your treatment, know the degree of resolution," Dr. Franklin says. "And then you want to obviously watch your horse very quickly and consistently, tracking and looking for any subtle problems."
Supporting the immune system after treatment is equally important. A horse that has been through EPM and its treatment has been through significant stress. The gut needs support through that recovery period and beyond. The feed room still needs to be secure. The hauling and competition schedule needs to be managed thoughtfully going forward.
And watch. Any return of the signs that preceded diagnosis warrants a call to the veterinarian.
Actionable Checklist: EPM Prevention and Response
- ✓ Secure the grain room so opossums and wildlife cannot enter
- ✓ Do not leave pet food or horse feed out around the clock near the barn
- ✓ Remove attractors that draw opossums to feed and water areas
- ✓ Support your horse's immune system with a forage-first diet and daily gut supplementation
- ✓ Build recovery time into the hauling and competition schedule
- ✓ Know your horse's baseline. Asymmetric weakness, stumbling, and rapid focal muscle atrophy are the signs that matter
- ✓ Do not rely on a blood test alone for diagnosis
- ✓ Work with your veterinarian on a full neurological exam when EPM is suspected
- ✓ Pursue early diagnosis and treatment. Outcome is strongly tied to how early treatment begins
- ✓ Use FDA-approved treatments under veterinary guidance
- ✓ Track recovery carefully and monitor for relapse after treatment ends
- ✓ Consider vitamin E supplementation during and after treatment in consultation with your veterinarian
We Asked the Vet: Your EPM Questions Answered
What is EPM?
EPM stands for equine protozoal myeloencephalitis. It is a neurological disease caused by the protozoan parasite Sarcocystis neurona, which infects the central nervous system and can affect any part of it from the brain to the end of the spinal cord. It is the most prevalent cause of neurologic disease in horses in the Americas.¹
How do horses get EPM?
Horses get EPM by ingesting feed or water contaminated with opossum feces that contain infective sporocysts of S. neurona. The opossum is the definitive host in the parasite's life cycle. Horses are a dead-end host and cannot spread EPM to other horses.
Is EPM contagious between horses?
No. Horses cannot spread the disease to other horses. No biosecurity measures between horses in the same barn are needed based on EPM alone.
Does a positive blood test mean my horse has EPM?
No. A positive blood test means the horse has been exposed to S. neurona. Because 50 to 90% of horses in the U.S. have been exposed, a positive test alone is not a diagnosis. A neurological examination, ruling out other causes, and in some cases cerebrospinal fluid testing are required for a meaningful diagnosis.
How common is EPM in performance horses?
Exposure is very common in all horses. Fewer than 1% develop clinical disease.¹ Performance horses on active show and haul schedules may be at higher risk of progressing from exposure to disease because travel, competition, and intense training all stress the immune system, which appears to be the key factor in whether exposure becomes active disease.
What are the signs of EPM?
The most consistent signs are asymmetric weakness, stumbling or poor foot placement, and focal muscle atrophy that can appear very quickly. Performance horses may show subtle signs first, such as reluctance to take one lead, loss of impulsion, or drifting, before more obvious neurological signs appear. Vague performance decline or a horse that just seems off is not a reliable clinical sign of EPM on its own.
Can EPM be treated?
Yes. Three FDA-approved medications are available. Treatment typically lasts one to six months. About two-thirds of treated horses show improvement and many return to their previous level of performance.² Some horses do not respond, and some that improve will relapse.
Will my horse fully recover from EPM?
Recovery varies and depends heavily on how early treatment began. The nervous system's ability to repair itself is limited compared to other tissues. Horses treated early with mild disease have the best prognosis. Horses with significant pre-existing neurological damage before treatment may have permanent deficits. This is why early recognition matters.
What role does the immune system play in EPM?
A stressed or compromised immune system appears to be the key factor in whether an exposed horse develops clinical EPM. Anything that supports immune function, including minimizing unnecessary stress, maintaining a forage-first diet, and supporting the gut microbiome daily, reduces the risk of progression from exposure to disease.
How do I prevent EPM?
Control opossum access to your feed environment. Secure the grain room, remove food attractors near the barn, and consider trapping. Simultaneously, support your horse's immune system through gut health and stress management. Exposure may be unavoidable in much of North America. Whether that exposure becomes disease is influenced by how well the immune system is supported.
When to Call Your Veterinarian
Contact your veterinarian if you observe asymmetric weakness, stumbling or abnormal foot placement, or any area of rapid muscle atrophy. Do not attempt to diagnose EPM from a blood test alone. A full neurological exam, ruling out other causes, and potentially cerebrospinal fluid testing are required. Early treatment significantly improves the odds of recovery. If your horse is a performance horse on an active schedule and shows any combination of the above signs, do not wait.
The Takeaway
EPM gets blamed for a lot in the performance horse world. Sometimes it earns that blame. Most of the time, it does not.
Exposure is common. Disease is not. The difference between a horse that gets exposed and one that develops clinical EPM may come down to the immune system holding the line.
Secure the feed room. Support the gut. Know what to watch for. And when the signs that matter show up, get your veterinarian involved early.
Shop Complete Gut Protection here.
Listen to the full EPM breakdown with Dr. Rob Franklin below.
At Four Sixes Equine Supplements, we focus on the whole horse. The science. What works and stands the test of time, not chasing trends or quick fixes.
Formulated by veterinarians. Trusted on the ranch and in the arena. Built for horses that work.
-The 6666 Equine Supplements Team
References
- Reed SM, Furr M, Howe DK, Johnson AL, MacKay RJ, Morrow JK, Pusterla N, Witonsky S. Equine Protozoal Myeloencephalitis: An Updated Consensus Statement with a Focus on Parasite Biology, Diagnosis, Treatment, and Prevention. J Vet Intern Med. 2016;30(2):491-502.
- MacKay RJ, Howe DK. Equine Protozoal Myeloencephalitis. Vet Clin North Am Equine Pract. 2022;38(2):249-268.
This content is intended for informational purposes only and does not constitute veterinary medical advice, diagnosis, or treatment. Always consult with a licensed veterinarian regarding the health and care of your horse.
