EPM Is Rising. Here's What Every Horse Owner Needs to Know.

EPM Is Rising. Here's What Every Horse Owner Needs to Know.

What the Science Says About Exposure, Risk, and How to Protect Your Horse

KEY TAKEAWAYS

• EPM is caused by Sarcocystis neurona, a protozoan carried and shed by opossums, and horses become infected by ingesting contaminated feed, hay, or water¹
• An estimated 50 to 90% of U.S. horses have been exposed to S. neurona, but fewer than 1% develop clinical disease¹
• Exposure does not equal EPM. The immune system plays a critical role in whether an exposed horse develops neurologic signs²
• Clinical cases rise in spring and summer and peak into fall, correlating with increased opossum activity and high-stress performance seasons
• Risk factors for developing clinical EPM include the presence of opossums, prior stressful health events, heavy exercise, hauling, surgery, injury, and foaling³
• Signs are often subtle before they become obvious. Knowing your horse's normal is your most important diagnostic tool
• EPM requires proper diagnosis and FDA-approved antiprotozoal treatment. Supportive management does not replace a veterinary treatment plan

QUICK ANSWER

EPM is a neurologic disease caused most commonly by Sarcocystis neurona, a protozoan carried by opossums and shed in their feces. Horses become infected when they ingest sporocysts from contaminated feed, hay, pasture, or water. Exposure is common across the U.S., but clinical disease is not. The horse's immune system is thought to be the key variable between exposure and disease. When something compromises that immune defense, whether by stress, illness, heavy workload, or other factors, the organism can reach the central nervous system and produce neurologic signs. Understanding the difference between exposure and disease, recognizing early signs, and keeping your horse's immune system and gut health supported year-round are the most important things a horse owner can do.

Three Letters No Horse Owner Wants to Hear

It does not matter how long you have been around horses. EPM stops you cold.

The signs can be easy to miss at first. A horse that seems a little off. A subtle change in gait. A muscle that looks different on one side. By the time it becomes obvious, the disease has often been progressing for a while.

Clinical cases of EPM rise in spring and summer and peak into fall, correlating directly with increased opossum activity and the stress that comes with hauling, showing, and pushing horses hard. That does not mean your horse will get EPM. But it does mean right now is the time to understand it.

What Is EPM?

Equine Protozoal Myeloencephalitis is a neurologic disease caused most commonly by Sarcocystis neurona, an organism carried by the Virginia opossum (Didelphis virginiana).¹ Opossums are the definitive host. They harbor the organism in their intestines and shed the infective sporocysts into the environment through their feces.

Horses become infected when they ingest those sporocysts from contaminated feed, hay, pasture, or water. Once inside the horse, the organism can migrate to the central nervous system, where inflammation and damage produce neurologic signs.

Here is the distinction that matters most: an estimated 50 to 90% of U.S. horses have been exposed to the organism, but only a small percentage develop clinical EPM.¹ Clinical manifestation occurs in fewer than 1% of infected horses.²

Exposure does not equal EPM. EPM is not contagious from horse to horse.

Why Some Horses Develop EPM and Others Do Not

This is the question the research is still working to answer fully.

Risk factors for the development of EPM include the presence of opossums and prior stressful health-related events.³ Horses under higher levels of stress or strain, including pregnancy, training, and competition, appear to be at higher risk of disease development.³

The working theory is that the horse's immune system is the primary gatekeeper. A horse with a strong, stable immune response is better equipped to keep the organism from reaching the central nervous system. When that defense is compromised, whether by stress, illness, heavy workload, or other factors, the organism has an opening.

Factors governing the occurrence of EPM are largely unknown, although horse immunity is thought to contribute to EPM pathogenesis.²

This is why immune support is not a peripheral conversation when it comes to EPM. It is central to it.

What to Watch For

Signs vary depending on where in the nervous system the damage occurs. They can be subtle long before they become obvious.

Inspect your horses and pay close attention to:

• Asymmetric muscle loss or weakness, particularly on one side of the body
• Incoordination or ataxia, stumbling, or dragging toes
• Changes in gait, stride length, or performance quality
• Difficulty backing or navigating inclines or uneven ground
• Changes in balance or proprioception
• Head tilt, facial asymmetry, or difficulty swallowing in more advanced cases
• Unexplained changes in attitude or willingness to work

These signs can mimic other neurologic and musculoskeletal conditions. That is why diagnosis requires a thorough neurologic exam, ruling out other causes, and antibody testing of blood and in some cases cerebrospinal fluid.

A positive blood test alone does not mean a horse has EPM. Exposure is common. Clinical disease is not. Context matters.

If you suspect anything neurologic, get your veterinarian involved early. With neurologic disease, timing matters more than most people realize.

Exposure vs. Clinical Disease: At a Glance

What You Can Do

You cannot eliminate opossum exposure. You can reduce the conditions that allow disease to develop and catch problems early when they do.

1. Know your horse's normal.

This is the most important thing on this list. Know how they walk, back, turn, and place their feet. Know what their muscle symmetry looks like on both sides. Noticing something is not quite right may be the first step toward getting your veterinarian involved before things progress.

2. Protect feed and water from wildlife.

Lock up feed rooms and clean up spilled grain promptly. Cover stored hay and discard old or spoiled feed. Implement pest control to deter opossums from your barn. Provide clean, fresh water and clean tanks regularly. Contamination happens at the source.

3. Support the immune system year-round.

A large portion of the immune system lives in the gut. Immune cells line the gut wall, interact with beneficial microbes, and help the body respond normally to environmental challenges. A horse with a stable gut environment is better equipped to maintain that immune defense year-round.

Probiotic Pellets support normal gut microbiome balance and immune function daily, providing Saccharomyces boulardii, prebiotics, and L-glutamine to help maintain the gut environment that supports normal immune signaling.

4. Be intentional around high-stress periods.

Heavy exercise, hauling, surgery, injury, and foaling are all identified risk factors for EPM development in exposed horses.³ These are the moments to be most attentive to subtle neurologic changes and to have immune and gut support in place going in.

Keep Equine Probiotic Paste on hand for concentrated support when stress hits fast. It delivers the same core ingredients as the daily pellets in a concentrated form, useful during hauling, shows, and any period of acute stress.

5. Stay current on routine care.

Regular veterinary wellness checks, routine deworming based on fecal egg counts, and appropriate vaccinations all contribute to a horse whose immune system is better equipped to handle environmental exposure. A horse carrying a heavy parasite burden is already starting behind.

6. Support antioxidant status.

Vitamin E plays an important neuroprotective role and is commonly recommended in neurologic cases. Work with your veterinarian on dosing specific to your horse's situation. Medical Muscle provides natural Vitamin E alongside astaxanthin to support normal antioxidant balance in nervous and muscular tissue. It does not treat EPM, but for horses under neurologic stress, supporting that antioxidant environment is part of sound management.

7. If your horse is being treated for EPM, increase gut protection.

Many antiprotozoal medications used to treat EPM, including sulfonamides and pyrimethamine, can cause gastrointestinal upset. Keeping the horse eating and the gut stable during treatment is critical and often overlooked. Extra Strength Probiotic Pellets provide a higher concentration of support to help maintain normal microbial balance during and after treatment.

Actionable Checklist: Protecting Your Horse From EPM Risk

✓ Know your horse's baseline. Gait, muscle symmetry, balance, attitude

✓ Secure feed rooms and water sources from wildlife access

✓ Clean up spilled grain and discard old or spoiled feed promptly

✓ Keep water tanks clean and refresh regularly

✓ Maintain daily gut and immune support year-round

✓ Increase gut support concentration during high-stress periods before they hit

✓ Stay current on deworming based on fecal egg counts

✓ Keep up with routine wellness exams and appropriate vaccinations

✓ If you notice any subtle neurologic change, call your veterinarian early

✓ If your horse is on EPM treatment, prioritize gut support alongside it

We Asked the Vets: Your EPM Questions Answered

What is EPM and how does a horse get it?
EPM is a neurologic disease caused most commonly by Sarcocystis neurona, a protozoan carried by opossums. Horses become infected by ingesting sporocysts shed in opossum feces through contaminated feed, hay, or water. Once inside the horse, the organism can migrate to the central nervous system and produce neurologic damage.

My horse tested positive for EPM antibodies. Does that mean they have EPM?
Not necessarily. A positive blood test indicates exposure to S. neurona, which is extremely common. An estimated 50 to 90% of U.S. horses have been exposed.¹ Diagnosis of clinical EPM requires neurologic signs, a thorough exam, and in many cases cerebrospinal fluid testing alongside blood. A positive blood test alone is not sufficient for diagnosis.

What does EPM look like?
Signs vary depending on where in the nervous system damage occurs. Common signs include asymmetric muscle loss or weakness, ataxia or incoordination, stumbling, dragging toes, changes in gait, difficulty backing, and in more advanced cases facial nerve abnormalities or difficulty swallowing. Signs can be subtle early on and are often mistaken for other conditions.

What are the risk factors for developing clinical EPM?
Exposure to opossum-contaminated environments is the baseline risk. Beyond that, prior stressful health events, heavy exercise, hauling, surgery, injury, foaling, and anything that challenges the immune system are identified risk factors for development of clinical disease in exposed horses.³

Is EPM contagious between horses?
No. EPM is not transmitted horse to horse. Infection comes from environmental exposure to opossum feces.

What does EPM treatment look like?
EPM requires proper diagnosis and treatment with FDA-approved antiprotozoal medications. Three are currently approved: ponazuril (Marquis), diclazuril (Protazil), and sulfadiazine/pyrimethamine (Re-Balance). Approximately 57 to 62% of treated horses show improvement.⁴ Treatment should always be directed by a veterinarian.

Can gut health really affect EPM risk?
The gut houses a significant portion of the immune system. Immune cells in the gut wall interact constantly with beneficial microbes and help regulate normal immune signaling. A stable gut environment supports the immune response that is thought to be the primary factor between exposure and disease. While no supplement prevents or treats EPM, maintaining gut and immune health year-round is a sound part of any management program.

What should I do if I suspect EPM?
Call your veterinarian. Do not wait to see if it resolves. With neurologic disease, early intervention produces meaningfully better outcomes than delayed treatment.

When to Call Your Veterinarian

Call your veterinarian immediately if your horse shows any unexplained change in gait, muscle symmetry, balance, or coordination. Subtle signs in a horse with known opossum exposure warrant evaluation. Do not attempt to diagnose or treat EPM on your own.

A positive blood antibody test alone is not a diagnosis. Your veterinarian will perform a neurologic exam and determine whether further testing, including cerebrospinal fluid analysis, is appropriate.

EPM requires a veterinary diagnosis and FDA-approved treatment. The management habits in this blog support the environment your horse lives in. They do not replace a veterinary treatment plan.

The Takeaway

Exposure is common. Disease is not. But when neurologic signs appear, time matters.

Know your horse's normal. Protect their feed and water. Support their immune system and gut health year-round. And if something looks off, call your veterinarian.

Shop gut and immune support here.

At FullBucket, our horses mean everything to us. If you have questions, just reply to this email. Every message is read by our veterinarian founders.

— The FullBucket Team