EPM in Horses: Symptoms, Testing and Treatment

By Dr Duncan Houston

EPM can cause serious neurologic signs in horses, but it is also one of the easiest diseases to overdiagnose if testing is used the wrong way.

Equine Protozoal Myeloencephalitis, usually called EPM, is one of the most discussed neurologic diseases in horses in North America.

It deserves attention because it can cause weakness, incoordination, muscle loss, cranial nerve signs, strange lameness patterns and progressive neurologic decline. But it also deserves caution because many healthy horses have been exposed to the organisms associated with EPM and may test positive on bloodwork without actually having active disease.

That is the clinical trap.

A positive blood test does not automatically mean EPM. A wobbly horse does not automatically mean EPM. And treating every suspicious horse without ruling out other causes can waste time, money and, more importantly, delay the real diagnosis.

Quick Answer

EPM is a neurologic disease of horses caused mainly by Sarcocystis neurona and less commonly by Neospora hughesi. Horses usually become exposed to S. neurona by ingesting feed, forage or water contaminated with opossum feces, but horses are dead-end hosts and do not spread EPM to other horses. (MSD Veterinary Manual)

EPM should be suspected when a horse has asymmetric neurologic signs such as incoordination, weakness, unusual lameness, muscle atrophy, facial droop, head tilt, difficulty swallowing or sensory changes. Diagnosis should combine a full neurologic exam, exclusion of other diseases and appropriate testing, ideally paired serum and cerebrospinal fluid testing rather than blood testing alone.

What Is EPM?

EPM stands for Equine Protozoal Myeloencephalitis.

It is a disease of the horse’s central nervous system, meaning the brain and spinal cord. Most cases are caused by the protozoal parasite Sarcocystis neurona. A smaller number of cases are associated with Neospora hughesi. (MSD Veterinary Manual)

The parasite can damage different parts of the nervous system, which is why EPM can look different from horse to horse. One horse may show hindlimb incoordination. Another may develop asymmetric muscle wasting. Another may show facial nerve signs, head tilt or difficulty swallowing.

That variability is part of what makes EPM so frustrating.

It can look like a spinal cord disease, a brain disease, a lameness problem, a cranial nerve problem or a vague performance issue. MSD Veterinary Manual describes EPM signs as highly variable and able to mimic other neurologic diseases, with common findings including asymmetric ataxia, limb weakness and regional neurogenic muscle atrophy. (MSD Veterinary Manual)

How Do Horses Get EPM?

For Sarcocystis neurona, opossums are the definitive host. In North America, that usually means the Virginia opossum. Opossums shed infective sporocysts in their feces. Horses become exposed when they ingest contaminated feed, hay, pasture, bedding or water.

The important point is this:

Horses do not transmit EPM to other horses.

They are considered incidental dead-end hosts. That means a horse with EPM is not contagious to the rest of the barn in the way a horse with strangles, influenza or EHV might be.

That changes the prevention strategy.

You do not need to disinfect the stable because the affected horse touched the wall. You need to reduce exposure to opossum feces around feed, water, hay storage and barn areas.

Why EPM Is Often Overdiagnosed

This is one of the most important parts of the article.

Many horses in EPM-endemic regions have been exposed to Sarcocystis neurona, but only a small percentage ever develop clinical disease. UC Davis states that an estimated 50 to 90% of horses in the United States have been exposed, while typically less than 1% develop EPM. (Center for Equine Health)

That means a positive blood test often only tells you one thing:

The horse has been exposed at some point.

It does not prove the organism is currently in the brain or spinal cord.

AAEP states that a positive serum test indicates exposure but does not confirm central nervous system infection, regardless of titer magnitude. It also notes that serologic screening of normal horses is not recommended.

This is where misdiagnosis happens.

A horse is a little uneven behind.
A blood test comes back positive.
Everyone says “EPM.”
The real problem is cervical arthritis, trauma, EHV, West Nile virus, kissing spines, lameness or another neurologic disease.

That is not a harmless mistake. EPM treatment can be expensive, and delayed diagnosis of the real condition can cost the horse valuable time.

What Are the Symptoms of EPM in Horses?

EPM signs depend on which part of the nervous system is affected.

Common signs include:

• Incoordination
• Ataxia, often asymmetric
• Weakness
• Unusual or inconsistent lameness
• Stiff or abnormal gait
• Muscle atrophy, often uneven from side to side
• Loss of muscle along the topline, hindquarters, shoulder or face
• Reduced or absent skin sensation
• Facial droop
• Drooping eyelid, ear or lip
• Head tilt
• Difficulty swallowing
• Abnormal eye movements
• Behaviour or awareness changes
• Seizures or collapse in severe cases
• Signs that worsen with stress or exercise

EDCC lists asymmetric ataxia, weakness, muscle atrophy, unusual lameness, altered sensation, behavioural changes and cranial nerve deficits as possible EPM signs. University of Minnesota also notes that vague lameness can be blamed on EPM, but more common causes of lameness should be ruled out first.

The key pattern vets look for is asymmetry.

EPM often affects one side or one region more than the other. A horse may have a weaker left hind, muscle loss over one gluteal region, facial droop on one side, or uneven neurologic deficits.

That does not prove EPM, but it raises the suspicion.

EPM Risk Framework

Risk level	What it looks like	What it may mean	What to do
Low risk	Horse is sound, bright, no neurologic signs, no muscle loss, no gait change	Exposure may be possible, but disease is not suggested	Do not screen normal horses with blood tests
Moderate risk	Mild unevenness, subtle weakness, vague lameness, no clear neurologic deficits	EPM is possible, but lameness or orthopedic disease may be more likely	Arrange a veterinary exam before testing
High risk	Asymmetric ataxia, weakness, muscle atrophy, facial droop, head tilt, difficulty swallowing	Neurologic disease is likely and EPM is one possible cause	Full neurologic exam and targeted testing
Critical	Rapid worsening, recumbency, seizures, inability to rise, severe dysphagia, severe weakness	Serious neurologic disease, trauma or infection possible	Urgent veterinary care immediately

The decision point is simple:

A positive test does not make the diagnosis. The neurologic exam drives the diagnosis.

When Is This an Emergency?

EPM can be slowly progressive, but any neurologic horse deserves respect.

Call your veterinarian urgently if your horse has:

• Sudden incoordination
• Rapidly worsening weakness
• Difficulty standing
• Recumbency
• Severe head tilt
• Difficulty swallowing
• Feed or water coming from the nostrils
• Facial paralysis
• Seizures
• Collapse
• Severe depression
• Fever with neurologic signs
• Urine dribbling or bladder dysfunction
• Recent trauma with neurologic signs
• Multiple horses with fever or neurologic signs

The most important point is this:

Do not assume EPM if the horse has fever, sudden severe neurologic signs or multiple horses affected.

EPM is not contagious horse to horse, so multiple febrile neurologic horses on one property should push EHV, West Nile virus, toxicities or other outbreak-type problems higher on the list. Horses with EPM are not considered contagious to neighbouring horses.

How Do Vets Diagnose EPM?

The best EPM diagnosis is not one test. It is a process.

AAEP states that the most accurate antemortem diagnosis relies on three criteria: clinical signs consistent with brain or spinal cord dysfunction confirmed by neurologic examination, exclusion of other potential causes, and immunodiagnostic testing on paired serum and cerebrospinal fluid samples to confirm intrathecal antibody production.

That means your vet is asking three questions:

1. Is this horse truly neurologic?

2. Could something else explain the signs better?

3. Do the test results support active central nervous system infection?

That is much stronger than “the blood test was positive.”

Why Blood Testing Alone Is Not Enough

Blood testing is useful, but it has limits.

A positive blood test means the horse has antibodies against the organism. That usually means exposure. It does not prove active EPM. Cornell states that testing of cerebrospinal fluid, with paired serum, is more predictive of active disease than serum testing alone. (Cornell Vet College)

AAEP also states that a negative serum test is usually useful for excluding EPM because it suggests the horse has not been exposed. Rarely, a very recently infected horse may show signs before seroconversion, so repeat testing in 10 to 14 days may be needed if suspicion remains high.

In practice:

• Positive blood test: exposure, not proof
• Negative blood test: EPM is much less likely
• Positive blood test plus convincing neuro signs: EPM becomes more plausible
• Positive blood test plus vague lameness: be careful

Blood tests are tools. They are not fortune tellers.

Why CSF Testing Is Stronger

CSF stands for cerebrospinal fluid. It surrounds the brain and spinal cord.

Testing CSF can provide stronger support for EPM because it looks closer to the actual nervous system. But even CSF testing is not perfect.

AAEP states that a positive CSF test is more likely to correlate with EPM than a positive serum test, but false positives can occur because antibodies may diffuse across the blood-brain barrier or enter the sample through blood contamination. AAEP also states that the most accurate method is quantitative serum and CSF testing with calculation of a serum:CSF titer ratio or specific antibody index.

This is why paired serum and CSF testing matters.

The goal is not just to find antibodies somewhere. The goal is to show evidence that antibodies are being produced within the central nervous system.

That is a much more meaningful clue.

What Else Can Look Like EPM?

This is where veterinary reasoning really matters.

EPM can mimic many neurologic and musculoskeletal diseases. Important rule-outs include:

• Cervical vertebral stenotic myelopathy, often called wobbler syndrome
• Cervical arthritis or spinal cord compression
• Equine herpesvirus myeloencephalopathy
• West Nile virus
• Eastern, Western or Venezuelan equine encephalitis where relevant
• Rabies risk where regionally relevant
• Trauma or vertebral fracture
• Equine degenerative myeloencephalopathy
• Botulism
• Tetanus
• Inner ear or vestibular disease
• Guttural pouch disease affecting cranial nerves
• Brain or spinal cord abscess
• Toxicities
• Severe lameness mimicking neurologic weakness
• Kissing spine or back pain
• Sacroiliac pain
• Peripheral nerve injury
• Neoplasia

MSD Veterinary Manual notes that EPM can mimic other common neurologic diseases, and EDCC notes that clinical signs can mimic most neurologic diseases as well as musculoskeletal problems. (MSD Veterinary Manual)

This is why I do not like diagnosing EPM from blood titers alone.

If the horse has neck pain, symmetric ataxia, fever, multiple affected horses, acute collapse, recent trauma, or signs that fit another disease better, those need to be worked through properly.

EPM vs Wobbler Syndrome

This is one of the most common diagnostic crossroads.

EPM often causes asymmetric deficits. Wobbler syndrome usually causes spinal cord compression in the neck and may produce more symmetric incoordination, although real cases are not always textbook.

Clues that may push suspicion toward cervical spinal cord disease include:

• Young large breed horse
• Symmetric hindlimb ataxia
• Neck pain or stiffness
• Worsening when the neck is flexed or extended
• Poor proprioception in all limbs
• Radiographic or myelographic cervical abnormalities

Clues that may push suspicion toward EPM include:

• Marked left to right asymmetry
• Focal muscle atrophy
• Cranial nerve signs
• Multifocal neurologic deficits
• A pattern not easily explained by one spinal cord compression site

Neither pattern is perfect. This is why exam quality matters more than guessing.

EPM vs EHV

EHV becomes more concerning when there is fever, recent travel, multiple horses with fever, rapid onset neurologic signs, urine dribbling, or an outbreak situation.

EPM is not contagious horse to horse. EHV can be.

So if several horses on one property develop fever or neurologic signs close together, do not just think EPM. Think infectious outbreak until proven otherwise.

This is where the wrong assumption can have major consequences.

How Is EPM Treated?

EPM treatment usually involves antiprotozoal medication plus supportive care.

The three commonly recognised FDA-approved treatment options in the United States are:

• Ponazuril
• Diclazuril
• Sulfadiazine and pyrimethamine

University of Minnesota lists these three FDA-approved EPM treatments and describes ponazuril and diclazuril as 28-day courses, while sulfadiazine and pyrimethamine is usually given for 90 to 270 days. (University of Minnesota Extension)

Ponazuril

Ponazuril is an oral paste indicated for EPM caused by Sarcocystis neurona. FDA documentation for Marquis states that treatment is given at 5 mg/kg daily for 28 days. (Animal Drugs at FDA)

A loading dose may be used by some veterinarians depending on the case and product protocol, but this should be directed by the treating vet.

Diclazuril

Diclazuril is available as pelleted top-dress treatment. The DailyMed label for Protazil states it is indicated for EPM caused by Sarcocystis neurona and is administered at 1 mg/kg once daily for 28 days. (DailyMed)

This can be easier for some horses because it is added to feed, but correct dosing still depends on accurate weight.

Sulfadiazine and pyrimethamine

ReBalance contains sulfadiazine and pyrimethamine and is indicated for EPM caused by Sarcocystis neurona. Its label states the usual treatment regimen ranges from 90 to 270 days and warns about possible bone marrow suppression, so CBC monitoring is recommended during treatment. (DailyMed)

This is a longer treatment course and requires careful veterinary monitoring.

Supportive Care for EPM

Medication targets the protozoal organism, but supportive care helps protect the horse during neurologic recovery.

Supportive care may include:

• Anti-inflammatory medication when appropriate
• Vitamin E for neurologic support
• Careful exercise restriction or controlled rehab
• Physical therapy
• Safe stable management to prevent falls
• Assisted feeding if swallowing is affected
• Monitoring hydration and manure output
• Repeat neurologic exams
• Adjusted workload during recovery

Some horses appear worse shortly after starting treatment. University of Minnesota notes that signs may worsen early in treatment because medication is killing the parasites causing disease. The ReBalance label also describes possible worsening neurologic deficits during treatment, likely due to inflammatory reaction to dying parasites in central nervous system tissue. (University of Minnesota Extension)

This does not mean owners should ignore worsening signs. It means worsening needs to be discussed with the treating veterinarian, not managed by stopping or changing medication randomly.

Can Horses Recover From EPM?

Yes, many horses improve with treatment, especially when the disease is recognised early.

EDCC reports that 60 to 70% of aggressively treated EPM cases show significant or complete reversal of symptoms, and many horses return to normal activity. It also reports relapse in approximately 10 to 20% of cases.

University of Minnesota gives a more cautious owner-facing outcome summary: about 60 to 70% improve, but only 15 to 25% recover completely, with relapse within two years in about 10 to 20% of cases. It also notes that starting treatment early gives the best results. (University of Minnesota Extension)

Prognosis is better when:

• Signs are mild
• Treatment starts early
• The horse remains standing
• Muscle atrophy is limited
• The diagnosis is accurate
• Rehabilitation is well managed
• The horse has no major concurrent disease

Prognosis is more guarded when:

• The horse is recumbent
• Signs are severe
• Dysphagia is present
• Muscle loss is advanced
• Signs have been present for a long time
• The horse relapses
• Another neurologic disease is present

The honest answer is this: EPM is treatable, but not every horse returns fully to previous performance.

What Should You Do Right Now?

If your horse has mild vague lameness

Do not jump straight to EPM.

Start with a proper veterinary examination and lameness assessment. University of Minnesota specifically warns that vague lameness may be blamed on EPM, but more common lameness causes should be thoroughly ruled out first. (University of Minnesota Extension)

If your horse has clear neurologic signs

Call your veterinarian and describe exactly what you see.

Useful details include:

• Which limb looks abnormal
• Whether signs are worse on one side
• Whether the horse is weak or uncoordinated
• Whether the horse can turn safely
• Whether muscle loss is present
• Whether the face is affected
• Whether swallowing is normal
• Whether the horse has fever
• Whether signs are worsening
• Whether other horses are affected

If your horse is unsafe to move

Do not force exercise, trailer travel or lunging.

Keep the horse in a safe, quiet area with good footing and minimal obstacles. A neurologic horse can fall, injure itself or injure handlers.

If testing is being discussed

Ask your vet what question the test is answering.

Good questions include:

• Does the neurologic exam fit EPM?
• Are we testing serum only or paired serum and CSF?
• What diseases are we ruling out first?
• Would a positive result change treatment?
• Would a negative result make EPM unlikely?
• Is the horse stable enough for CSF collection?

If treatment starts

Track changes carefully.

Monitor:

• Gait
• Strength
• Ataxia grade
• Muscle atrophy
• Appetite
• Ability to swallow
• Behaviour
• Falls or near falls
• Response during the first one to four weeks
• Any worsening after starting medication

Do not judge the entire outcome after three days. But also do not ignore worsening. Neurologic cases need close communication with the vet.

Common Mistakes With EPM

Mistake 1: Diagnosing EPM from a blood test alone

A positive serum test indicates exposure, not central nervous system infection. AAEP is very clear on this point.

Mistake 2: Treating vague lameness as EPM

Many horses with subtle unevenness have orthopedic pain, not protozoal neurologic disease.

Mistake 3: Forgetting EHV, West Nile virus and trauma

A febrile, acutely neurologic horse needs broader thinking. EPM is only one possibility.

Mistake 4: Waiting too long when signs are clearly neurologic

Early treatment improves the chance of recovery. University of Minnesota notes that early treatment gives the best results. (University of Minnesota Extension)

Mistake 5: Assuming a treated horse is cured forever

Relapse can occur. EDCC and University of Minnesota both report relapse in approximately 10 to 20% of cases.

Mistake 6: Ignoring barn hygiene and wildlife access

Prevention should focus on reducing opossum access to feed and water. EDCC recommends keeping grains in rodent-proof containers and forage in enclosed facilities.

Mistake 7: Using unapproved or random treatments

EPM treatment should be directed by a veterinarian. Correct drug choice, duration and monitoring matter.

How To Reduce the Risk of EPM

You cannot eliminate every wildlife exposure, and opossums are part of the ecosystem. The practical goal is to make your horse’s feed and water less likely to be contaminated.

Prevention steps include:

• Store grain in animal-proof containers
• Keep feed rooms clean and closed
• Clean up spilled grain quickly
• Avoid leaving pet food out in barn areas
• Secure garbage bins
• Keep hay and forage stored where wildlife access is limited
• Reduce clutter where wildlife can hide
• Check feed rooms for signs of wildlife access
• Prevent opossums from entering feed storage areas
• Clean water troughs regularly
• Avoid feeding directly on contaminated ground where practical

University of Minnesota recommends making the farm less attractive to opossums by keeping feed enclosed, improving sanitation, checking facilities for wildlife signs, securing garbage and avoiding pet food around horse feeding areas. (University of Minnesota Extension)

EDCC also notes that prophylactic antiprotozoal medication may be considered for selected high-risk horses, but this should be prescribed by a veterinarian.

That is not a green light to medicate every horse on the farm.

It is a risk-based decision for specific situations.

Myth vs Reality

Myth	Reality
“A positive blood test means my horse has EPM.”	A positive serum test means exposure, not proof of active central nervous system infection.
“EPM spreads from horse to horse.”	Horses are dead-end hosts and do not transmit EPM to other horses.
“Any weird lameness is probably EPM.”	Lameness is common. Orthopedic causes should be ruled out before blaming EPM.
“A negative blood test is useless.”	A negative serum test is actually useful because it makes EPM less likely in most cases.
“Treatment always cures EPM completely.”	Many horses improve, but complete recovery is less common and relapse can occur.
“Opossums should be blamed for everything.”	Opossums are the definitive host, but practical prevention is about protecting feed and water from fecal contamination.

Frequently Asked Questions

Can horses give EPM to other horses?

No. Horses are dead-end hosts and do not transmit EPM to other horses. Exposure usually occurs when horses ingest feed, water or forage contaminated with opossum feces containing infective sporocysts.

Does a positive blood test mean my horse has EPM?

No. A positive blood test means the horse has been exposed to the organism. It does not prove that the parasite is causing disease in the brain or spinal cord. AAEP states that positive serum testing does not confirm central nervous system infection.

What is the best test for EPM?

The strongest antemortem approach combines a neurologic examination, exclusion of other diseases and paired serum plus cerebrospinal fluid testing to assess antibody production in the central nervous system. Cornell notes that CSF testing with paired serum is more predictive of active disease than serum alone. (Cornell Vet College)

Can EPM be cured?

Many horses improve with treatment, especially when treated early. EDCC reports that 60 to 70% of aggressively treated cases show significant or complete reversal of signs, but relapse may occur in 10 to 20% of horses.

Should I treat for EPM without testing?

Sometimes treatment may begin when the neurologic exam strongly supports EPM and other urgent diseases have been considered, but this should be a veterinary decision. Treating based on a positive blood test alone risks missing other serious causes of neurologic signs.

The Bottom Line

EPM is real. It is serious. And it can absolutely change a horse’s life.

But it is also one of the easiest equine neurologic diseases to overcall.

The biggest mistake is treating a positive blood test instead of treating the horse in front of you. The diagnosis should start with a careful neurologic exam, then rule out other diseases, then use the right testing to support or weaken the suspicion.

Think of EPM when a horse has asymmetric ataxia, weakness, focal muscle loss, facial nerve signs, head tilt, difficulty swallowing or progressive neurologic deficits.

Be cautious when the only evidence is vague lameness and a positive blood test.

Act urgently if the horse is rapidly worsening, recumbent, unable to swallow, febrile with neurologic signs, or unsafe to move.

The best outcome comes from getting the diagnosis right early, starting appropriate treatment when justified, managing recovery carefully, and reducing opossum fecal contamination around feed and water.

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If you are unsure whether your horse’s weakness, unusual gait, muscle loss or neurologic signs could be EPM or something else, ASK A VET™ can help you organise the signs, track progression and decide when veterinary care should not wait.