EPM in Horses: Symptoms, Testing and Why Blood Tests Can Mislead

By Dr Duncan Houston

EPM is a real and serious neurologic disease, but it is also one of the easiest equine conditions to overdiagnose.

Equine Protozoal Myeloencephalitis, usually called EPM, is one of the most discussed neurologic diseases in horses in North America. It can cause weakness, incoordination, muscle loss, facial nerve signs, strange lameness patterns, and progressive neurologic decline.

But there is a major diagnostic trap.

Many healthy horses have antibodies to the organisms linked with EPM. That means they have been exposed at some point. It does not automatically mean they have active disease in the brain or spinal cord.

That distinction matters because treating a horse for EPM based on a blood test alone 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 by ingesting feed, hay, pasture, or water contaminated with opossum feces, but horses are dead-end hosts and do not spread EPM to other horses. (MSD Veterinary Manual)

A positive blood test does not confirm EPM. It usually means exposure. Diagnosis should be based on a neurologic examination, compatible clinical signs, exclusion of other diseases, and, where possible, paired serum and cerebrospinal fluid testing to look for evidence of antibody production within the central nervous system.

What Is EPM?

EPM stands for Equine Protozoal Myeloencephalitis.

It is a disease affecting the central nervous system, meaning the brain and spinal cord. Most cases are caused by Sarcocystis neurona, while Neospora hughesi is a less common cause. Because the organisms can affect different areas of the nervous system, the signs can vary widely from horse to horse. (MSD Veterinary Manual)

EPM can look like:

• Hindlimb weakness
• Incoordination
• Muscle wasting
• Facial nerve dysfunction
• Head tilt
• Difficulty swallowing
• Strange or inconsistent lameness
• Poor performance
• Progressive neurologic decline

The clinical challenge is that many of these signs can also be caused by other diseases. MSD Veterinary Manual notes that EPM can mimic other common neurologic diseases and that the most common findings include asymmetric ataxia, weakness, and regional neurogenic muscle atrophy. (MSD Veterinary Manual)

In plain terms: EPM is possible when a horse is neurologic, but it is rarely the only possibility.

How Do Horses Get EPM?

For Sarcocystis neurona, opossums are the definitive host. Opossums shed infective sporocysts in their feces. Horses become exposed when they ingest contaminated feed, hay, water, bedding, or pasture. (MSD Veterinary Manual)

The horse is considered an aberrant or dead-end host. That means horses do not normally form the infectious life stage needed to spread the parasite onward, and they do not pass EPM directly to other horses. (MSD Veterinary Manual)

This matters for prevention.

You do not control EPM by isolating the affected horse like you would with strangles or EHV. You control exposure risk by reducing opossum access to feed, hay, water, and stable areas.

Why EPM Is Overdiagnosed

This is the most important part of the article.

A blood test for EPM detects antibodies. Antibodies mean the horse has been exposed to the organism at some point. They do not prove that the organism is currently causing disease in the brain or spinal cord.

AAEP states clearly that a positive serum test indicates exposure but does not confirm central nervous system infection, regardless of titer size. AAEP also states that screening normal horses with serology is not recommended.

A large seroprevalence study found that 5,250 healthy horses across 18 states had high rates of antibody exposure: 78% were seropositive for S. neurona, 34% were seropositive for N. hughesi, and 31% were seropositive for both. These were healthy horses, which is exactly why a positive blood test alone is so misleading. (PubMed)

Cornell also notes that about 50% of horses in the United States may have been exposed to S. neurona, while fewer than 1% of exposed horses develop clinical EPM. (Cornell Vet College)

That is the diagnostic trap.

A horse can be positive on bloodwork and have neck arthritis.
A horse can be positive on bloodwork and have EHV.
A horse can be positive on bloodwork and have wobblers.
A horse can be positive on bloodwork and have a hindlimb lameness problem.

The test may be real. The diagnosis may still be wrong.

What Are the Symptoms of EPM in Horses?

EPM signs can be subtle or severe. They often affect one side more than the other.

Common signs include:

• Asymmetric ataxia, meaning one side looks more uncoordinated
• Hindlimb weakness
• Stumbling
• Toe dragging
• Abnormal gait
• Unusual or inconsistent lameness
• Muscle wasting, often uneven from side to side
• Loss of topline or hindquarter muscle
• Facial droop
• Drooping eyelid, ear, or lip
• Head tilt
• Difficulty swallowing
• Poor balance
• Reduced skin sensation
• Abnormal sweating
• Behaviour change
• Seizures or collapse in severe cases

University of Minnesota lists incoordination, weakness, abnormal gait, muscle atrophy, drooping eyes, ears or lips, difficulty swallowing, seizures, collapse, abnormal sweating, sensory loss, and head tilt as possible signs. It also warns that vague lameness is sometimes blamed on EPM, but more common lameness causes should be ruled out first. (University of Minnesota Extension)

The key clinical pattern is asymmetry.

A horse with one hindlimb weaker than the other, one-sided muscle wasting, or facial signs on one side may fit EPM better than a horse with vague, symmetrical stiffness.

But asymmetry still does not prove EPM. It only raises suspicion.

EPM Risk Framework

Risk level	What it looks like	What it may mean	What to do
Low risk	Horse is bright, coordinated, no muscle loss, no gait change, no neurologic signs	Exposure may be possible, but disease is not suggested	Do not screen a normal horse with blood testing
Moderate risk	Mild unevenness, vague lameness, subtle weakness, no clear neurologic deficits	EPM is possible, but orthopedic disease may be more likely	Start with a veterinary exam and lameness assessment
High risk	Asymmetric ataxia, weakness, muscle wasting, 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, collapse, inability to swallow, severe weakness, fever with neurologic signs	Serious neurologic disease, trauma, infection, or outbreak disease possible	Urgent veterinary care immediately

The decision point is simple:

A positive blood test does not diagnose EPM. A proper neurologic workup drives the diagnosis.

When Is This an Emergency?

Call your veterinarian urgently if your horse has:

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

EPM is usually not contagious between horses, so multiple horses with fever or neurologic signs should raise concern for other diseases such as EHV, West Nile virus, toxicity, botulism, or other outbreak-style problems. Horses do not shed the EPM organism during or after clinical disease.

Do not assume every neurologic horse has EPM.

That assumption can be dangerous.

How Do Vets Diagnose EPM?

The best diagnosis is not based on one test. It is based on a process.

AAEP states that the most accurate antemortem diagnosis relies on:

• Clinical signs consistent with brain or spinal cord dysfunction
• Exclusion of other potential causes
• Paired serum and CSF testing to assess intrathecal antibody production

Intrathecal antibody production means evidence that antibodies are being produced within the central nervous system, rather than simply circulating in the blood after previous exposure.

A proper EPM workup may include:

• Full physical examination
• Full neurologic examination
• Lameness examination where needed
• Bloodwork
• Serum antibody testing
• Cerebrospinal fluid testing
• Serum:CSF ratio or antibody index
• Imaging if cervical spine disease, trauma, or other causes are suspected
• Testing for EHV, West Nile virus, or other neurologic diseases where appropriate

In practice, the vet is trying to answer three questions:

Is this horse genuinely neurologic?
Does the pattern fit EPM?
Have more likely or more urgent causes been ruled out?

Why Blood Testing Alone Is Not Enough

A serum blood test is useful, but it is limited.

A positive serum result means exposure. It does not confirm active central nervous system disease. MSD Veterinary Manual states that positive or high serum titers have limited diagnostic value because they do not clearly distinguish exposed horses from horses with active clinical EPM. (MSD Veterinary Manual)

A negative serum test is often more useful. AAEP states that a negative serum test generally has a high negative predictive value, meaning it is useful for excluding EPM as the cause of disease. Rarely, a recently infected horse may show signs before antibodies are detectable, so repeat testing 10 to 14 days later may be needed if suspicion remains high.

Owner translation:

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

Blood tests should support the diagnosis. They should not replace clinical reasoning.

Why CSF Testing Is Stronger

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

Testing CSF gives stronger evidence because it is closer to the central 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 due to antibody diffusion across the blood-brain barrier or blood contamination of the sample. The most accurate method is paired serum and CSF quantitative testing with a serum:CSF titer ratio or specific antibody index.

Cornell notes that serum:CSF titer ratios are very predictive, and that ratios below 100 strongly correlate with EPM for certain testing methods, with reported sensitivity of 83% and specificity of 97% for that ratio approach. (Cornell Vet College)

This is why a spinal tap can be worth discussing when the diagnosis is uncertain and treatment decisions are expensive or risky.

It is not because vets want to make things complicated.

It is because guessing wrong can cost the horse time.

What Else Can Look Like EPM?

EPM has a large differential list.

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 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
• Toxicity
• Severe lameness mimicking neurologic weakness
• Back pain or sacroiliac pain
• Peripheral nerve injury
• Neoplasia

MSD Veterinary Manual states that EPM clinical signs can mimic other common neurologic diseases and can be highly variable. (MSD Veterinary Manual)

This is the veterinary judgement that matters:

If the horse has fever, multiple horses are affected, signs appeared suddenly after travel, or the horse has trauma history, EPM should not be the only thing on the list.

EPM vs Wobbler Syndrome

Wobbler syndrome is one of the most common EPM lookalikes.

EPM often causes asymmetric signs, such as one-sided weakness, one-sided muscle atrophy, or cranial nerve deficits.

Wobbler syndrome often causes spinal cord compression in the neck, with incoordination that may affect all four limbs or the hindlimbs more obviously.

Clues that may point more toward cervical spinal cord disease include:

• Young large-breed horse
• More symmetrical ataxia
• Neck stiffness or pain
• Poor proprioception in multiple limbs
• Signs worsened by neck position
• Cervical radiographic or myelographic changes

Clues that may point more toward EPM include:

• Marked left-to-right asymmetry
• Focal muscle loss
• Facial nerve signs
• Head tilt
• Multifocal neurologic signs
• Pattern that does not fit one spinal cord compression site

Neither pattern is perfect. That is why the exam matters.

EPM vs EHV

EHV becomes more concerning when there is:

• Fever
• Recent travel
• Multiple horses with fever
• Rapid onset neurologic signs
• Urine dribbling
• Known outbreak exposure
• Recent show, clinic, sale, or transport history

EPM does not spread horse to horse. EHV can.

So if multiple horses on a property are febrile, weak, wobbly, or neurologic, do not settle on EPM because one horse has a positive blood test. EHV or another infectious outbreak disease may need urgent control.

How Is EPM Treated?

EPM treatment usually involves antiprotozoal medication plus supportive care.

University of Minnesota lists three FDA-approved treatment options:

• Ponazuril, a 28-day paste course
• Diclazuril, a 28-day pelleted product added to feed
• Sulfadiazine and pyrimethamine, an oral liquid suspension commonly used for 90 to 270 days

Supportive nursing care or hospitalization may be needed in more severe cases. (University of Minnesota Extension)

Supportive care may include:

• Anti-inflammatory medication when appropriate
• Vitamin E for neurologic support
• Safe stall management
• Controlled rehabilitation
• Physical therapy
• Assistance with feeding or swallowing if affected
• Monitoring for falls or worsening deficits
• Repeat neurologic examinations

Some horses may look worse shortly after treatment begins because medication is killing organisms and inflammation may change. University of Minnesota notes that signs may appear to worsen after initial treatment for this reason. (University of Minnesota Extension)

That does not mean owners should ignore worsening signs. It means worsening should be discussed with the treating veterinarian.

Can Horses Recover From EPM?

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

University of Minnesota reports that about 60 to 70% of treated horses improve, but only about 15 to 25% recover completely. It also reports relapse within two years in about 10 to 20% of cases, and notes that early treatment 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
• Diagnosis is accurate
• Rehabilitation is careful
• No major concurrent disease is present

Prognosis is more guarded when:

• The horse is recumbent
• Signs are severe
• Swallowing is affected
• Muscle loss is advanced
• Signs have been present for a long time
• The horse relapses
• Another neurologic disease is also 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 vague lameness

Do not jump straight to EPM.

Book a veterinary exam and lameness assessment. University of Minnesota specifically warns that vague lameness is sometimes blamed on EPM, but more common causes of lameness should be ruled out first. (University of Minnesota Extension)

If your horse has clear neurologic signs

Call your veterinarian.

Useful details to record:

• Which limb looks abnormal
• Whether signs are worse on one side
• 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
• Whether there has been recent travel or trauma

If your horse is unsafe to move

Do not force exercise, lunging, or trailer travel.

Keep the horse in a safe area with good footing, minimal obstacles, and calm handling until your veterinarian advises what to do next.

If testing is being discussed

Ask your vet:

• Does the neurologic exam fit EPM?
• Are we testing serum only or serum plus CSF?
• What diseases need to be ruled out first?
• Would a positive blood test actually change the plan?
• Would a negative blood test make EPM unlikely?
• Is the horse stable enough for CSF collection?

If treatment starts

Track changes carefully.

Monitor:

• Gait
• Strength
• Ataxia
• Muscle loss
• Appetite
• Swallowing
• Behaviour
• Falls or near falls
• Response during the first 1 to 4 weeks
• Any worsening after starting medication

Do not stop treatment early because the horse looks better after a few days. Also do not ignore worsening. EPM cases need close communication with the treating vet.

Common Mistakes With EPM

Mistake 1: Diagnosing EPM from a blood test alone

A positive serum test means exposure, not confirmed central nervous system disease. This is the most common overdiagnosis trap.

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, trauma, and wobblers

A neurologic horse needs a full differential list. A positive blood test should not shut down the rest of the workup.

Mistake 4: Waiting too long when signs are clearly neurologic

Early treatment gives the best chance of improvement. University of Minnesota notes that the best results occur when treatment starts early. (University of Minnesota Extension)

Mistake 5: Assuming treatment guarantees a full cure

Many horses improve, but complete recovery is less common and relapse can occur. (University of Minnesota Extension)

Mistake 6: Ignoring opossum access to feed and water

Prevention focuses on reducing contamination of feed and water by opossum feces. Feed storage and barn hygiene matter. (University of Minnesota Extension)

Mistake 7: Using random or unapproved treatments

EPM medication should be prescribed and monitored by a veterinarian. The wrong drug, dose, duration, or diagnosis can create expensive failure.

How To Reduce the Risk of EPM

You cannot eliminate every wildlife exposure. Opossums are part of the ecosystem.

The realistic goal is to reduce contamination of feed, water, and stable areas.

Practical steps include:

• Store grain in animal-proof containers
• Keep feed rooms clean and closed
• Clean up spilled grain quickly
• Avoid leaving pet food in barn areas
• Secure garbage bins
• Keep hay stored where wildlife access is limited
• Reduce clutter where wildlife can hide
• Check feed rooms for droppings or chewed material
• Block obvious wildlife entry points
• Clean water troughs regularly
• Avoid feeding directly on contaminated ground where practical
• Keep barn cat and dog food away from horse feed areas

University of Minnesota recommends making the farm less attractive to opossums by securing feed, keeping facilities tidy, checking for wildlife signs, blocking entry points, securing garbage, and avoiding pet food near horse feeding areas. (University of Minnesota Extension)

This is not about declaring war on opossums.

It is about not letting them treat the feed room like a 24-hour buffet.

Myth vs Reality

Myth	Reality
“A positive blood test means my horse has EPM.”	A positive serum test means exposure. It does not prove active disease.
“EPM spreads from horse to horse.”	Horses are dead-end hosts and do not spread EPM to other horses.
“Any strange lameness is probably EPM.”	Orthopedic causes are often more likely and should be ruled out.
“CSF testing is unnecessary.”	Paired serum and CSF testing gives stronger evidence than serum alone.
“Treatment always cures EPM.”	Many horses improve, but complete recovery is less common and relapse can occur.
“A negative blood test is useless.”	A negative serum test is often useful because it makes EPM less likely.

Frequently Asked Questions

Does a positive blood test mean my horse has EPM?

No. A positive blood test means your horse has been exposed to the organism. It does not prove the parasite is causing disease in the brain or spinal cord. AAEP and MSD both warn that serum positivity alone does not confirm active EPM.

Can horses give EPM to other horses?

No. Horses are dead-end hosts and do not transmit EPM directly to other horses. Exposure usually occurs when horses ingest feed, water, hay, or pasture contaminated with opossum feces. (MSD Veterinary Manual)

What is the best test for EPM?

The strongest practical approach is a neurologic examination plus paired serum and CSF testing to calculate a serum:CSF ratio or antibody index. This helps determine whether antibodies are being produced within the central nervous system.

Can EPM be cured?

Some horses recover well, especially with early treatment, but not all return completely to normal. University of Minnesota reports that about 60 to 70% improve, 15 to 25% recover completely, and 10 to 20% may relapse within two years. (University of Minnesota Extension)

Should I treat for EPM without CSF testing?

Sometimes treatment may begin when the neurologic signs strongly fit EPM and other urgent diseases have been considered, but this should be a veterinary decision. Treating based on bloodwork alone risks missing other serious conditions.

The Bottom Line

EPM is real, serious, and worth treating early when the diagnosis fits.

But it is also one of the most overdiagnosed neurologic diseases in horses.

The biggest mistake is treating a positive blood test instead of diagnosing the horse in front of you. Many healthy horses have antibodies. Very few exposed horses develop clinical disease. That means the neurologic examination, pattern of signs, differential diagnoses, and serum plus CSF interpretation matter far more than a blood result by itself.

Think about EPM when a horse has asymmetric ataxia, weakness, focal muscle wasting, 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 goal is not just to treat EPM.

The goal is to make sure EPM is actually the disease you are treating.

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If you are unsure whether your horse’s weakness, unusual gait, muscle loss, facial changes, 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.