EPM in Horses: Symptoms, Diagnosis, and Why It’s Often Misdiagnosed

By Dr Duncan Houston

Equine Protozoal Myeloencephalitis, or EPM, is one of the most talked-about neurologic diseases in horses.

It is also one of the most misunderstood.

Many horses are tested. Many test positive. Far fewer actually have the disease.

That gap between exposure and true disease is where most mistakes happen. Horses get labeled with EPM based on a blood test alone, while the real cause of their problem is missed.

This is not just a diagnostic detail. It directly affects treatment decisions, cost, and outcomes.

Quick Answer

EPM is a neurologic disease caused by protozoa that infect the brain and spinal cord. It typically causes asymmetric weakness, incoordination, and muscle loss. Diagnosis cannot be made from a blood test alone because many healthy horses test positive. Accurate diagnosis requires a neurological exam, appropriate testing such as CSF analysis, and ruling out other causes of neurologic disease before starting treatment.

What Causes EPM?

EPM is most commonly caused by:

• Sarcocystis neurona

• less commonly Neospora hughesi

Horses become infected by ingesting the organism, usually through feed or water contaminated by opossum feces.

Once inside the body:

• the organism enters the bloodstream

• migrates to the central nervous system

• damages the brain and spinal cord

Important point

Exposure is common. Disease is not.

Why EPM Is So Often Misunderstood

This is the single most important concept.

A large proportion of horses in endemic areas have been exposed to Sarcocystis neurona.

That means:

• many horses will test positive

• most of those horses will never develop EPM

Clinical insight

A positive test shows exposure.
It does not confirm disease.

What Does EPM Actually Look Like?

EPM is a neurologic disease, not a lameness problem, even though it is often mistaken for one.

Hallmark feature

Asymmetry

Signs are often worse on one side of the body.

Common Signs

• incoordination (ataxia)

• abnormal gait

• stumbling

• muscle atrophy, often uneven

• weakness in one limb more than the others

• difficulty turning or backing

• cranial nerve deficits such as:

  • facial asymmetry

  • difficulty swallowing

  • head tilt

Clinical insight

If signs are perfectly symmetrical, EPM becomes less likely.

How Fast Does EPM Progress?

Progression varies.

Early stage

• subtle coordination issues

• mild weakness

• performance changes

Progressive stage

• worsening ataxia

• muscle loss

• more obvious neurologic deficits

Advanced stage

• inability to stand

• severe neurologic dysfunction

Time-based guidance

If neurologic signs are worsening over days to weeks, this should always trigger a full workup, not assumptions.

Why Blood Tests Alone Are Misleading

Blood tests measure antibodies.

They tell you:

• the horse has been exposed

They do not tell you:

• whether the organism is in the central nervous system

• whether the horse has active disease

Clinical reality

Many healthy horses will test positive.

Treating every positive horse would mean treating many horses that do not have EPM.

The More Accurate Approach: CSF Testing

Cerebrospinal fluid (CSF) testing is more useful because it reflects what is happening in the central nervous system.

Best diagnostic approach

• paired serum and CSF testing

• interpretation of the ratio between them

Limitations

• requires a spinal tap

• not always practical in the field

• still not perfect

Clinical takeaway

Diagnosis is never based on one test alone.

What a Proper Diagnosis Looks Like

EPM is a clinical diagnosis supported by testing, not the other way around.

Step 1: Neurological exam

• assess coordination

• evaluate strength

• check cranial nerve function

Step 2: Rule out other causes

• imaging if needed

• bloodwork

• viral testing

Step 3: Targeted testing

• serum and CSF analysis

Clinical insight

If you skip step 2, you risk treating the wrong disease.

Conditions That Mimic EPM

This is where most misdiagnosis occurs.

Common differentials include:

• cervical vertebral malformation (wobbler syndrome)

• equine herpesvirus (EHV-1)

• West Nile virus

• equine encephalitis

• trauma

• metabolic disease

• orthopedic problems presenting as abnormal gait

Decision checkpoint

If a horse is labeled as EPM without ruling these out, the diagnosis is incomplete.

Severity Framework: How Concerning Are the Signs?

Mild

• subtle incoordination

• minimal weakness

• stable signs

Action:
Full workup needed before treatment decisions

Moderate

• obvious gait abnormality

• muscle asymmetry

• worsening signs

Action:
Diagnostic testing and treatment planning

Severe

• significant weakness

• difficulty standing

• cranial nerve involvement

Action:
Urgent veterinary care and aggressive treatment

Critical

• recumbency

• inability to rise

• rapid progression

Action:
Emergency intervention

When Should You Treat?

Treatment should begin when:

• clinical signs are consistent with EPM

• other causes have been reasonably ruled out

• diagnostic evidence supports CNS involvement

Clinical insight

Treating based on a positive blood test alone is poor medicine.

Treatment Options

Common medications include:

• ponazuril

• diclazuril

• sulfonamide and pyrimethamine combinations

Treatment duration typically extends for several weeks.

Supportive care may include:

• anti-inflammatories

• vitamin E supplementation

• rehabilitation

What Outcomes Can You Expect?

Best case

• early diagnosis

• mild signs

• good response to treatment

Outcome:
Return to normal function possible

Moderate case

• more established neurologic damage

Outcome:
Improvement likely, but deficits may remain

Severe case

• advanced neurologic disease

Outcome:
Guarded to poor

Relapse risk

Relapse can occur, especially under stress.

Common Mistakes Owners Make

• assuming a positive blood test equals EPM

• treating without a full workup

• ignoring asymmetry as a key clue

• missing other neurologic diseases

• delaying diagnosis

The biggest mistake is skipping clinical reasoning.

What Should You Do Next?

If your horse shows neurologic signs:

1. Treat it as a neurologic problem first, not a specific disease

2. Arrange a veterinary exam promptly

3. Do not rely on blood tests alone

4. Ask what has been ruled out, not just what has been found

5. Start treatment only when the diagnosis makes sense

Frequently Asked Questions

Can a horse test positive and not have EPM?

Yes. This is very common.

Is EPM the most common cause of neurologic signs?

No. It is one of several important causes.

Can horses recover?

Many improve with treatment, especially if diagnosed early.

Should every positive horse be treated?

No. Treatment should be based on clinical signs and diagnostics.

What matters most?

Correct diagnosis before treatment.

Final Thoughts

EPM is a real disease, but it is also an easy diagnosis to misuse.

The problem is not that it is rare.
The problem is that it is over-attributed.

The most important step is not testing.
It is thinking.

If the clinical picture fits, investigate properly.
If it does not, keep looking.

That is what protects the horse.

If you are unsure whether your horse’s signs are consistent with EPM or want help interpreting test results and deciding on treatment, ASK A VET™ can help guide a clear and accurate next step.