Equine Protozoal Myeloencephalitis (EPM) in Horses by a Vet – 2025 Diagnosis & Prevention Guide 🧠🐎

By Dr Duncan Houston, BVSc

Introduction

Equine Protozoal Myeloencephalitis (EPM) is a serious neurologic disease affecting horses in North America. However, in 2025, it's also considered one of the most over-diagnosed equine conditions. With multiple lookalike illnesses, incomplete testing options, and expensive treatment, EPM demands a thoughtful diagnostic process. In this guide, Dr Duncan Houston explains how to distinguish EPM from other diseases, test accurately, treat effectively, and prevent exposure.

What Is EPM? 🦠

• Caused primarily by the protozoa Sarcocystis neurona, occasionally by Neospora hughesi.
• Transmitted via fecal contamination—opossums are the definitive host.
• Horses ingest infective sporocysts while grazing or eating contaminated feed or water.
• Organisms travel to the brain and spinal cord, causing inflammation and nerve damage.

Common Symptoms of EPM 🧠

Clinical signs vary but often include:

• Unusual or inconsistent lameness
• Incoordination (ataxia)
• Muscle atrophy—especially asymmetric
• Facial nerve deficits (drooping eyelids, lips)
• Head tilt, difficulty swallowing, behavioral changes
• Worsening symptoms under stress

Other Conditions That Mimic EPM 🌀

Veterinarians must rule out other neurologic and systemic diseases first:

• Wobbler Syndrome: Cervical spinal cord compression
• Equine Herpesvirus (EHV-1): Neurologic form
• West Nile Virus
• Eastern/Western Encephalitis
• Trauma or vertebral fractures
• Rabies

Diagnostic Testing Options 🔬

1. Blood Testing (Serum Antibody Titers)

• Fast and non-invasive
• Drawback: Many healthy horses test positive due to prior exposure—not current disease
• False positives are common

2. Cerebrospinal Fluid (CSF) Testing

• Provides stronger evidence of CNS infection
• Best approach compares CSF to blood antibody levels (antibody index)
• Drawback: Invasive and can still result in false positives

The Clinical Diagnosis Approach 🧠🩺

• Comprehensive neurological exam by your vet
• Evaluate gait, posture, cranial nerve function, and muscle tone
• Rule out other diseases with imaging, bloodwork, and titers
• If all other diseases are excluded and EPM is likely—treatment may begin without definitive testing

Why Misdiagnosis Is a Problem 💸

• Treatment for EPM is expensive—often $800–$1600 for a 1–2 month course
• Misdiagnosis wastes time and money
• Meanwhile, the real underlying disease (e.g., EHV-1 or trauma) may worsen without appropriate care

Approved EPM Treatments 💊

Antiprotozoal drugs are effective but require prolonged use:

• Marquis® (ponazuril): 28-day treatment with loading dose
• Protazil® (diclazuril): Alfalfa-based pelleted formula—easy to feed
• ReBalance® (sulfadiazine + pyrimethamine): Daily oral liquid—requires longer duration

Supportive therapies may include:

• Anti-inflammatories (e.g., flunixin, dexamethasone)
• Vitamin E for neurological support
• Physical therapy or assisted rehab

Prognosis 📈

• Early treatment offers best results—over 60% of horses improve significantly
• Severely affected horses may recover partially but have permanent deficits
• Relapse is possible—especially in high-stress or immunocompromised horses

Prevention Tips 🛡️

• Keep opossums away from feed and water areas—cover bins, clean spilled grain
• Regularly clean feed rooms, pastures, and barns
• Minimize stress: trailer travel, overtraining, or injury can lower immunity
• Manage turnout to reduce wildlife access

Case Example: Misdiagnosed EPM

1. A horse showed mild incoordination and was treated for EPM based on blood titers alone
2. No improvement after 4 weeks—veterinary neurologist found cervical arthritis compressing spinal cord
3. Correct diagnosis led to anti-inflammatory therapy and exercise restriction—horse improved

FAQs About EPM

Q: Can horses transmit EPM to each other?

A: No. Horses are dead-end hosts. They acquire it from contaminated environments—especially opossum feces.

Q: Is EPM curable?

A: Many horses improve or return to function—but some require long-term support or have lingering deficits.

Q: Should I treat without testing?

A: Always rule out other diseases first. Work with your vet to assess risk, costs, and alternatives.

Conclusion

Equine Protozoal Myeloencephalitis can be devastating—but it’s often confused with other neurological conditions. Accurate diagnosis, appropriate testing, and early treatment improve the odds for recovery. Prevention through wildlife control and stress reduction remains essential, especially in high-risk areas.

Suspect EPM or seeing unexplained lameness or neurologic signs? Reach out to Ask A Vet via AskAVet.com or through our app 📱 for real-time help from Dr Duncan Houston and our veterinary team. 🩺🐴