🧠 Vet’s 2025 Guide to Equine Protozoal Myeloencephalitis (EPM)

By Dr Duncan Houston BVSc

1. Introduction: Why EPM Matters in 2025

Equine protozoal myeloencephalitis (EPM) is one of the most frequently diagnosed neurologic diseases in horses. While many horses are exposed to the parasites behind EPM, a smaller percentage develop clinical disease. As a vet, helping owners recognize early signs and act quickly is essential to protecting neural health and preserving performance. This guide offers an in-depth update for 2025, including causes, diagnostics, treatments, rehab, and prevention.🐴🧬

2. What Causes EPM?

EPM is caused by protozoan parasites: Sarcocystis neurona (most common) and Neospora hughesi :contentReference[oaicite:3]{index=3}. The opossum is the definitive host, shedding infective sporocysts in feces. Horses are incidental hosts and become infected by ingesting contaminated hay, water, or feed :contentReference[oaicite:4]{index=4}. While up to 50–90% of horses show exposure via antibodies, clinical disease occurs in fewer than 1% :contentReference[oaicite:5]{index=5}.

3. How the Parasite Reaches the Brain & Spinal Cord

After ingestion, sporocysts travel through the bloodstream and cross into the central nervous system. The protozoa invade neurons and glial cells, triggering neural damage and inflammation. Clinical signs depend on lesion location :contentReference[oaicite:6]{index=6}.

4. Clinical Signs: The “Three As” and More

• Asymmetry: Uneven gait, weaker on one side :contentReference[oaicite:7]{index=7}.
• Ataxia: Incoordination & stumbling, worsened on slopes or head elevation :contentReference[oaicite:8]{index=8}.
• Atrophy: Muscle wasting over topline or limbs :contentReference[oaicite:9]{index=9}.
• Other signs: facial paralysis, head tilt, difficulty swallowing, seizures, abnormal sweating, sensory deficits :contentReference[oaicite:10]{index=10}.

5. Diagnosis: A Multi‑Step Process

Diagnosing EPM is complex—no single definitive test while alive. Instead veterinarians use:

• Physical & neurologic exam: Detect asymmetry, ataxia, muscle tone changes.
• Ruling out mimics: Such as wobblers, equine herpesvirus, WNV, EEE/WEE, etc. :contentReference[oaicite:11]{index=11}.
• Serum and CSF antibody testing: Evaluates exposure vs. central nervous infection via serum:CSF ratio :contentReference[oaicite:12]{index=12}.
• Advanced imaging/testing: MRI, CT, nuclear scintigraphy, PCR as needed :contentReference[oaicite:13]{index=13}.
• AAEP guidelines: Provide structured diagnostic and treatment recommendations :contentReference[oaicite:14]{index=14}.

6. Treatment Options

6.1 FDA‑approved Antiprotozoals

• Ponazuril (Marquis®): Oral paste daily for 28–90+ days; often gold-standard :contentReference[oaicite:15]{index=15}.
• Diclazuril (Protazil®): Pelleted feed additive, 28‑day course; alternative option :contentReference[oaicite:16]{index=16}.
• ReBalance® (sulfadiazine + pyrimethamine): Oral suspension 90–270 days; used cautiously in breeding mares :contentReference[oaicite:17]{index=17}.

6.2 Adjunctive Therapies

• NSAIDs or corticosteroids to reduce CNS inflammation. Antioxidants like vitamin E may support neural recovery :contentReference[oaicite:18]{index=18}.
• Physical rehab—controlled hand-walking, proprioception exercises, hydrotherapy.
• Rest & stall environment supportive of recovery.

7. Prognosis & Follow‑Up

With timely diagnosis and treatment, 60–80% of horses show significant improvement; only ~25% fully resolve :contentReference[oaicite:19]{index=19}. Relapses occur in ~10–20% within 2–3 years :contentReference[oaicite:20]{index=20}. Careful monitoring, repeat testing, and ongoing support improve long-term outcomes.

8. Prevention: Blocking the Life Cycle

• Secure feed bins & prevent opossum contamination :contentReference[oaicite:21]{index=21}.
• Avoid feeding on ground; cover hay & water troughs :contentReference[oaicite:22]{index=22}.
• Clean up wildlife carcasses; manage opossum populations humanely :contentReference[oaicite:23]{index=23}.
• Maintain overall health: stress reduction, balanced diet, routine vet checks :contentReference[oaicite:24]{index=24}.

9. Case Study: “Echo” Reclaims Her Stride

“Echo,” a 4‑year‑old Thoroughbred mare, developed uneven hindlimb stride and topline atrophy. Serum:CSF testing confirmed EPM. She completed 60‑day ponazuril protocol with NSAIDs, stall rest, and rehabilitation. Within 12 weeks, coordination and topline musculature returned. Careful management prevented relapse.

10. Ask A Vet Support 🩺

At Ask A Vet, we support owners through every step—neurologic evaluation, sample interpretation, treatment recommendations, rehab protocols, and follow‑up testing. If EPM strikes, we’re here to guide and reassure. Download the Ask A Vet app for expert veterinary support anytime, anywhere.

11. Quick Reference Table

12. Final Thoughts

EPM is a serious brain and spinal disease, but with proper diagnosis, treatment, and preventive practices, many horses can make dramatic recoveries. Early recognition and collaboration with your vet—and tools like Ask A Vet—helps secure your horse’s neurological health and lifelong performance. ❤️

Download the Ask A Vet app today for personalised guidance and 24/7 veterinary support through every step of your horse’s EPM journey.