EPM in Horses by a Vet – 2025 Guide to Diagnosis Challenges, Symptoms & Overdiagnosis Risks 🧠🐴
In this article
EPM in Horses by a Vet – 2025 Guide to Diagnosis Challenges, Symptoms & Overdiagnosis Risks 🧠🐴
By Dr Duncan Houston, BVSc
Introduction
Equine protozoal myeloencephalitis (EPM) is the most frequently diagnosed infectious neurological disease in North America, yet it remains one of the most overdiagnosed and misunderstood equine conditions. Caused by two protozoa—Sarcocystis neurona and Neospora hughesi—EPM affects the central nervous system and produces subtle but serious signs, including one-sided incoordination and muscle wasting.
What Causes EPM? 🧬
- Sarcocystis neurona (primary cause)
- Neospora hughesi (less common)
- Transmitted via contaminated feed or water from opossum feces
- Not directly contagious between horses
Key Clinical Signs of EPM ⚠️
- Asymmetrical incoordination (ataxia)—more severe on one side
- Muscle atrophy (wasting), often unilateral
- Gait abnormalities—stumbling, lameness
- Facial nerve paralysis, head tilt, behavior changes (in advanced cases)
Challenges in Diagnosis 🔍
Blood Tests
- Detect antibodies against Sarcocystis or Neospora
- Only show exposure—not confirmation of disease
- Commonly lead to false positives
Important Study Results (5,000 Horses)
- 78% tested positive for Sarcocystis neurona
- 34% positive for Neospora hughesi
- 31% positive for both—but none had symptoms
Why So Many False Positives?
- Many horses develop antibodies but never get sick
- Blood testing is too sensitive for diagnosing active disease
- Misinterpreting results leads to unnecessary treatment
Spinal Fluid Testing (CSF) 🧠
- More accurate than blood tests
- Measures antibody levels in cerebrospinal fluid
- Still not perfect—risk of contamination and sampling difficulties
When Is EPM the Real Diagnosis? ✅
EPM should only be diagnosed when:
- Neurological signs match the typical asymmetrical pattern
- Blood or CSF tests support suspicion
- Other causes are ruled out:
- Wobbler syndrome
- Herpes virus
- West Nile virus
- Trauma or tumors
Why EPM Is Overdiagnosed in 2025 ⚠️
- Symptoms are vague and mimic other diseases
- Many horses are diagnosed and treated solely based on blood tests
- Expensive medications used unnecessarily
Treatment of EPM 💊
- Antiprotozoal medications like:
- Diclazuril
- Ponazuril
- Sulfonamides with pyrimethamine
- NSAIDs and Vitamin E for support
- Early treatment improves success, but relapses can occur
Case Example: Asymmetrical Weakness in a 9-Year-Old Gelding
- Horse showed left hind limb incoordination, dragging toe
- Blood titer high for Sarcocystis neurona
- CSF confirmed active antibodies > diagnosis made
- Treated with ponazuril for 30 days
- Recovered enough for light trail use, but retained mild weakness
FAQs About EPM
Q: Does a positive blood test mean my horse has EPM?
A: No—most horses test positive due to prior exposure. Less than 1% develop disease.
Q: Can horses recover fully?
A: Some do with early treatment; others may retain mild neurological signs permanently.
Q: Should I treat based on a blood test alone?
A: No. Clinical signs and confirmatory CSF results are important before starting treatment.
Conclusion
Equine Protozoal Myeloencephalitis is a complex disease with subtle signs and diagnostic challenges. In 2025, clinical signs and spinal fluid testing remain the most reliable tools to confirm diagnosis. Blood tests alone are misleading and have contributed to overdiagnosis and overtreatment. If your horse shows asymmetrical neurological signs, involve your vet early to pursue appropriate diagnostics and exclude other conditions.
Need help confirming a possible EPM diagnosis or interpreting test results? Visit AskAVet.com or download our app 📱 to consult Dr Duncan Houston for expert guidance. 🩺🐴
