EPM in Horses by a Vet – 2025 Diagnostic Challenges & Symptom Guide 🧠🐴
In this article
EPM in Horses by a Vet – 2025 Diagnostic Challenges & Symptom Guide 🧠🐴
By Dr Duncan Houston, BVSc
Introduction
Equine Protozoal Myeloencephalitis (EPM) is the most common infectious neurological disease in horses in North America. It can be devastating, but it is also one of the most overdiagnosed conditions due to misleading blood test results and variable symptoms. In this 2025 vet-led guide, we explore EPM’s causes, signs, and diagnostic limitations, emphasizing why clinical judgment remains essential for accurate diagnosis and effective care.
What Causes EPM? 🦠
- Two protozoal parasites:
- Sarcocystis neurona – most common
- Neospora hughesi – less common
- Spread through ingestion of infected opossum feces in feed, hay, or water
- Protozoa migrate through the bloodstream to the central nervous system, damaging the brain and spinal cord
Typical Symptoms 🧠
Symptoms vary depending on which parts of the CNS are affected. However, the hallmark is asymmetrical neurological signs.
- Incoordination (ataxia)
- Muscle atrophy—especially in the hindquarters
- Abnormal gait or lameness not linked to limbs
- One-sided weakness—one leg worse than the other
- Facial nerve paralysis or head tilt
- Difficulty swallowing or behavioral changes
Diagnostic Testing: Why It's Tricky ⚠️
- Blood tests: Measure antibody titers—not actual infection or disease
- Spinal fluid (CSF) tests: More accurate, but still not perfect
- Gold standard: Paired serum and CSF ratio, but this requires a spinal tap, which is invasive and may not be feasible in the field
Study Insights: High Seropositivity, Low Disease 📊
- Study of 5,000 horses across 18 states found:
- 78% had antibodies to Sarcocystis neurona
- 34% positive for Neospora
- 31% had both
- None of these horses had EPM symptoms
This confirms that a positive test does not mean active disease. In fact, less than 1% of seropositive horses ever develop clinical EPM.
Why EPM Is Overdiagnosed 🧪
- Vague or variable symptoms like stumbling, behavioral changes, or fatigue may be attributed to EPM without ruling out other causes
- Owners may request EPM treatment based on a positive blood test alone
- But only clinical signs + targeted testing justify treatment
Conditions That Mimic EPM 🔍
- Cervical vertebral malformation (Wobbler syndrome)
- Equine herpesvirus (EHV-1)
- West Nile Virus
- Eastern/Western Encephalitis
- Trauma, rabies, or metabolic disorders
Best Diagnostic Approach in 2025 ✅
- Neurological exam: Check for proprioception, strength, cranial nerve deficits
- Paired serum/CSF testing: Best indicator of CNS infection
- Full workup: Rule out other causes before assuming EPM
Treatment Options 💊
- Ponazuril (Marquis®) – paste form, 28-day course
- Diclazuril (Protazil®) – alfalfa-based pellet
- Sulfonamide + pyrimethamine (ReBalance®) – longer duration
- Supportive therapy: Vitamin E, anti-inflammatories, and rehab
Prognosis 🩺
- Early treatment = better chance of recovery
- Some horses return to full work, others have residual deficits
- Relapse is possible, especially under stress
Case Example: A Cautionary Tale
- A 10-year-old gelding had muscle asymmetry and occasional stumbles
- Owner requested EPM blood test—positive
- Vet performed spinal tap—normal CSF: no active infection
- Further diagnostics revealed cervical arthritis (Wobbler syndrome)
- Horse was treated with anti-inflammatories and rehab—improved without EPM treatment
FAQs About EPM Testing
Q: My horse is positive on a blood test. Does it have EPM?
A: Not necessarily. Most horses are seropositive without clinical disease. Only symptomatic horses with CSF confirmation should be treated.
Q: Can a horse recover from EPM?
A: Yes—with early treatment, many improve. Some may have lasting deficits.
Q: Should all positive horses be treated?
A: No. Treating without symptoms is unnecessary and expensive. Diagnosis should be based on exam and diagnostics—not just titers.
Conclusion
EPM is a complex neurological disease that demands careful, clinical diagnosis—not just a blood test. In 2025, veterinarians are emphasizing symptom-based evaluation, supported by diagnostic testing. If your horse is showing signs of uncoordinated movement or muscle loss, work closely with your vet to explore all possible causes before beginning costly treatment for EPM.
Need help diagnosing or managing EPM? Visit AskAVet.com or use our app 📱 to consult with Dr Duncan Houston and our neurology team for accurate diagnosis and therapy plans. 🩺🐴
