Blood Testing for EPM in Horses by a Vet – 2025 Diagnosis Limits, Accuracy & Clinical Clues 🧠🐴
In this article
Blood Testing for EPM in Horses by a Vet – 2025 Diagnosis Limits, Accuracy & Clinical Clues 🧠🐴
By Dr Duncan Houston, BVSc
Introduction
Equine protozoal myeloencephalitis (EPM) is the most frequently diagnosed infectious neurological disease in horses across North America. Caused by protozoal parasites Sarcocystis neurona and Neospora hughesi, EPM affects the brain and spinal cord. But diagnosing EPM remains tricky in 2025—because many horses test positive without being sick. This guide explores the limitations of blood testing and how to focus on clinical signs when evaluating a possible EPM case.
What Is EPM? 🧬
- Neurological disease caused by protozoa—Sarcocystis neurona or Neospora hughesi
- Transmitted when horses ingest infected opossum feces
- Protozoa travel through the blood to the central nervous system
Symptoms of EPM ⚠️
- Asymmetrical incoordination (ataxia)
- Muscle atrophy—often more severe on one side
- Gait abnormalities or stumbling
- Facial nerve issues or behavior changes
- Head tilt or difficulty swallowing (in advanced cases)
Blood Testing for EPM 🧪
What Blood Tests Measure
- They detect antibodies—a sign that the horse has been exposed to the organism
- They do not confirm active neurological disease
Study Findings (5,000+ Horses)
- 78% were positive for Sarcocystis neurona
- 34% were positive for Neospora hughesi
- 31% were positive for both
- None of these horses had any EPM symptoms
This confirms that a positive blood test does not mean the horse has EPM.
Why False Positives Occur
- Many horses are exposed without ever developing disease
- Blood tests may be overly sensitive
- Over-diagnosis and over-treatment are common as a result
Spinal Fluid Testing 🧠
- CSF (cerebrospinal fluid) tests are more accurate than blood tests
- Used to compare CSF antibody levels with blood titers
- Can still produce false positives or sampling complications
Accurate EPM Diagnosis in 2025 🩺
- Combine testing with clinical signs
- Only pursue EPM treatment if:
- Neurological signs match typical EPM patterns
- Blood/CSF testing supports suspicion
- Other causes (Wobbler, EHV, West Nile, trauma) are ruled out
Why EPM Is Overdiagnosed 🔍
- Many neurological signs are vague or overlap with other diseases
- Clients and vets may assume a positive titer equals disease
- Unnecessary treatment can cost over $800/month and delay proper care
Case Example: Non-EPM Diagnosis
- Horse showed hindlimb weakness and poor coordination
- Blood test was positive for Sarcocystis neurona
- Spinal tap was negative, and further diagnostics revealed Wobbler syndrome
- Horse improved with neck injections—not EPM treatment
FAQs About EPM Testing
Q: If my horse tests positive on blood but has no symptoms, should I treat?
A: No. Only treat EPM when there are matching neurological signs and testing supports it.
Q: What’s the best way to confirm EPM?
A: A combination of neurological exam + CSF analysis + exclusion of other diseases.
Q: Should I retest after treatment?
A: Usually not needed unless symptoms return. Titers can remain positive after recovery.
Conclusion
Blood testing alone is not enough to diagnose EPM. In 2025, equine neurologists stress the importance of correlating test results with actual clinical signs. Many healthy horses will test positive for Sarcocystis or Neospora, but only a small fraction ever develop disease. To avoid misdiagnosis, prioritize careful evaluation, targeted testing, and clear neurological signs before pursuing costly EPM treatment.
Need help diagnosing a neurological issue or interpreting test results? Visit AskAVet.com or download our app 📱 to consult with Dr Duncan Houston for equine neurology support. 🩺🐴
