Equine Protozoal Myeloencephalitis (EPM) in Horses: Signs, Diagnosis, and Treatment
- See your vet immediately if your horse develops new incoordination, weakness, muscle loss, facial droop, trouble swallowing, or other neurologic changes.
- EPM is a neurologic disease most often caused by *Sarcocystis neurona* after a horse ingests infective sporocysts in feed or water contaminated by opossum feces.
- Diagnosis usually combines a neurologic exam, ruling out other causes of ataxia, and blood plus cerebrospinal fluid testing to look for intrathecal antibody production.
- FDA-approved treatment options include ponazuril, diclazuril, or sulfadiazine/pyrimethamine, often with short-term anti-inflammatory support and rehabilitation planning.
- Many horses improve with treatment, but full recovery is not guaranteed and relapses can happen, so early veterinary care matters.
What Is Equine Protozoal Myeloencephalitis (EPM) in Horses?
Equine protozoal myeloencephalitis, or EPM, is an infectious neurologic disease that affects the brain and spinal cord. In North America, most cases are linked to the protozoan Sarcocystis neurona. A smaller number are associated with Neospora hughesi. Horses are considered dead-end hosts, which means they do not spread the disease to other horses.
EPM can look different from one horse to the next because the parasite may damage different parts of the central nervous system. Some horses show mild, vague gait changes at first. Others develop obvious weakness, muscle atrophy, cranial nerve deficits, or trouble standing. Signs are often asymmetric, which is one clue that makes your vet think about EPM.
This condition is serious, but it is not hopeless. Many horses improve with treatment, especially when care starts early. Even so, some horses are left with lasting neurologic deficits, and relapse is possible. That is why any new neurologic sign in a horse deserves prompt veterinary attention.
Symptoms of Equine Protozoal Myeloencephalitis (EPM) in Horses
- Ataxia or incoordination
- Weakness in one or more limbs
- Abnormal gait, stiffness, or unexplained lameness-like movement
- Muscle atrophy along the topline, hindquarters, shoulder, or face
- Head tilt or poor balance
- Facial paralysis or drooping ear, eyelid, or lip
- Difficulty swallowing
- Loss of sensation or abnormal sweating
- Seizures, collapse, or inability to rise
EPM is often called a "master of disguise" because it can mimic lameness, cervical vertebral stenotic myelopathy, trauma, equine herpesvirus myeloencephalopathy, and other neurologic problems. Signs may come on gradually or appear more suddenly, and they are often worse on one side of the body.
See your vet immediately if your horse shows any new neurologic change, especially stumbling, weakness, facial droop, trouble swallowing, collapse, or rapid worsening. Horses with severe ataxia can injure themselves quickly, so safe confinement and urgent veterinary guidance are important while you wait for evaluation.
What Causes Equine Protozoal Myeloencephalitis (EPM) in Horses?
Most EPM cases are caused by Sarcocystis neurona. Horses become infected by ingesting sporocysts in feed or water contaminated with opossum feces. In the United States, the opossum is the known definitive host for S. neurona. A smaller number of cases are linked to Neospora hughesi.
Exposure is common, but disease is much less common. Cornell notes that about half of horses in the U.S. have been exposed to S. neurona, yet fewer than 1% of exposed horses develop clinical EPM. That means a positive blood test alone does not prove that EPM is causing your horse's signs.
Researchers do not think every exposed horse gets sick for the same reason. Parasite strain, dose of exposure, stress, travel, concurrent illness, and immune status may all play a role. What matters most for pet parents is that EPM is not contagious from horse to horse. An affected horse does not shed the organism to stablemates.
How Is Equine Protozoal Myeloencephalitis (EPM) in Horses Diagnosed?
Your vet starts with a full history and neurologic exam. Because EPM can affect almost any part of the central nervous system, there is no single sign that confirms it. Your vet will also consider other causes of ataxia and weakness, such as cervical spinal cord compression, equine herpesvirus myeloencephalopathy, trauma, equine degenerative myeloencephalopathy, rabies, and other infectious or inflammatory diseases.
Testing usually includes blood work and EPM antibody testing on serum and cerebrospinal fluid (CSF). Merck and Cornell both emphasize that the most useful support for diagnosis comes from evidence of intrathecal antibody production, often assessed with serum:CSF titer ratios or CSF index methods. A positive serum test mainly shows exposure, not necessarily active disease. Blood contamination of a CSF sample can also create misleading results, so sample quality matters.
In some horses, your vet may recommend cervical radiographs, ultrasound-guided CSF collection, PCR in select acute cases, or referral imaging and hospitalization. Recheck exams are important because response to treatment, progression of signs, and safety risks all influence next steps. Diagnosis is often a combination of pattern recognition, exclusion of look-alike conditions, and targeted lab testing rather than one standalone test.
Treatment Options for Equine Protozoal Myeloencephalitis (EPM) in Horses
Spectrum of Care means you have options. Here are treatment tiers at different price points.
Budget-Conscious Care
- Farm-call exam and neurologic assessment
- Serum EPM testing, with referral discussion if signs are significant
- One FDA-approved oral antiprotozoal chosen by your vet, often ponazuril or diclazuril for 28 days, or sulfadiazine/pyrimethamine when a longer lower-upfront-medication plan fits the case
- Short course of anti-inflammatory medication if your vet feels it is appropriate
- Strict exercise restriction, safer footing, and basic home monitoring for falls, appetite, and swallowing
Recommended Standard Treatment
- Comprehensive neurologic exam and baseline blood work
- Serum plus CSF EPM testing or serum:CSF ratio testing through a reference lab
- FDA-approved antiprotozoal treatment: ponazuril 5 mg/kg PO every 24 hours for 28 days, diclazuril 1 mg/kg PO every 24 hours for 28 days, or sulfadiazine/pyrimethamine PO every 24 hours for at least 90 days, as directed by your vet
- Adjunctive care that may include flunixin, phenylbutazone, vitamin E, and tailored nursing recommendations
- Recheck exam to assess gait, safety, and whether treatment duration or rehab needs adjustment
Advanced / Critical Care
- Hospitalization or referral-center evaluation for severe ataxia, recumbency risk, cranial nerve signs, or swallowing problems
- CSF collection, expanded neurologic workup, and imaging or additional infectious disease testing to rule out look-alike conditions
- FDA-approved antiprotozoal treatment plus intensive supportive care, such as IV fluids, assisted feeding, sling support, wound prevention, and close monitoring
- Short-term corticosteroids in selected severe brain-involved cases if your vet determines benefits outweigh risks, plus anti-inflammatory support and vitamin E
- Structured rehabilitation and longer-term reassessment if the horse survives the acute phase
Cost estimates as of 2026-03. Actual costs vary by location, clinic, and individual case.
Questions to Ask Your Vet About Equine Protozoal Myeloencephalitis (EPM) in Horses
Bring these questions to your vet appointment to get the most out of your visit.
- Based on my horse's neurologic exam, how strongly do you suspect EPM versus another cause of ataxia?
- Do you recommend serum testing only, or serum plus CSF testing for my horse's case?
- Which FDA-approved medication fits my horse best, and how long do you expect treatment to last?
- What side effects or monitoring needs should I watch for with ponazuril, diclazuril, or sulfadiazine/pyrimethamine?
- Is my horse safe to trailer, turn out, or hand-walk right now, or should activity be restricted?
- What signs would mean this is becoming an emergency, such as trouble swallowing, falling, or inability to rise?
- Should we add anti-inflammatory support, vitamin E, or rehabilitation exercises?
- What is the realistic prognosis for return to comfort, turnout, or athletic work in my horse's specific case?
How to Prevent Equine Protozoal Myeloencephalitis (EPM) in Horses
There is no proven vaccine for EPM. Merck notes that a previously marketed conditionally approved vaccine is no longer offered, and AAEP states that an effective vaccine has not been developed. Prevention focuses on lowering exposure rather than eliminating risk completely.
The most practical step is reducing contact between opossums and horse feed or water. Store grain and supplements in secure containers, keep hay and feed in enclosed areas when possible, clean up spilled grain, remove fallen fruit, and avoid leaving pet food, bird seed, or garbage where opossums can access it. Water sources should also be kept as clean as possible.
AAEP and Merck both note that preventive antiprotozoal use has been studied, especially in higher-risk settings, but there is no standard evidence-based protocol that guarantees protection. For that reason, preventive medication should be a case-by-case conversation with your vet, not a routine assumption. Good overall health, stress reduction, and prompt evaluation of any neurologic sign are still the most useful real-world prevention tools.