High severity

EPM (Equine protozoal myeloencephalitis)

EPM is equine protozoal myeloencephalitis, usually Sarcocystis neurona (sometimes Neospora hughesi). Owner-reported signs are often asymmetric ataxia, muscle atrophy, weakness, or incoordination — a horse that looks lame but is neurologic. It is not ordinary lameness, not Lyme disease, not shivers, and not locked stifle. A photograph cannot diagnose EPM. A still cannot tell EPM from other neurologic disease, Lyme, ordinary lameness, or shivers. A still photo is especially weak here: this is a movement / neurologic problem. Diagnosis is a vet exam plus testing, not a photo. The gait scan is a first look at the movement to share with a vet, not an EPM test, and there is no EPM scanner.

Symptoms to watch for

Asymmetric ataxia or incoordination — one side worse than the other — the movement is the clue, not a still photo

Muscle atrophy, often on one side of the rump or along the topline — a reason to book a vet, not a photo verdict

Weakness or a horse that looks lame but is neurologic — a clue, not a diagnosis

May look normal in a still; the first sign is often a change in the way they move — that is a vet visit, not a photograph

A gait that can look like ordinary lameness, Lyme, shivers, or other neurologic disease — a still cannot tell them apart

A still photo is especially weak here: this is a movement / neurologic problem — a reason to book a vet, not a photo verdict

Common causes

Equine protozoal myeloencephalitis, usually Sarcocystis neurona (sometimes Neospora hughesi) — exposure and testing are clues, not a photo diagnosis
Not ordinary lameness, not Lyme disease, and not shivers — a still cannot tell EPM from those
Not locked stifle (upward fixation of the patella) — that is a different problem and needs a vet
A still cannot tell EPM from other neurologic disease, Lyme, ordinary lameness, or shivers — those need a vet exam plus testing

Prevention

Watch for asymmetric ataxia, muscle atrophy, weakness, or a horse that looks lame but is neurologic, and book a vet rather than waiting on a photo
Do not work through a new incoordination, weakness, or asymmetric hitch hoping it will settle — that is a vet visit, not a photo
Do not treat a photo, a still of a lame or wobbly stride, or a scan result as a diagnosis — a still is especially weak on a movement / neurologic problem
Do not treat a photo, a still of an asymmetric stride, or a scan result as a diagnosis
A gait clip can help you and the vet look at the movement — it does not confirm or rule out EPM, and it cannot tell EPM from other neurologic disease, Lyme, ordinary lameness, or shivers

What to do

1
Book a vet visit if the horse is asymmetrically ataxic, weak, losing muscle on one side, or looks lame but is neurologic. Diagnosis is a vet exam plus testing — not a photograph.
2
Do not treat a still of a lame or wobbly stride as a verdict. A still cannot tell EPM from other neurologic disease, Lyme, ordinary lameness, or shivers, and a still photo is especially weak here.
3
If you record a gait clip, film walk and trot on a hard, level surface from the side and from the front — that is a video, not a still, and it is a first look to share, not a diagnosis.
4
Use a gait scan only as a first look at the movement to share, not as an EPM test. There is no EPM scanner.
5
If EPM is confirmed, follow the plan your vet prescribes. Do not start a treatment protocol from a photo.
6
Keep the vet on the same page. Share what you see; do not treat a photo as a verdict.

When to call the vet

Pick up the phone if…

Schedule a vet visit if the horse is asymmetrically ataxic, weak, losing muscle on one side, or looks lame but is neurologic, or if the horse looks normal at rest but the hitch appears when you ask them to walk and trot. Call sooner if the horse will not bear weight, is collapsing, or is in obvious distress. A photograph cannot diagnose EPM. A still cannot tell EPM from other neurologic disease, Lyme, ordinary lameness, or shivers — a still photo is especially weak here. Diagnosis is a vet exam plus testing, not a photo. The gait scan is a first look at the movement, not an EPM test, and there is no EPM scanner.

Get a first look at the movement

Scan the gait in HorseCare. 5 free scans this month, then Pro.

Scan the gait

5 free scans this month, then Pro. Scans run in the HorseCare app.

Educational content only. Not a substitute for veterinary advice.