📋 At a Glance
⚠️ Signs Requiring Veterinary Contact
- Any sudden onset of incoordination, stumbling, or inability to place feet normally
- Hindlimb weakness or ataxia — particularly if asymmetric (one side worse than the other)
- Muscle wasting that develops over days to weeks without a corresponding limb injury
- Difficulty swallowing, drooping of the lip, eyelid, or ear — facial nerve involvement
- A horse that is unable to rise after lying down or is falling
- Head tilting, circling, or apparent vestibular dysfunction
- Progressive worsening of any gait abnormality over days — EPM can advance rapidly without treatment
Understanding EPM — Disease Mechanism
Sarcocystis neurona completes its life cycle in the opossum. Opossums shed sporocysts in their feces, which can contaminate feed, hay, and water sources accessed by horses. When horses ingest these sporocysts, the protozoa migrate through the intestinal wall, enter the bloodstream, and in susceptible horses, invade the central nervous system — particularly the brain and spinal cord.
Once in the nervous system, S. neurona destroys neurons and causes focal areas of inflammation and necrosis (tissue death). The location and extent of nervous tissue damage determines the clinical signs — which is why EPM produces such variable presentations. Damage to the cervical spinal cord produces hindlimb then forelimb ataxia; brainstem involvement produces cranial nerve deficits; cerebral involvement (less common) produces behavior changes, seizures, or visual deficits.
Most horses exposed to S. neurona (seropositive) do not develop clinical disease — the immune system successfully contains the infection in the majority of cases. It is horses with compromised immune function (stress, concurrent disease, transport, overtraining) that are most likely to develop progressive neurological disease.
AAEP Neurological Grading — Communicating Severity
Veterinarians use a standardized 5-grade neurological scoring scale to assess severity and track progression or improvement over time. Understanding these grades helps horse owners communicate clearly about what they are observing.
| Grade | Description | What You Might Observe |
|---|---|---|
| Grade 1 | Deficit barely detectable; inconsistent at normal paces | Very subtle stumbling or subtle unevenness that appears intermittently |
| Grade 2 | Deficit detectable at normal movement; worse with manipulation (head elevation, circling, inclines) | Consistent but mild ataxia; clearer on tight circles or backing |
| Grade 3 | Deficit pronounced at normal movement; horse stumbles spontaneously | Obvious incoordination at walk; may stumble on straight line; potentially unsafe to ride |
| Grade 4 | Horse stumbles spontaneously; falls with inclines, circling, or obstacles | Significant risk of injury; do not ride; emergency veterinary evaluation |
| Grade 5 | Recumbent or near-recumbent; cannot or will not rise | Medical emergency; immediate veterinary attention required |
Diagnosis — The Challenge of EPM
EPM diagnosis is one of the more complex areas of equine medicine because exposure to S. neurona is extremely common, while clinical disease is relatively uncommon. A positive serum antibody test confirms that the horse has been exposed — but many horses are seropositive without ever developing neurological disease. A negative test makes EPM much less likely but does not completely rule it out.
Cerebrospinal fluid (CSF) testing — obtained by spinal tap at the atlanto-occipital space — is more specific for active CNS infection than serum testing, because the blood-brain barrier limits normal antibody movement into the CSF. An elevated CSF:serum antibody ratio suggests active CNS involvement rather than mere peripheral exposure.
In practice, many horses are treated empirically based on clinical presentation, neurological grade, geographic exposure, and exclusion of other neurological conditions — without confirmatory CSF testing. Your veterinarian determines the appropriate diagnostic and treatment approach for your horse's specific situation.
Treatment and Recovery Expectations
Three FDA-approved protocols are available for EPM treatment. All involve daily oral administration for 28–90 days depending on the protocol chosen. Response to treatment is variable: some horses improve dramatically within weeks; others show minimal response; a small percentage continue to progress despite treatment.
Treatment response depends on the severity of neurological damage at the time treatment begins — which is the strongest argument for early recognition and intervention. Horses treated when grade 1–2 have significantly better outcomes than horses treated at grade 3–4 after extensive neurological damage has accumulated.
Post-Treatment Considerations
- Neurological deficits may improve gradually over months — improvement can continue up to a year after treatment completes
- Some horses that improve cannot fully return to their prior use — residual deficits may be subtle but affect safety for riding
- Relapse risk exists — stress events (transport, illness) may reactivate latent infection; discuss long-term monitoring with your vet
- Opossum control on the property reduces re-exposure risk
Prevention — Reducing Exposure Risk
Because there is no fully effective EPM vaccine currently available for commercial use in the US, prevention focuses on reducing environmental exposure to opossum feces.
Opossum Exclusion and Farm Management
- Secure all feed and hay storage — opossums attracted to grain, sweet feed, and hay; secure lids prevent fecal contamination
- Cover water tanks when not in use — automatic waterers should be checked regularly for contamination
- Check hay for opossum feces before feeding — fecal pellets are easily visible on light-colored hay
- Eliminate attractants — fallen fruit, open compost, accessible garbage — that bring opossums near the barn
- Motion-activated lighting and exclusion fencing may reduce opossum traffic in barn areas
✅ Action Steps — While Contacting Your Vet
- Contact your veterinarian immediately — neurological disease requires urgent evaluation and is dangerous for horses and handlers
- Restrict movement until examined — a neurologically affected horse is at high fall and injury risk
- Secure safely in a well-bedded stall away from obstacles, fencing, water tanks, and other horses
- Record a video if possible — documenting the specific gait abnormality and which limbs are affected is very helpful for your veterinarian
- Document recent stressors — transport, illness, competition, vaccination, or management changes that preceded onset
- Do not ride a horse with suspected neurological disease — safety risk for horse and rider
📋 Prevention & Long-Term Management Discussion Points
- Opossum exclusion measures on the property — securing feed and water sources
- Serum vs. CSF testing — discuss with your vet which approach is appropriate for your horse's presentation
- Treatment protocol selection — ponazuril vs. diclazuril vs. sulfadiazine/pyrimethamine; discuss cost, compliance, and efficacy considerations
- Monitoring neurological grade during treatment — objective improvement tracking
- Long-term relapse monitoring — stress events and immune-compromising situations are highest-risk periods
Questions to Ask Your Veterinarian
- Given this horse's clinical signs and history, what is your clinical suspicion for EPM vs. other neurological conditions?
- Do you recommend serum testing alone, or is CSF testing appropriate for this case?
- Which treatment protocol do you recommend, and what is the expected treatment duration and monitoring schedule?
- What neurological grade do you assign at this examination, and how does that affect prognosis?
- What signs of improvement — or worsening — should I monitor for, and when should I call you?
- After treatment, what level of work is this horse realistically expected to return to?
