Equine EPM at a Glance
EPM Diagnostic Confidence
Comparison
| Test | Sample | Accuracy | Notes |
|---|---|---|---|
| Blood Serology | Serum | Moderate (60–70%) | Detects exposure, not active disease |
| CSF Western Blot | Cerebrospinal fluid | High (85–90%) | Gold standard — requires spinal tap |
| CSF PCR | CSF | Very high (95%) | Detects parasite DNA directly |
| MRI/Imaging | Brain/spinal cord | High | Rules out other neurological causes |
EPM Diagnosis & Treatment
EPM Prevention Checklist
| Minimize opossum access to feed and stalls | |
| Use opossum-proof feed storage containers | |
| Keep hay stored in enclosed, opossum-proof area | |
| Remove standing water and brush piles | |
| Test new horses for EPM exposure before mixing | |
| Wash water buckets and feed tubs regularly | |
| Seal openings in barn walls and rafters | |
| Consult vet at first sign of ataxia or weakness |
Equine protozoal myeloencephalitis (EPM) is a neurological disease of horses caused by the protozoan parasite Sarcocystis neurona (and less commonly Neospora hughesi). The definitive host is the Virginia opossum (Didelphis virginiana) -- horses acquire the infection by ingesting feed or water contaminated with opossum feces containing infective sporocysts. Not all exposed horses develop disease; an estimated 50--90% of horses in North America have antibody evidence of exposure, but clinical EPM affects only a small percentage. Diagnosis requires cerebrospinal fluid (CSF) analysis combined with serum testing; treatment with antiprotozoal medications is successful in many cases, though neurological deficits may persist.
| Diagnostic Tool | What It Confirms | Key Limitation |
|---|---|---|
| Serum antibody (Western blot) | Exposure to S. neurona | 50-90% of healthy horses test positive — not diagnostic alone |
| CSF antibody index (SAI) | Active CNS involvement | Requires anesthesia for CSF collection — definitive but invasive |
| Clinical grade 0-5 scale | Disease severity and prognosis | Cannot distinguish EPM from Wobbler/EHM without lab confirmation |
The Opossum Connection
Sarcocystis neurona has a two-host life cycle. In the wild, opossums serve as definitive hosts and shed sporocysts in their feces after eating infected intermediate hosts (marine mammals, skunks, raccoons, armadillos, and cats). Horses are accidental 'dead-end' hosts -- they ingest sporocysts from feed or water contaminated by opossum feces, the sporocysts migrate to neural tissue, and the parasite cannot complete its life cycle from horses. Prevention strategies focus on reducing opossum access to feed storage and water sources: secure all hay and grain in opossum-proof containers, use covered water troughs, and remove food sources that attract opossums (fallen fruit, unsecured garbage, open compost).
Clinical Signs
EPM causes progressive neurological signs reflecting damage in the brain, brainstem, or spinal cord. Signs vary depending on which neural structures are affected, and the presentation is often asymmetric (one side more affected than the other), which is a distinguishing feature from other neurological diseases. Signs include: stumbling and incoordination (ataxia) that worsens on inclines or when the horse's head is elevated; muscle atrophy, typically asymmetric; weakness in one or more limbs; facial nerve deficits (asymmetric ear position, drooping lip, difficulty swallowing); and in severe cases, the horse may fall. A grading scale (0 to 5) is used clinically to rate severity: Grade 1 is subtle signs detectable only on careful neurological examination; Grade 4--5 is severe ataxia with fall risk.
Diagnosis
EPM diagnosis requires integrating clinical signs with laboratory testing. Serum antibody testing (SAC test, Western blot) indicates exposure but not active disease -- positive serum antibodies alone are not diagnostic because exposure rates are high. The most specific diagnostic approach is comparing antibody titers in CSF versus serum using the Sarcocystis neurona antibody index (SAI) or neospora antibody index: CSF antibodies in an affected horse are elevated relative to serum, confirming central nervous system involvement. CSF collection requires lumbosacral or atlanto-occipital tap under general anesthesia or heavy sedation. Ruling out cervical vertebral stenotic myelopathy (CVSM/Wobbler syndrome), equine herpesvirus myeloencephalopathy (EHM), and cervical trauma is essential before committing to EPM treatment.
Treatment
Three antiprotozoal treatments are FDA-approved for EPM in the US: ponazuril (Marquis), diclazuril (Protazil), and a combination of sulfadiazine and pyrimethamine (ReBalance). Treatment courses typically run 28 days. Approximately 60--70% of treated horses improve by at least one neurological grade; 25--30% return to normal function; 10--20% do not respond or worsen. Anti-inflammatory treatment (flunixin meglumine or dexamethasone short-term) may be added initially to reduce inflammation in severely affected horses. Horses that relapse after treatment may benefit from a longer treatment course or investigation of immunosuppression as a contributing factor.
Prognosis and Management
Horses with Grade 1--2 EPM that receive prompt treatment have the best outcomes. Grade 3--4 horses have more variable prognoses and may retain deficits affecting athletic performance. Horses with Grade 5 (recumbent, unable to rise) have a poor prognosis for return to function. EPM does not protect against reinfection -- horses can acquire a second infection after treatment. Farms with heavy opossum populations and recurrent EPM cases should consider aggressive feed security modifications and potentially consulting with a wildlife management professional.
EPM pathogenesis and life cycle are from Dubey et al. (2001, 2015) peer-reviewed research on Sarcocystis neurona published in Veterinary Parasitology. Diagnostic criteria follow the AAEP EPM consensus guidelines. Treatment efficacy data are from FDA registration studies for approved antiprotozoal products. Clinical grading scale is the standardized Mayhew equine neurological examination scale.