EHM (Equine Herpesvirus) at a Glance
EHV-1 Clinical Outcomes
Comparison
| Feature | EHV-1 Respiratory | EHV-1 Neurologic (EHM) | Key Difference |
|---|---|---|---|
| Signs | Fever, cough, nasal discharge | Ataxia, urine retention, recumbency | Neuro signs = emergency |
| Onset | 2–7 days post exp. | 5–10 days post fever | Often follows fever spike |
| Contagious | Extremely (via nasal shed) | Same virus strain | Same transmission route |
| Treatment | NSAIDs, rest, support | Aggressive: antivirals, sling | Requires hospital-level care |
EHM Outbreak Response
EHM Biosecurity Checklist
| Isolate any horse with fever of unknown origin | |
| Take daily temperature of all barn horses | |
| Test suspect cases with nasopharyngeal PCR | |
| Quarantine exposed horses for 21 days minimum | |
| Use dedicated equipment for each horse group | |
| Disinfect stalls with accelerated hydrogen peroxide | |
| Limit barn visitors during outbreak | |
| Vaccinate stable against EHV-1? (ask vet about risk) |
Equine herpesvirus myeloencephalopathy (EHM) is a neurological syndrome caused by equine herpesvirus type 1 (EHV-1) that affects the spinal cord and brain vasculature, causing ataxia, hindlimb weakness, bladder dysfunction, and in severe cases recumbency and death. Unlike EHV-1 respiratory or abortion strains, the neuropathogenic strain (distinguished by an N752 mutation in the DNA polymerase gene) causes vasculitis and thrombosis in CNS blood vessels rather than just upper respiratory infection. EHM occurs sporadically but can affect multiple horses at a facility simultaneously, making it a significant biosecurity concern.
| EHM Phase | Post-Exposure Timing | Key Signs |
|---|---|---|
| Respiratory onset | Days 3–7 | Fever 102–106°F, nasal discharge, reduced appetite |
| Neurological onset | Days 6–10 | Second fever spike; symmetric hindlimb ataxia; bladder atony |
| Critical / recumbent | Days 7–14+ | Cannot rise; poor prognosis — intensive referral care required immediately |
How EHV-1 Becomes Neurological
EHV-1 is endemic in horse populations worldwide; most adult horses have been exposed. The virus establishes latency in trigeminal ganglia and lymph nodes, reactivating under stress (shipping, competition, illness, weaning). In most reactivations, horses shed virus nasally for 7 to 10 days and recover without neurological signs. In a subset of horses -- more commonly those exposed to the N752 neuropathogenic strain -- the virus infects endothelial cells of CNS vasculature, triggering inflammation, platelet aggregation, and ischemic infarcts in the spinal cord and brainstem.
Clinical Signs and Timeline
Initial signs are identical to standard EHV-1 respiratory infection: fever (102 to 106 degrees F), nasal discharge, and reduced appetite appearing 3 to 7 days after exposure. Neurological signs develop 6 to 10 days post-exposure as a second fever spike: symmetric ataxia beginning in the hindlimbs (like "walking on ice"), hindlimb weakness, urine dribbling from bladder atony, tail weakness, and reduced perineal sensation. Severely affected horses become recumbent within 24 to 48 hours of neurological onset; recumbent horses have a guarded to poor prognosis.
A key clinical marker: EHM typically affects hindlimbs symmetrically and is non-painful, distinguishing it from cervical spinal cord compression (wobbler syndrome) or trauma, which are often asymmetric or painful on palpation.
Diagnosis
Presumptive diagnosis is based on clinical signs plus PCR confirmation of EHV-1 in nasal swabs or whole blood. PCR can differentiate the neuropathogenic N752 strain from standard EHV-1 strains. Cerebrospinal fluid analysis shows xanthochromia (yellow-tinged CSF from hemoglobin breakdown) with elevated protein and minimal cell increase -- a characteristic pattern. Paired serum neutralization titers showing a 4-fold rise confirm active infection retrospectively.
Treatment and Biosecurity
No antiviral therapy has FDA approval for EHV-1 in horses. Valacyclovir (27 to 40 mg/kg orally every 8 hours for 7 days, then tapered) has demonstrated antiviral activity in pharmacokinetic studies and is used off-label at many referral hospitals. Supportive care includes IV fluids, NSAIDs (flunixin meglumine 1.1 mg/kg IV every 12 hours), bladder catheterization for atonic bladders, and deep bedding for recumbent horses with sling support.
Biosecurity is critical. Affected horses shed EHV-1 nasally and should be immediately isolated in a separate airspace (not just a different stall). Any horse with fever on the same premises should be quarantined. USDA and AAEP recommend a minimum 21-day isolation from the last fever case before ending a facility quarantine. No currently available EHV-1 vaccine reliably prevents EHM; modified-live EHV vaccines are contraindicated in outbreak settings.
Sources: American Association of Equine Practitioners (AAEP) EHV-1 Outbreak Management Guidelines (2012); Pusterla N et al., "Equine herpesvirus-1 myeloencephalopathy," Veterinary Journal (2009); USDA APHIS Equine Herpesvirus guidelines.