📋 At a Glance
⚠️ Signs Requiring Veterinary Contact
- Fever above 101.5°F — particularly in a horse that has recently traveled to an event
- Any neurological signs following a recent respiratory illness — ataxia, hindlimb weakness, urinary incontinence
- Multiple horses at a facility developing fever or respiratory signs simultaneously
- Fever in a pregnant mare — EHV-1 can cause late-term abortion with or without prior respiratory signs
- Hindlimb weakness, stumbling, or urinary incontinence developing in a horse post-travel
- Neurological EHM at an event or barn — notify event organizers, facility management, and state veterinarian
EHV-1 vs. EHV-4 — Why the Distinction Matters
EHV-4 is the more common type and primarily causes upper respiratory infection — fever, nasal discharge, and cough in young horses. Most adult horses have significant immunity from prior exposure and rarely develop significant disease. EHV-4 does not cause neurological disease and very rarely causes abortion.
EHV-1 is the clinically more significant type. It shares the same respiratory route as EHV-4 but additionally causes abortion in mares (via placentitis and fetal infection), neonatal foal death, and — in its neuropathogenic form — Equine Herpesvirus Myeloencephalopathy (EHM). The EHM neuropathogenic strain differs from the non-neuropathogenic EHV-1 strains by a single mutation in the DNA polymerase gene that can be detected by laboratory analysis.
EHM — The Neurological Form
EHM represents the most serious clinical consequence of EHV-1 infection. In EHM, the virus infects the vascular endothelium of the central nervous system, producing vasculitis (blood vessel inflammation) and subsequent ischemic damage to spinal cord and brain tissue. The resulting neurological signs reflect the pattern and extent of CNS ischemia.
EHM can range from mild hindlimb ataxia to complete inability to stand and requires immediate veterinary attention. It can progress rapidly over 24–48 hours. Survival rates vary significantly depending on the severity at presentation and the supportive care available — horses at grade 4–5 neurological severity have substantially higher mortality.
| Feature | Respiratory EHV-1 | EHM (Neurological EHV-1) |
|---|---|---|
| Signs | Fever, nasal discharge, cough, lymph node enlargement | Ataxia, hindlimb weakness, urinary incontinence, recumbency (severe) |
| Onset after exposure | 3–8 days | 4–10 days (fever often precedes neurological signs by 1–2 days) |
| Transmission risk | High — nasal secretions peak during fever | High — same as respiratory; neurological signs are not the infectious phase |
| Prognosis | Self-limiting; most recover in 2–3 weeks | Variable — mild: good; moderate-severe: guarded to poor |
| Required management | Isolation, supportive care, monitoring | Intensive veterinary care; stall rest; deep bedding; IV fluids/anti-inflammatories |
EHV and Reproductive Loss in Mares
EHV-1 is a significant cause of abortion in mares, typically occurring in the last trimester (8–11 months of gestation). Abortion typically occurs suddenly, often without prior signs of illness in the mare — the placenta is usually intact (indicating non-contagious abortion in most cases), and the aborted fetus shows characteristic pathological changes. In rare cases, multiple mares in a herd abort in sequence — a 'storm' of EHV-1 abortion.
Vaccination at months 5, 7, and 9 of pregnancy with EHV-1 (rhinopneumonitis) vaccine is the primary prevention strategy for EHV-1 abortion. While no vaccine completely eliminates risk, evidence suggests vaccination reduces the frequency and severity of EHV-1 abortion events.
Biosecurity — Outbreak Response
When EHV is suspected or confirmed at a facility, the response must be immediate and comprehensive. Your veterinarian and state animal health officials will direct specific protocols. General principles:
Immediate Biosecurity Actions
- Isolate affected horses and implement strict contact precautions
- Temperature monitor all horses at the facility twice daily — fever typically precedes neurological signs
- Stop all horse movement on and off the property immediately
- Notify event facilities your horses attended in the past 14 days — they may have exposed horses
- Contact the state veterinarian if EHM is confirmed — required in most jurisdictions
- Dedicated personnel, equipment, and PPE for each management group
Vaccination — Protection and Realistic Expectations
EHV vaccines are available in two formats: modified-live intranasal (EHV-1 only) and killed intramuscular (EHV-1/4 combination). Both are recommended for horses with exposure risk from events, breeding, or new horse contact. Importantly, no vaccine currently provides complete protection against EHM — vaccinated horses can still develop the neurological form, though some evidence suggests vaccination may reduce severity in some cases.
Discussion with your veterinarian about the appropriate vaccine type, schedule, and frequency for your horse's specific exposure risk is more valuable than a general vaccination recommendation.
✅ Action Steps — While Contacting Your Vet
- Monitor temperature twice daily for 14 days after any event, sale, or new horse contact
- Contact your veterinarian immediately if fever develops post-travel or post-event
- Implement isolation immediately for any horse with fever or neurological signs
- Notify the state veterinarian if EHM is confirmed — reportability enables industry monitoring and protection
- Notify facilities your horses attended in the past 2 weeks
📋 Prevention & Long-Term Management Discussion Points
- Vaccination schedule — discuss appropriate EHV vaccination frequency with your vet based on your horse's event schedule
- Temperature monitoring program — establish monitoring routine for post-event horses
- Event biosecurity — minimizing nose-to-nose contact, not sharing equipment, and handwashing between horses
- New horse quarantine — isolate all new arrivals for minimum 21 days
- Pregnant mare protocols — EHV-1 vaccination timing at months 5, 7, and 9 of gestation
Questions to Ask Your Veterinarian
- Given my horse's competition schedule, what EHV vaccination protocol do you recommend?
- Is there any post-event monitoring protocol you recommend after major competitions?
- If we have an EHM case on the property, what does the isolation protocol look like specifically?
- Should pregnant mares on this property have EHV vaccination now regardless of timing in the schedule?
- What are the current regional EHV activity patterns I should be aware of?
