Equine Herpesvirus in Horses by a Vet – 2025 Guide to Strains, Symptoms & Prevention 🧬🐴

By Dr Duncan Houston, BVSc

Introduction

Equine herpesvirus (EHV) is one of the most significant infectious diseases in the modern horse world. It causes outbreaks of respiratory illness, abortion in pregnant mares, and a potentially fatal neurologic form. In 2025, accurate understanding and strict biosecurity remain your best tools for prevention. With misinformation often circulating online, this guide clarifies what we know and how to respond to this evolving viral threat.

What Is Equine Herpesvirus (EHV)? 🦠

• There are multiple types—EHV-1 and EHV-4 are most relevant to equine health.
• EHV-1: Causes respiratory illness, late-term abortion, and neurological disease.
• EHV-4: Primarily causes respiratory disease—especially in young horses.
• Like human herpesviruses, EHV can remain dormant (latent) and reactivate under stress.

Transmission 🧬

• Spread through respiratory secretions—nose-to-nose contact, coughing, shared buckets or tack.
• Also spreads via fomites (tack, clothing, grooming tools) and hands.
• Highly contagious—outbreaks can affect entire barns quickly.

Clinical Forms of EHV 😷

1. Respiratory Disease

• Fever, nasal discharge, cough, lethargy
• Most common in foals, weanlings, and young horses
• Highly contagious, typically self-limiting with supportive care

2. Abortion

• Usually occurs in the final trimester (7–11 months gestation)
• Often no warning signs—abortion can be sudden and without fever
• Mares may remain otherwise healthy but shed virus

3. Neurologic Disease (EHM – Equine Herpes Myeloencephalopathy)

• Unsteady gait, hind limb weakness, urinary incontinence
• Can progress to recumbency or death
• Caused by neurotropic strains of EHV-1

Why Diagnosis Can Be Tricky 🧪

• Bloodwork, nasal swabs, or PCR tests can detect the virus
• Latent infections may reactivate silently
• Fever may be the only early clue—take rectal temps during outbreaks

Vaccines for EHV 💉

Vaccination is an essential part of control—but it has limits:

• Reduces shedding and severity of respiratory disease and abortion
• Does not prevent neurologic disease (EHM)
• Use vaccines with a high antigen load—ask your vet for best brands
• Vaccination schedule: typically every 6 months for performance horses; pregnant mares receive boosters at 5, 7, and 9 months

Should You Vaccinate During an Outbreak? ❓

• If a horse is already exposed, do not vaccinate
• Vaccination during active infection may worsen immune response or delay identification of sick horses

Quarantine & Biosecurity 🛑

• Isolate new or returning horses for at least 14–21 days
• Isolate any horse with a fever or nasal discharge immediately
• Use separate equipment, buckets, and handlers for each group
• Disinfect tack, stalls, grooming tools, and wash racks daily

What to Do During an Outbreak 📋

• Record temperatures twice daily for all horses
• Notify your veterinarian and stable manager immediately
• Stop horse movement on and off the property—event cancelations may be required
• Support sick horses with fluids, anti-inflammatories, and stall rest

Recovery & Monitoring ⏳

• Horses shedding virus must be isolated for 21–28 days
• Perform repeat PCR tests to confirm clearance if required by events or transport regulations

Long-Term Carriers 🔄

• Many horses are latently infected—they harbor the virus for life and may shed it again under stress
• Focus on managing stress and maintaining a strong immune system year-round

Case Example: EHV-1 Show Outbreak

1. At a 3-day show, 2 horses spiked fevers on day two
2. One developed hindlimb incoordination and urinary dribbling
3. Entire event was shut down, with 35 horses quarantined on-site
4. Early detection limited spread; only 3 horses became ill, and no deaths occurred

FAQs About EHV

Q: Can I tell if a horse is shedding EHV?

A: Not always. Fever may be the only symptom. PCR testing is the best tool to confirm active infection.

Q: Is there a “safe” time to compete after exposure?

A: Horses should test negative by PCR and show no fever for at least 21 days before resuming events.

Q: Should I vaccinate during an outbreak?

A: Only unexposed horses should receive boosters. Exposed or symptomatic horses should not be vaccinated during active disease periods.

Conclusion

Equine herpesvirus is a persistent challenge—but one that can be managed with smart, science-based strategies. Vaccines reduce disease severity, and strict quarantine is your strongest tool in stopping viral spread. In 2025, awareness, biosecurity, and timely veterinary input remain the foundation of outbreak control and horse protection.

Want help planning your vaccine schedule or managing a quarantine protocol? Contact Ask A Vet at AskAVet.com or through our app 📱 to consult directly with Dr Duncan Houston and our team. 🩺🐴