Equine Herpesvirus in Horses by a Vet – 2025 Guide to Respiratory vs. Neurological Disease 🧬🐴

By Dr Duncan Houston, BVSc

Introduction

Equine Herpesvirus-1 (EHV-1) is a well-known threat in equine populations worldwide. It’s infamous for causing two distinct disease forms: a mild respiratory illness and a devastating neurological condition known as equine herpesvirus myeloencephalopathy (EHM). In 2025, researchers are still uncovering why some horses progress to EHM while others recover uneventfully. Understanding risk factors, early signs, and immune responses is key to managing outbreaks.

The Two Forms of EHV-1 Disease 🔍

1. Respiratory Form

• Typically affects younger horses
• Usually mild and self-limiting
• Symptoms:
  • Fever (usually two spikes)
  • Nasal discharge
  • Mild cough
  • Depression and poor appetite

2. Neurological Form (EHM)

• More common in older horses, mares, pregnant or lactating horses
• Can cause paralysis, incontinence, or death
• Symptoms:
  • Hindlimb weakness or ataxia
  • Incoordination
  • Loss of tail tone or bladder control
  • Recumbency or inability to stand

How EHV-1 Progresses Through the Body 🧠

• Initial infection begins in the respiratory tract
• Virus enters the lymph nodes and then bloodstream (viremia)
• In some horses, virus attacks blood vessels in the spinal cord and brain
• Leads to vasculitis, blood clots, and tissue destruction

Why Some Horses Develop Neurological Signs 🔬

• Researchers believe differences in immune response determine disease outcome
• Study from Michigan State showed:
  • Young horses: typically had two fever spikes and developed respiratory disease
  • Neurological cases: showed only the second fever spike

Risk Factors for EHM

• Age – more common in mature horses
• Sex – females more likely than males
• Reproductive status – pregnant or lactating mares at increased risk
• Breed – more common in:
  • Warmbloods
  • Thoroughbreds
  • Quarter Horses
  • Paints
  • Standardbreds
  • Appaloosas
  • Fjords, Lipizzaners, Draft horses, Spanish breeds

Diagnosis of EHV-1 & EHM 🩺

• PCR testing: Nasal swabs and blood samples for viral DNA
• Fever + hindlimb weakness = test immediately
• Always isolate symptomatic horses pending results

Treatment and Supportive Care 💊

• No specific antiviral approved, but acyclovir or valacyclovir may be used off-label
• Supportive therapy includes:
  • Anti-inflammatories (NSAIDs)
  • IV fluids, slings, bladder support
  • Nursing care for recumbent horses

Prevention: The Key to Control 🛡️

Vaccination

• Reduces severity of respiratory signs and viral shedding
• Vaccines are not labeled to prevent the neurological form (EHM)
• Use high-antigen vaccines before travel or breeding season

Biosecurity

• Isolate new arrivals for 3 weeks
• Disinfect water buckets, tack, and trailers
• Separate horses by age and risk group
• Take rectal temperatures twice daily during outbreaks

FAQs About EHV-1 and EHM

Q: Can a vaccinated horse still develop neurological EHV?

A: Yes. Current vaccines are not labeled for EHM prevention—only for respiratory protection.

Q: Can EHV-1 spread through the air?

A: Yes—primarily through nasal secretions and close contact. Also on tack, clothing, and shared items.

Q: How long does the virus survive in the environment?

A: For several days on surfaces. Disinfect with bleach or accelerated hydrogen peroxide.

Conclusion

Equine herpesvirus is a persistent and unpredictable threat. While many horses recover from the respiratory form, the neurological version (EHM) can have devastating outcomes, including paralysis and euthanasia. In 2025, researchers continue to explore how immune response influences disease progression. Until more answers are found, the best protection is early detection, strict biosecurity, and strategic vaccination.

Worried about herpesvirus at your barn? Visit AskAVet.com or download our app 📱 to speak directly with Dr Duncan Houston for custom vaccine and outbreak plans. 🩺🐴