Vet Guide: Herpes Virus Vaccination in Horses 2025 🧬🐴

By Dr Duncan Houston, BVSc

Equine herpesvirus (EHV) is a common and serious threat—especially the neurological form of EHV-1, which has caused significant outbreaks and fatalities at major equestrian events. Despite available vaccines, prevention of the neurologic form remains a challenge. 🧠⚠️

In this 2025 vet-reviewed article, Dr Duncan Houston explores the facts about EHV vaccination, explains what current vaccines can and cannot do, and shares key findings from recent research. 💉🐎

1. What Is Equine Herpesvirus? 🧬

Equine herpesvirus type 1 (EHV-1) causes several disease forms in horses:

• 🌬️ Respiratory illness (fever, nasal discharge, cough)
• 🍼 Abortion in pregnant mares
• 🧠 Neurological disease (also known as Equine Herpes Myeloencephalopathy, or EHM)

The neurologic form (EHM) is rare but devastating, potentially resulting in hind limb paralysis, incontinence, and death. ⚠️

2. What Do Vaccines Actually Protect Against? 💉

Current EHV vaccines are only labeled to protect against:

• 🦠 Respiratory disease (EHV-1 and EHV-4)
• 👶 Abortion from EHV-1 infection

No EHV vaccine is approved for protection against the neurologic form (EHM). Cases of EHM have occurred in fully vaccinated horses. 🧪

3. Does Vaccination Increase Risk of EHM? 🤔

This has been debated. Some speculate that vaccination might increase the risk of EHM in certain situations due to immune system response modulation—but there’s no definitive proof of this, and studies show mixed outcomes. 📉📈

What’s clear: vaccination may not prevent EHM—but it can reduce virus shedding and clinical signs, especially when using high-antigen vaccines. 🧬✅

4. Oklahoma State Study: What We Learned 🧪📊

Researchers at Oklahoma State vaccinated 6 aged mares three times with Pneumabort-K, a high-antigen EHV vaccine. Another 6 mares were left unvaccinated. All horses were then exposed to a neurologic strain of EHV-1. Results:

• 5 of 6 unvaccinated horses developed severe neurological signs
• Only 1 of 6 vaccinated horses had severe symptoms

This suggests that while vaccines may not prevent infection, they may reduce the severity of disease when using high-antigen products. 📉🧠

5. Recommended Vaccine Strategy for EHV 💡

Even without neurologic strain protection, vaccination is still essential. A smart EHV vaccine protocol includes:

• 📅 Boosters every 6 months for show or travel horses
• 💉 Pregnant mares: Pneumabort-K at 5, 7, and 9 months gestation
• 🏇 Use high-antigen vaccines like Pneumabort-K or Calvenza in at-risk horses

6. Biosecurity Still Matters 🔐

Since vaccination isn’t a complete shield, focus on preventing exposure:

• 🚫 Avoid nose-to-nose contact at shows
• 🧴 Sanitize hands and equipment between horses
• 📈 Take rectal temps twice daily during travel
• 🏥 Quarantine new arrivals for 14 days

7. Summary Table: EHV Vaccine Facts 📋✅

8. Final Thoughts from Dr Duncan Houston 💬

While no vaccine guarantees full protection against neurologic herpes, smart use of high-antigen vaccines and strict biosecurity protocols can help reduce disease severity and spread. Stay informed, vaccinate strategically, and always isolate sick or new horses. 🧠🐴💉

Need help planning your EHV vaccination protocol or biosecurity measures? Visit AskAVet.com or download the Ask A Vet app 📲 for equine vaccine plans tailored to your horse’s lifestyle and risk level.

— Dr Duncan Houston, BVSc