Equine Herpes Virus: What Every UK Horse Owner Must Know

Equine herpes virus (EHV) is one of the most widespread yet potentially devastating diseases affecting UK horses. While most horse owners have heard of it, many underestimate its serious consequences – from abortion storms on stud farms to life-threatening neurological disease. Understanding EHV and the crucial role of vaccination could save your horse’s life and protect the broader equine community.

What is Equine Herpes Virus?

EHV is a family of viruses that infect horses, with two types of primary concern:

EHV-4: Primarily causes respiratory disease EHV-1: Causes respiratory disease, abortion in pregnant mares, and devastating neurological disease

Here’s the sobering reality: Some studies suggest infection rates of dormant EHV could be as high as 80-90% worldwide. Once infected, horses harbour the virus for life. It lies dormant in nerve tissue, reactivating during times of stress – transport, competition, illness, pregnancy – potentially causing disease or silent viral shedding that infects other horses.

EHV-1 vs EHV-4: Understanding the Difference

EHV-4: The Respiratory Troublemaker

Primary effects:

Respiratory disease (coughing, nasal discharge, fever)
Mainly affects young horses
Generally less severe
Rarely causes abortion or neurological problems

Clinical signs:

Fever (38.5-40.5°C)
Nasal discharge (clear, becoming thicker)
Soft, moist cough
Enlarged lymph nodes
Lethargy and reduced performance
Usually resolves in 1-3 weeks

Think of EHV-4 as the troublesome respiratory virus that disrupts training schedules and spreads through groups of horses, but rarely causes life-threatening complications.

EHV-1: The Dangerous One

Primary effects:

Respiratory disease (similar to EHV-4)
Abortion in pregnant mares (late-term, often 7-11 months gestation)
Neurological disease (Equine Herpesvirus Myeloencephalopathy or EHM)
Neonatal foal death

What makes EHV-1 more dangerous:

Unlike EHV-4, which mainly stays in the respiratory tract, EHV-1 can enter the bloodstream and travel to:

The pregnant uterus, damaging the placenta and causing abortion
Blood vessels in the spinal cord and brain, causing neurological disease
Throughout the body, causing more severe systemic illness

Abortion:

Typically occurs 2-4 weeks after initial infection
Mare usually shows no warning signs before aborting
Can affect multiple mares simultaneously (“abortion storms”)
Devastating emotional and financial loss

Neurological disease (EHM) signs:

Fever, followed by neurological symptoms
Incoordination and wobbliness (especially hindquarters)
Weakness, difficulty standing
Loss of tail tone
Urinary incontinence or retention
In severe cases: inability to rise, recumbency
Can be fatal or cause permanent damage

Prognosis for EHM:

Mild cases: 70-80% recovery
Moderate cases: 50-60% recovery
Severe (recumbent) cases: 20-30% recovery, often requiring euthanasia

The unpredictability and severity of EHV-1 make it one of the most feared diseases in the equine industry.

How EHV Spreads

EHV spreads horse-to-horse through several routes:

Direct contact:

Nose-to-nose contact (most efficient transmission)
Respiratory secretions from coughing/sneezing

Indirect contact:

Shared water buckets, feed bins, and equipment
Contaminated hands, clothing, and tack
Stable surfaces and trailers

Aborted material:

Foetuses and placental membranes contain extremely high viral loads
Major source of contamination during abortion outbreaks

Silent shedding:

Healthy-appearing horses can shed virus during stress-induced reactivation
This makes EHV particularly difficult to control

Environmental survival:

Virus survives several days to weeks in the environment
Longer survival in cool, damp conditions (typical UK climate)
Shorter survival in hot, dry, sunny conditions

Unlike influenza, EHV doesn’t travel long distances on air currents – it requires closer contact or contaminated surfaces to spread.

The UK Context

EHV is endemic in the UK horse population, meaning it’s constantly present. Several factors keep it a significant concern:

High horse density: UK has one of Europe’s highest horse populations per square kilometre
Active competition scene: Weekly travel and mixing at events
Busy breeding industry: Concentrated populations of pregnant mares
Cool, damp climate: Favours environmental virus survival
Young horse populations: Racing and sport horse industries produce large groups of susceptible youngsters

Recent UK outbreaks have demonstrated that EHV-1 remains a serious threat, with neurological cases resulting in deaths and permanent disabilities, and abortion storms affecting multiple mares on breeding establishments.

Clinical Signs Summary

Respiratory disease (both EHV-1 and EHV-4):

Fever
Nasal discharge
Coughing
Lethargy
Reduced appetite and performance

Abortion (EHV-1 only):

Late-term pregnancy loss (7-11 months)
Usually no warning signs in mare
Can affect multiple mares

Neurological disease (EHV-1 only):

Incoordination and weakness
Difficulty urinating
Loss of tail tone
In severe cases: inability to stand

The Cost Equation

Vaccination Costs

EHV vaccination is affordable and represents excellent value:

Primary course (3 injections):

Approx £50 per injection
Total initial cost: up to Approx £150

Ongoing protection:

Six-monthly boosters: Approx £50 per dose
Annual cost: Approx £100

Pregnant mares (essential protocol):

Boosters at 5, 7, and 9 months gestation
Additional cost during pregnancy: Approx £150
Critical for abortion prevention

The Benefits of Vaccination

EHV vaccination doesn’t prevent infection (the virus remains latent in previously infected horses), but provides crucial benefits:

What vaccination does:

Reduces disease severity significantly
Decreases viral shedding (less transmission to other horses)
Reduces respiratory disease incidence and duration
Dramatically reduces abortion risk (60-80% protective efficacy in properly vaccinated mares)
May reduce neurological disease risk and severity
Protects individual horses and the broader community

Vaccination is most effective when:

Primary course is completed properly (3 doses)
Boosters are given on schedule (six-monthly for high-risk, annual for others)
Pregnant mares receive boosters at 5, 7, and 9 months gestation
Combined with good biosecurity practices

UK Vaccination Protocols

Standard Protocol (Non-Breeding Horses)

Primary course:

First injection (from 5 months of age)
Second injection (4-6 weeks later)

Ongoing boosters:

Six-monthly for competition horses, young stock, high-density yards
Annual for lower-risk leisure horses

Pregnant Mare Protocol (Essential)

During pregnancy:

Booster at 5 months gestation
Booster at 7 months gestation
Booster at 9 months gestation

This protocol is critical for any pregnant mare and should be non-negotiable for breeding operations.

Competition and High-Risk Situations

Ensure vaccination within past 6 months before competitions
Some international events require proof of EHV vaccination
Consider timing boosters before high-risk travel or events

Biosecurity: The Essential Partner to Vaccination

Vaccination alone isn’t enough – biosecurity measures are critical:

Daily practices:

Monitor temperatures (normal: 37.5-38.5°C)
Don’t share water buckets, feed bins, or equipment
Wash hands between handling horses
Maintain good stable ventilation

New arrivals and returning horses:

Quarantine for 14-28 days
Monitor temperature twice daily
House separately with dedicated equipment
Handle isolated horses last

High-risk situations:

Minimize nose-to-nose contact at events
Don’t share water or equipment at shows
Monitor temperature daily for 7-14 days after competitions
Consider brief isolation after high-risk events

Breeding farms (critical):

Require vaccination proof for visiting mares
Quarantine new arrivals
Vaccinate all pregnant mares following protocol
Isolate immediately if abortion occurs
Proper disposal of aborted material (extreme viral loads)

During outbreaks:

Stop all horse movements on/off property
Isolate affected horses immediately
Twice-daily temperatures for all horses (21-28 days minimum)
Thorough disinfection (1% bleach, phenolic disinfectants)
Notify others who may have been exposed

Who Needs EHV Vaccination?

Essential for:

All pregnant mares (following pregnancy protocol)
Competition and event horses (six-monthly)
Young horses, who are exposed to other “well travelled” horses, or on yards with a high input / output
Horses on breeding farms or stud operations
Horses in training yards or high-density environments
Stallions at stud

Strongly recommended for:

All other horses, regardless of use
Leisure horses (annual boosters)
Elderly horses (maintain protection despite age)

The reality: Given the high prevalence of latent infection and unpredictable reactivation, virtually every horse benefits from EHV vaccination.

Common Questions

Q: My horse never leaves home. Does it still need vaccination? A: Yes. You can bring virus home on clothing/equipment, stress can reactivate latent virus already in your horse, and visitors/service providers can introduce disease.

Q: Will vaccination prevent my mare from aborting? A: Vaccination reduces abortion risk by 60-80% but doesn’t provide 100% protection. However, unvaccinated mares face substantially higher risk.

Q: Can vaccinated horses still develop neurological disease? A: Yes, though evidence suggests vaccination may reduce risk and severity. This unpredictability is why biosecurity remains critical alongside vaccination.

Q: When should I vaccinate my young horse? A: Begin at 5 months of age (after maternal antibody decline) with a two-dose primary course, then six-monthly boosters through high-risk years.

The Bottom Line

EHV represents a significant, ongoing threat to UK horses. While we cannot eliminate the virus from the equine population, we can dramatically reduce its impact through:

Vaccination:

Affordable (Approx £100 annually if doing 6 monthly boosters)
Effective (reduces disease severity, shedding, and abortion risk)
Essential for pregnant mares (specific pregnancy protocol)
Recommended for virtually all horses

Biosecurity:

Quarantine new arrivals and returning horses
Temperature monitoring during high-risk periods
Don’t share equipment
Rapid isolation of sick horses

Community responsibility:

Honest disclosure about disease status
Respect quarantine and movement restrictions
Educate yourself and others
Follow venue and event biosecurity guidelines

Take Action Today

Check your horse’s vaccination status:

Review passport – when was the last EHV vaccination?
If overdue or never vaccinated – call your vet this week
Set calendar reminders for future boosters
If breeding – ensure pregnancy protocol is followed

For pregnant mares:

Book vaccinations at 5, 7, and 9 months gestation now
Don’t miss doses – set multiple reminders
This is non-negotiable for protecting your foal investment

Implement basic biosecurity:

Monitor temperature when indicated
Have a plan for isolating sick horses
Know your vet’s emergency contact details
Educate anyone who handles your horses

Conclusion

EHV may be widespread and impossible to eliminate, but it’s far from unmanageable. For Approx £100 per year ( less than a month’s feed for most horses!) vaccination provides substantial protection against respiratory disease, dramatically reduces abortion risk in pregnant mares, and may protect against the devastating consequences of neurological disease.

Combined with sensible biosecurity practices, vaccination remains your best defense against this persistent threat. The cost of prevention is minimal; the cost of disease – in veterinary bills, lost foals, permanent disability, and heartbreak – can be catastrophic.

Talk to your vet about your horse’s EHV vaccination status today. Ensure pregnant mares follow the critical 5-7-9 month protocol. Stay vigilant with biosecurity. Protect your horse, protect others’ horses, and support a healthier UK equine community.

Prevention isn’t just smart – it’s essential.

Equine Herpes Virus