Bit & Bridle

Equine Herpesvirus (EHV)

Other Names

  • Equine Rhinopneumonitis
  • Equine Herpes Myeloencephalopathy (EHM) — the neurological form
  • EHV-1, EHV-4 (the most clinically significant strains)

What is Equine Herpesvirus (EHV)?

Equine herpesvirus (EHV) is a family of closely related viruses that are endemic in horse populations worldwide. Unlike many equine infectious diseases, EHV establishes lifelong latent infection — the virus persists in the horse’s nervous system and lymphoid tissues indefinitely and can reactivate during periods of stress or immunosuppression, causing the horse to shed virus without showing obvious clinical signs.

Nine equine herpesviruses have been identified, but EHV-1 and EHV-4 are the most clinically important:

  • EHV-1: Causes respiratory disease, abortion in mares, and the neurological syndrome equine herpes myeloencephalopathy (EHM). Arguably the most serious of the equine herpesviruses due to its potential to cause fatal neurological disease and devastating abortion storms.
  • EHV-4: Primarily causes respiratory disease (rhinopneumonitis), especially in young horses. Rarely associated with abortion or neurological disease.
  • EHV-3: Causes equine coital exanthema, a venereal disease affecting the external genitalia of stallions and mares.

What Causes EHV and How Does it Spread?

EHV is spread through:

  • Direct contact: Nose-to-nose contact with an infected horse or one that is shedding virus asymptomatically.
  • Respiratory droplets and aerosol: The virus is shed from the respiratory tract during active infection and can be transmitted by close aerosol contact within a stable environment.
  • Indirect contact (fomites): Contaminated clothing, equipment, tack, water buckets, and human hands can transmit the virus between horses. The virus can survive on surfaces for days.
  • Reactivation of latent infection: This is a key feature of EHV epidemiology. Horses that recovered from previous infection carry the virus in a dormant state. Stress — weaning, transport, competition, mixing of horses, concurrent illness — can trigger reactivation and shedding, making apparently healthy horses a source of infection.
  • Aborted foetuses and placental material: EHV-1 abortion produces enormous quantities of virus in foetal fluids and placenta — one of the most infectious biological materials in equine medicine. Safe, rapid disposal and biosecurity around aborting mares is critical.

Clinical Presentations of EHV

Respiratory Disease

The most common presentation, particularly in younger horses:

  • Fever (often biphasic)
  • Nasal discharge — initially watery, becoming mucopurulent
  • Mild cough
  • Depression and reduced appetite
  • Enlarged submandibular lymph nodes

Most horses recover from respiratory EHV within 1–3 weeks with supportive care, though secondary bacterial infections can complicate recovery. Foals and immunocompromised horses may develop more severe pneumonia.

Abortion

EHV-1 abortion is a devastating economic and welfare event. Mares typically abort in the last trimester (7–11 months gestation) without prior warning signs — no preparatory mammary development or premature milk production. The foetus is usually born dead or dies shortly after. Occasionally, a live but weak, immunocompromised foal (the “EHV foal”) is born and survives briefly.

Abortion storms — multiple mares aborting within a short period — represent some of the most dramatic and economically damaging outbreaks in thoroughbred and warmblood breeding operations.

Equine Herpes Myeloencephalopathy (EHM)

EHM is a neurological syndrome caused by EHV-1, characterised by vasculitis (inflammation of blood vessels) within the spinal cord and brain. It can occur as a sporadic individual case or as a cluster outbreak, and even vaccinated horses can be affected.

Signs of EHM:

  • Sudden onset of hindlimb ataxia (wobbliness), progressing to weakness or paralysis
  • Urinary incontinence
  • Loss of tail tone
  • Inability to rise in severe cases
  • Some horses show concurrent respiratory signs or fever preceding neurological signs by 6–10 days

Mortality in EHM outbreaks is significant — reported fatality rates range from 20–40% in severely affected horses. Horses that survive may be left with residual neurological deficits.

Diagnosis

  • PCR testing: Nasopharyngeal swabs or blood samples (buffy coat) tested by PCR are the most sensitive method for detecting active EHV-1 infection. In suspected EHM cases, samples should be taken urgently.
  • Serology: Rising antibody titres between acute and convalescent samples confirm recent infection, but this takes 2–3 weeks and does not help with acute management.
  • Cerebrospinal fluid (CSF) analysis: May show abnormalities consistent with vasculitis in EHM cases.
  • Post-mortem examination: Foetal and placental samples in abortion cases provide high viral loads and allow rapid PCR confirmation.

Treatment

There is no specific licensed antiviral treatment for EHV in horses, but management and supportive care are critical:

  • Supportive care: Rest, good nutrition, and nursing care for respiratory cases. Most recover without specific medication.
  • NSAIDs: To reduce fever and inflammation in respiratory disease.
  • Antivirals: Valacyclovir (an oral prodrug of acyclovir) has been used in EHM cases and some studies suggest benefit in reducing viral replication. It is used off-licence and its efficacy remains debated, but it is widely prescribed in severe cases given the limited alternatives.
  • Intensive nursing care for EHM: Recumbent horses require support slings, bladder catheterisation (for urinary incontinence), pressure sore prevention, and intensive monitoring. Horses unable to rise after 2–3 days carry a poor prognosis.
  • Corticosteroids: Sometimes used in severe EHM to reduce vasculitis, though this is controversial as steroids can theoretically promote viral replication.

Biosecurity for EHV Outbreaks

EHV outbreaks — particularly involving EHM or abortion — require immediate and rigorous biosecurity:

  • Isolate affected horses in a separate building with dedicated staffing and equipment
  • In suspected EHM outbreaks, create “wet zones” (isolation) and “dry zones” (clean areas) with strict movement protocols
  • Pregnant mares should be segregated from horses arriving from outside the yard at all times as standard practice
  • Notify your vet immediately and consider reporting to the Animal Health Trust’s equine disease surveillance network
  • Restrict horse movements on and off the property until the outbreak is controlled
  • In the event of an abortion, treat all foetal material as highly infectious: double-bag for disposal, disinfect the stable thoroughly, and do not allow other horses near the box

Prevention and Vaccination

EHV vaccination is available in the UK. Products include Duvaxyn EHV 1,4 and Pneumabort-K (licensed specifically for abortion prevention in mares).

Current vaccines help reduce the risk and severity of respiratory disease and reduce (but do not eliminate) the risk of EHV-1 abortion. Unfortunately, vaccines do not reliably prevent EHM — vaccinated horses can still develop the neurological form. Research is ongoing to develop improved vaccines.

BEVA recommends vaccination against EHV for:

  • Pregnant mares: at 5, 7, and 9 months gestation to reduce abortion risk
  • Horses in competition, on busy yards, or frequently mixing with unknown horses: 6-monthly boosters
  • Youngstock on busy yards

When to Call the Vet

Contact your vet urgently if:

  • A mare aborts, particularly in the last trimester
  • A horse shows sudden hindlimb ataxia or weakness — especially if there has been recent mixing with other horses
  • Multiple horses on a yard develop respiratory signs, fever, or neurological symptoms within a short period
  • You suspect an EHV outbreak at a competition or event yard

Frequently Asked Questions About Equine Herpesvirus

Can EHV be transmitted to humans?

No. Equine herpesviruses do not infect humans. Horse handlers are not at risk of contracting EHV, though standard hygiene practices are always advisable when handling sick horses.

Can a horse that has had EHV be a carrier for life?

Yes. Like all herpesviruses, EHV establishes latent infection that persists in the horse’s nervous system and lymphoid tissue indefinitely. The horse can reactivate and shed the virus asymptomatically, particularly during periods of stress, without showing any clinical signs. This is one reason EHV is so difficult to eradicate from a population.

Is EHM contagious between horses?

EHM itself (the neurological syndrome) is not directly contagious — it is an abnormal immune/vascular response to EHV-1 infection in an individual horse. However, horses with EHM are infected with EHV-1 and are therefore shedding virus that can cause infection (including potentially EHM) in other horses. Strict isolation and biosecurity during EHM cases is essential.

How does EHV differ from equine influenza?

Both viruses cause respiratory disease, but equine influenza is caused by an influenza virus (Orthomyxoviridae) whereas EHV is a herpesvirus. EHV additionally causes abortion and neurological disease. Vaccination schedules are separate. Both can cause outbreaks in horses mixing at competition venues, though the clinical and biosecurity implications are quite different.