Bit & Bridle

Equine Rhinopneumonitis (ER)

Other Names

  • Equine Herpesvirus (EHV-1 and EHV-4)
  • Viral rhinopneumonitis
  • Equine viral rhinotracheitis

What is Equine Rhinopneumonitis?

Equine rhinopneumonitis (ER) is a collective term for a group of disease syndromes caused by two closely related herpesviruses — equid herpesvirus-1 (EHV-1) and equid herpesvirus-4 (EHV-4). These viruses are among the most significant pathogens in equine medicine worldwide, causing a spectrum of disease that includes respiratory illness, reproductive losses in mares, neonatal foal disease, and — particularly with EHV-1 — devastating neurological complications.

Both EHV-1 and EHV-4 are herpesviruses, meaning they establish lifelong latent infections in their hosts after initial exposure. Most horses become infected early in life, and the viruses periodically reactivate throughout the horse’s lifetime — often asymptomatically — providing ongoing opportunities for transmission to other horses.

EHV-4 is primarily associated with respiratory disease — the “rhinopneumonitis” component — and is a common cause of upper respiratory illness particularly in young horses. EHV-1 is responsible for all four manifestations of the disease complex: respiratory illness, abortion (sometimes in devastating outbreak form), neonatal foal pneumonitis, and myeloencephalopathy (neurological disease). EHV-1-related outbreaks at equestrian events have caused significant disruption to competition calendars globally in recent years.

Causes

EHV-1 and EHV-4 are both members of the Alphaherpesvirinae subfamily (unlike EHV-5, which is a gammaherpesvirus). They are highly contagious and spread through several routes:

  • Direct nose-to-nose contact: The primary transmission route. Infected horses shed large amounts of virus in nasal secretions during active respiratory episodes.
  • Indirect contact: Contaminated equipment including shared water and feed buckets, headcollars, bits, grooming tools, rugs, and tack. Humans can also carry virus on their hands, clothing, and footwear between horses.
  • Aborted fetuses and placentas: The fetus, placenta, and fetal fluids from an EHV-1 abortion contain extremely high concentrations of virus and are a major source of environmental contamination. Ingestion or inhalation of contaminated material by in-contact mares is a significant transmission route.
  • Aerosolisation within stables: Within the confined environment of a stable or barn, virus can spread via respiratory aerosols for some distance.
  • Stress-induced reactivation and shedding: Horses under stress — from transport, competition, change of yard, or concurrent illness — may reactivate latent virus and become transiently infectious to other horses even without showing clinical signs themselves.

Symptoms and Signs

Clinical presentation depends on the form of disease:

  • Respiratory disease (both EHV-1 and EHV-4): Fever (often biphasic — two temperature spikes several days apart), nasal discharge (initially clear then mucopurulent), lethargy, mild cough, and reduced appetite. Young horses are most severely affected. In otherwise healthy adult horses, recovery is typically complete within 1–3 weeks, though a period of reduced performance may persist.
  • Abortion (EHV-1): Typically occurs in the second half of pregnancy (after 7 months), often without preceding illness in the mare. The mare appears to abort without warning. In outbreak situations, multiple mares may abort within a short period — this is sometimes called an “abortion storm”. The fetus is typically expelled fresh with an intact placenta.
  • Neonatal foal disease (EHV-1): Foals infected in utero or at birth may develop severe pneumonia within days of birth, progressing rapidly. They may be born weak, develop respiratory distress, diarrhoea, and neurological signs. Mortality is high.
  • Equine herpes myeloencephalopathy (EHM) — EHV-1: Neurological signs including hindlimb and/or forelimb incoordination, difficulty urinating and defaecating (bladder and bowel dysfunction), recumbency, and in severe cases an inability to rise. See our detailed entry on Equine Myeloencephalopathy (EHM).

Diagnosis

  • Nasal swabs and PCR testing: Real-time PCR testing of nasopharyngeal swabs or whole blood provides rapid, sensitive, and specific detection of EHV-1 and EHV-4. This is the investigation of choice during an acute outbreak. A specific PCR assay can also differentiate between neuropathogenic (D752) and non-neuropathogenic strains of EHV-1, which has important implications for outbreak management.
  • Serology: Paired serum samples (taken 2–3 weeks apart) showing a rising antibody titre support recent active infection. However, many horses have background titres from vaccination or previous infection, limiting the specificity of a single sample.
  • Post-mortem examination: Histopathology and virus isolation from aborted fetuses and placentas definitively confirms EHV-1 abortion.
  • CSF analysis: In horses with suspected EHM, cerebrospinal fluid analysis and PCR testing can support the diagnosis of neurological disease.

Treatment

  • Respiratory disease: Treatment is primarily supportive — rest, nursing care, anti-inflammatory medication (NSAIDs), and good nutrition. Broad-spectrum antibiotics are indicated if secondary bacterial pneumonia develops. Most horses recover fully from uncomplicated respiratory disease.
  • Antiviral therapy: Valacyclovir (prodrug of acyclovir) has in vitro activity against EHV-1 and is used in some neurological cases to reduce viral replication, though the evidence for clinical benefit in horses is limited. It should be administered as early as possible in the course of neurological disease.
  • EHM management: See our detailed entry on Equine Myeloencephalopathy for management of neurological disease. Intensive nursing care, bladder management, and corticosteroids are central to management.
  • Isolation: Strict isolation of any horse suspected to have EHV-1 or EHV-4 is essential during an outbreak to prevent further spread. This typically involves box rest with separate equipment, protective clothing for handlers, and restriction of horse movement.

Prevention

  • Vaccination: Vaccines are available against EHV-1 and EHV-4 and are recommended for all horses, particularly those travelling, competing, or living in mixed yards. Licensed vaccines in the UK include Duvaxyn EHV 1,4 and Pneumabort K+1b. Vaccines reduce the severity of respiratory disease, reduce viral shedding, and help reduce the incidence of EHV-1 abortion. They do not reliably prevent the neurological form (EHM). Vaccination schedules typically involve primary courses followed by boosters every 6 months in high-risk horses (broodmares, youngstock, competition horses).
  • Strict quarantine: New arrivals should be isolated for a minimum of 21 days before mixing with resident horses. During this period they should be monitored daily for fever and respiratory signs.
  • Biosecurity at events: Avoid nose-to-nose contact between horses at shows, competitions, and sales. Use your own equipment and do not share tack, buckets, or grooming tools. Wash your hands between handling different horses.
  • Broodmare management: Keep pregnant mares in small, stable groups away from horses that travel or compete. Immediately isolate any aborting mare and her contaminated environment.
  • Temperature monitoring: Routine daily temperature taking in horses at high-risk establishments can identify early febrile cases before clinical signs develop, enabling rapid isolation.

Prognosis

Prognosis depends on the clinical form. Uncomplicated respiratory disease in healthy adult horses carries an excellent prognosis. Abortion is emotionally and economically significant, but the mare’s prognosis for future breeding is generally good. Neonatal EHV-1 disease carries a guarded to poor prognosis in severely affected foals. EHM carries a variable prognosis — mildly affected horses that remain standing typically recover well, but horses that become recumbent carry a much poorer prognosis and many do not recover to soundness.

When to Call the Vet

Contact your veterinarian immediately if you suspect EHV-1 on your premises — particularly if a mare aborts unexpectedly, if multiple horses develop fever and respiratory signs simultaneously, or if any horse develops neurological signs. EHV-1 outbreaks can spread rapidly through a horse population. Do not move horses off the premises until you have veterinary advice. In the UK, significant outbreaks should be reported to the Animal Health Trust and the HBLB for national surveillance. For related reading, see our entries on Equine Myeloencephalopathy and equine infectious disease control. The British Equine Veterinary Association provides guidance on EHV biosecurity protocols.

Frequently Asked Questions

How often should horses be vaccinated against EHV?

For most horses, EHV vaccination is recommended every 6 months — particularly those in high-risk situations including broodmares, competition horses, youngstock, and horses on busy livery yards. Some vaccination protocols recommend more frequent vaccination (every 3 months) for broodmares and horses at highest risk of exposure. Your veterinarian will advise on the most appropriate schedule based on your horse’s individual risk profile.

If my horse is vaccinated against EHV, will it be protected against the neurological form?

Current licensed EHV vaccines in the UK are not labelled as protecting against the neurological form of EHV-1 (EHM). However, vaccination may still help by reducing the overall viral load during active infection, which may lessen the severity of any neurological involvement. Vaccination remains strongly recommended despite this limitation, as it significantly reduces respiratory disease and abortion. Research into more effective vaccines against neuropathogenic EHV-1 strains is ongoing.

How long should a horse be isolated after an EHV diagnosis?

Horses with confirmed or suspected EHV-1 infection should remain isolated until they have been fever-free for at least 21 days and two consecutive PCR nasal swabs (taken 7 days apart) are negative. The exact protocol should be agreed with your veterinarian. Horses that have been in close contact with an EHV-1 case should be monitored with daily temperature checks for 21 days and isolated immediately if fever develops.

Is the EHV-1 neuropathogenic strain different from the respiratory strain?

Yes, partially. The neuropathogenic potential of EHV-1 is strongly associated with a specific mutation (D752) in the viral DNA polymerase gene, leading to a variant sometimes called the “neuropathogenic strain”. However, the neurological disease (EHM) can also occur — less commonly — with the more typical strain. PCR assays that differentiate between strains are now routinely available and are used during outbreak investigations to assess the risk of neurological disease in in-contact horses.