Bit & Bridle

Equine Viral Pneumonitis (EVP)

Other Names

  • Equine Viral Rhinopneumonitis (EVRP)
  • Equine Herpesvirus Infection (EHV-1, EHV-4)
  • Equine Rhinopneumonitis
  • Herpesvirus Abortion

What is Equine Viral Pneumonitis (EVP)

Equine viral pneumonitis (EVP) is a respiratory disease caused by equine herpesviruses (EHV), primarily EHV-1 and EHV-4. It is one of the most economically significant and widespread equine infectious diseases globally, presenting in three distinct clinical syndromes: respiratory disease, abortion (EHV-1 primarily), and equine herpesvirus myeloencephalopathy (EHM) — a neurological syndrome caused by EHV-1. All three forms arise from infection with the same virus family, though different strains and host immune responses determine which syndrome manifests.

EVP most commonly affects young horses — weanlings, yearlings, and horses in their first year of competition — who lack protective immunity. However, horses of all ages can be infected, particularly during periods of stress, competition travel, or when exposed to recently introduced animals. EHV is characterised by latency: after primary infection, the virus establishes a dormant state within sensory ganglia and lymphoid tissue, from which it can reactivate under stress — a key feature that explains the persistence of the virus within horse populations.

EVP has significant implications for horse welfare and the equestrian industry, with major EHV outbreaks capable of causing widespread abortion storms in breeding operations and neurological disease in competition horses.

Causes of Equine Viral Pneumonitis

Five equine herpesviruses have been identified (EHV-1 through EHV-5), of which EHV-1 and EHV-4 are the most clinically significant:

  • EHV-4 (Equine rhinopneumonitis virus): The primary cause of respiratory disease (viral pneumonitis). Principally causes upper respiratory tract signs in young horses. Less likely than EHV-1 to cause abortion or neurological disease.
  • EHV-1 (Equine abortion virus): Causes all three clinical syndromes — respiratory disease, abortion, and EHM. A neuropathogenic strain of EHV-1 carrying a specific mutation (ORF30 D752) is associated with EHM outbreaks.
  • Transmission: EHV spreads primarily by direct contact with infected nasal secretions, aborted foetuses, placental material, and foetal fluids. Indirect transmission via contaminated tack, clothing, hands, and equipment also occurs. Aerosolised droplets can spread infection over short distances. Latently infected horses that reactivate under stress are a significant source of infection.
  • Risk factors: Stress (transport, competition, weaning, change of environment), overcrowding, poor biosecurity, mixing horses from multiple sources, and immunosuppression all increase the risk of clinical disease following EHV exposure.

Symptoms and Signs

Clinical signs vary depending on the syndrome:

Respiratory syndrome (EVP):

  • Fever, often biphasic (two peaks), with temperatures reaching 38.5–41°C
  • Serous to mucopurulent nasal discharge
  • Dry, harsh cough
  • Pharyngeal lymph node enlargement
  • Depression and reduced appetite
  • Mild to moderate signs of lower respiratory involvement in severe cases
  • Most horses recover without complications within 1–3 weeks

Abortion syndrome:

  • Sudden abortion without premonitory signs, typically in the last trimester (months 7–11) of pregnancy
  • Stillbirth or birth of a weak, non-viable foal
  • The mare typically shows no signs of respiratory disease
  • Aborted foetuses are often fresh and autolytic changes are absent

Neurological syndrome (EHM):

  • Ataxia (incoordination) of the hind limbs, progressing to the forelimbs in severe cases
  • Weakness and inability to rise in severely affected horses
  • Urinary incontinence and retention
  • Faecal retention
  • Loss of perineal and tail tone
  • Altered mentation in some horses
  • Fever may precede neurological signs by 6–10 days

Diagnosis

Clinical suspicion should be confirmed with laboratory testing, particularly during outbreaks:

  • Virus isolation: Nasopharyngeal swabs or buffy coat samples submitted during the febrile phase (within the first 24–48 hours of fever) may yield EHV in tissue culture. Highly specific but requires specialist laboratory conditions.
  • PCR: Polymerase chain reaction on nasal swabs, EDTA blood, or cerebrospinal fluid (CSF) is the most sensitive and rapid method for confirming active EHV infection. Identifies EHV-1 vs. EHV-4 and detects the neuropathogenic EHV-1 variant.
  • Serology: Paired serum samples (acute and convalescent, 14–21 days apart) showing a fourfold rise in antibody titre confirm recent infection. Not useful for rapid diagnosis.
  • Post-mortem (aborted foetuses): Histopathology of foetal tissues (liver, lung, spleen) typically reveals characteristic intranuclear inclusion bodies and hepatic necrosis in EHV-1 abortion. Virus isolation and PCR from foetal tissues confirm diagnosis.
  • CSF analysis: In EHM cases, CSF may show xanthochromia (yellow discolouration), elevated protein, and variable cell changes reflecting vasculitis-induced changes in the central nervous system.
  • Haematology: Leucopenia (reduced white blood cell count) is a common finding during the viraemic phase of EHV-1 infection, as the virus targets lymphocytes.

Treatment

There is no specific antiviral treatment licensed for use in horses in the UK, though supportive care and management are important:

  • Valaciclovir and acyclovir: Antiviral drugs active against herpesviruses. Oral valaciclovir has been used in horses with EHM, as it achieves better bioavailability than acyclovir in this species. Use is off-licence in the UK, and evidence for efficacy in EHM remains limited. Specialist consultation is advised.
  • Anti-inflammatory drugs: NSAIDs (flunixin meglumine, phenylbutazone) reduce fever and systemic inflammation.
  • Corticosteroids: May be considered in EHM cases to reduce vasculitis-mediated neurological damage, though their use is controversial and not universally recommended.
  • Intensive nursing care for EHM: Recumbent horses require deep bedding, regular turning, bladder catheterisation (for urinary retention), manual evacuation of faeces, and prevention of pressure sores. Many horses with EHM that can maintain a sternal or standing position recover, given time and excellent nursing.
  • Respiratory support: Oxygen supplementation for horses with severe lower respiratory involvement.
  • Biosecurity: Isolating affected horses is critical to preventing spread. Strict hygiene — dedicated equipment, protective clothing, and minimising horse movement — must be enforced during an outbreak.

Prevention

Prevention relies on vaccination and stringent biosecurity:

  • Vaccination: Vaccines against EHV-1 and EHV-4 are available (e.g., Rhinomune, Pneumabort-K, Prevacin, Duvaxyn EHV 1,4). While vaccination does not prevent infection or eliminate latency, it reduces the severity of respiratory disease and the incidence of abortion. Regular booster vaccination of mares in late pregnancy is recommended to reduce abortion risk. Vaccination does not reliably prevent EHM.
  • Biosecurity at competitions and yards: Strict biosecurity at equestrian events — including temperature monitoring of horses on arrival, isolation of new arrivals, and restrictions on horse-to-horse contact — is essential. Many competition organisers now require proof of vaccination.
  • Quarantine of new arrivals: Newly arrived horses should be isolated for a minimum of 3 weeks and monitored for clinical signs before mixing with resident horses.
  • Stress reduction: Minimising the impact of transport, competition, and management changes reduces the risk of EHV reactivation from latency.
  • Outbreak management: When EVP is confirmed, affected premises should be quarantined, movement of horses should be restricted, and your official vet informed. Contact APHA if an abortion occurs — equine abortion is reportable in the UK.

Prognosis

Respiratory EVP in otherwise healthy horses carries a good prognosis, with most animals recovering fully within 2–4 weeks. Abortion due to EHV-1 is devastating for breeding operations but does not typically affect the mare’s long-term health or fertility. EHM carries a variable prognosis — horses with mild ataxia that remain standing have a reasonable chance of recovery, while those that become recumbent have a poor prognosis despite intensive nursing. Horses that survive EHM may have permanent neurological deficits. The risk of EHM in an exposed population is relatively low (approximately 1–10% of exposed horses in an outbreak), but the severity when it occurs makes prevention of paramount importance.

When to Call the Vet

Contact your vet promptly if:

  • Multiple horses on a yard develop fever, nasal discharge, or respiratory signs simultaneously — this pattern is characteristic of an infectious outbreak.
  • A pregnant mare aborts or produces a stillborn or weak foal.
  • Any horse develops hind limb ataxia, weakness, or neurological signs.
  • You have reason to believe your horses have been exposed to a known EHV case at a competition or event.

Report equine abortions to your vet and, in the UK, to the APHA. For outbreak guidance, see the BEVA EHV guidance page. For related respiratory conditions see our guides on equine pneumonia and equine sinusitis.

Frequently Asked Questions

What is the difference between EHV-1 and EHV-4?

EHV-4 (equine rhinopneumonitis virus) is predominantly a respiratory pathogen, causing upper respiratory tract disease mainly in young horses. EHV-1 (equine abortion virus) is more clinically versatile — it causes respiratory disease, abortion in mares, and the potentially fatal neurological syndrome EHM. A specific mutation in EHV-1 (ORF30 D752) is associated with an increased risk of EHM. PCR testing can distinguish between EHV-1 and EHV-4 and identify the neuropathogenic EHV-1 variant.

Can vaccinated horses still catch EHV?

Yes. Available EHV vaccines reduce the severity of disease and the risk of abortion but do not provide sterile immunity. Vaccinated horses can still become infected, shed virus, and develop clinical signs — though disease is typically milder. Vaccination against EHV-1 does not reliably prevent EHM. This means vaccination should be viewed as risk reduction rather than complete protection, and biosecurity measures remain essential even in vaccinated populations.

How long should a horse with EVP be rested?

A minimum of 3 weeks of rest following resolution of clinical signs is generally recommended, with a gradual return to work thereafter. Return to work should be guided by clinical examination and, ideally, haematological confirmation that values have returned to normal. Horses returned to work too early risk secondary bacterial pneumonia, prolonged recovery, and potential spread to other horses. Any horse with EHM requires a longer, vet-guided recovery period.

What should I do if there is an EHV outbreak at a competition I have attended?

Monitor your horse’s temperature twice daily for 14 days after the event. If your horse develops fever (above 38.5°C) or any other signs of illness, isolate it immediately and contact your vet. Inform your yard manager so that other horses that attended the event can also be monitored. Do not move horses off the premises until you have veterinary guidance. Check the BEVA and governing body websites for outbreak updates and specific advice for affected premises.