Bit & Bridle

Equine Viral Arteritis (EVA)

Other Names

EVA, equine arteritis virus infection, equine typhoid (historical), pink eye (not to be confused with conjunctivitis), abortion plague (historical).

What is Equine Viral Arteritis?

Equine viral arteritis (EVA) is a contagious viral disease of horses caused by Equine arteritis virus (EAV), a member of the family Arteriviridae. It affects horses, donkeys, and other equids worldwide and is of major concern to the horse breeding industry because of its ability to cause abortion in pregnant mares and establish persistent infection in the reproductive tracts of stallions.

The disease was first described in the 1950s following a serious outbreak of respiratory illness and abortion at a Standardbred stud farm in Bucyrus, Ohio, USA, where the virus was subsequently isolated. Since then, EVA has been reported across North America, Europe, Asia, and beyond. Seroprevalence (the proportion of horses with antibodies indicating past exposure) varies considerably by country and breed — Standardbreds and Warmbloods show notably higher seroprevalence than Thoroughbreds in many surveys.

In the UK, EVA is a notifiable disease. This means that if you suspect a horse has EVA, you are legally required to notify the Animal and Plant Health Agency (APHA). The Horserace Betting Levy Board (HBLB) and BEVA publish codes of practice for EVA control in the Thoroughbred and sport horse industries.

The economic impact of EVA outbreaks can be enormous. Abortion storms on stud farms, temporary breeding bans on seropositive stallions, and the inability to export affected horses to countries with strict EVA regulations cause significant financial and logistical disruption to breeding operations every year.

What Causes Equine Viral Arteritis?

Equine arteritis virus (EAV) is a positive-sense, single-stranded RNA virus. It has a lipid envelope, which makes it susceptible to most common disinfectants (including hypochlorite bleach, quaternary ammonium compounds, and alcohol-based products) and unstable outside a host. The virus does not survive long in the environment under normal conditions.

Transmission occurs via several routes:

  • Respiratory route: Direct contact with respiratory secretions from acutely infected horses — nasal discharge, coughing, and exhaled aerosol. This is the primary route during outbreaks in co-mingled horses at racing yards, shows, and sales.
  • Venereal route: Semen from persistently infected (carrier) stallions is the most significant route of transmission in breeding populations. The virus replicates in the accessory sex glands of carrier stallions and is shed in semen indefinitely without causing the stallion any clinical signs. Mares bred to carrier stallions become infected and can then transmit the virus to in-contact horses via the respiratory route.
  • Indirect contact: Contaminated equipment, clothing, bedding, and shared water sources can facilitate transmission during acute outbreaks, though this is a less significant route than direct respiratory or venereal contact.

Approximately 30–35% of stallions that recover from acute EVA infection become persistently infected carriers. Testosterone appears to be required for persistent infection — geldings and mares clear the virus. The risk of a stallion becoming a carrier is not predictable from the severity of his acute illness; stallions with mild acute infection can still become persistent carriers.

Symptoms and Signs

Clinical presentation varies enormously — from subclinical infection (no visible signs) to severe systemic illness. Many naturally infected horses show no signs at all, which complicates disease control. When clinical signs do occur, they typically appear 3–14 days after exposure and may include:

General signs:

  • Fever (39–41°C), often the first sign
  • Depression and lethargy
  • Loss of appetite
  • Stiffness and reluctance to move

Respiratory signs:

  • Nasal discharge (initially watery, may become mucopurulent)
  • Conjunctivitis — reddening and discharge from the eyes, giving rise to the historical name “pink eye”
  • Coughing
  • Mild to moderate respiratory distress in severe cases

Oedema (swelling):

  • Periorbital oedema — swelling around the eyes, giving a characteristic “puffy eyed” appearance
  • Oedema of the limbs, particularly the hind limbs
  • Scrotal and preputial oedema in stallions
  • Oedema along the ventral abdomen (belly)

Skin signs:

  • Urticarial skin rash (hives) on the neck, shoulder, or flank in some horses

Reproductive effects:

  • Abortion in pregnant mares — may occur during acute illness (1–3 weeks after infection) or up to several months later. Abortion rates in unvaccinated, seronaive mares can reach 50–70% in some outbreak scenarios.
  • Temporary infertility in stallions during acute infection due to scrotal oedema and elevated testicular temperature

Severe/fatal cases: Rare, but neonatal foals infected in utero or shortly after birth may develop severe, rapidly fatal pneumonia. Young foals and immunocompromised horses are at higher risk of severe disease.

Diagnosis

Given EVA’s notifiable status in the UK, prompt and accurate diagnosis is essential. Your vet will consider the clinical picture alongside laboratory confirmation.

Virus isolation and PCR: Nasopharyngeal swabs, conjunctival swabs, and blood samples taken during the acute febrile phase (first 7 days of illness) are the most reliable samples for virus detection via PCR. Semen samples from suspect carrier stallions are tested by PCR. Aborted foetal material can also be submitted for testing.

Serology: A significant (fourfold or greater) rise in antibody titres between paired samples taken 14–21 days apart confirms recent infection. Single serology tests indicate exposure but cannot distinguish recent from historical infection. Seronegative horses with no history of vaccination that subsequently seroconvert are highly suspicious.

Virus neutralisation (VN) test: The gold-standard serological test. Used to determine whether a stallion is a carrier — carrier stallions are seropositive (have antibodies) and shed virus in semen confirmed by PCR.

In the UK, testing protocols for stallions before breeding season are set out in the HBLB Codes of Practice, which are available on the HBLB website. All stallions standing at stud should be tested annually. See also our entry on equine encephalomyelitis for another significant neurological and respiratory disease affecting horses.

Treatment

There is no specific antiviral treatment for EVA. Management is supportive and focuses on reducing clinical signs and preventing complications.

Supportive care:

  • Rest — affected horses should be removed from work immediately
  • Non-steroidal anti-inflammatory drugs (NSAIDs) such as phenylbutazone or flunixin meglumine to reduce fever, pain, and inflammation
  • Supportive nutrition and hydration
  • Cold hydrotherapy or support bandaging for significant limb oedema

Antimicrobials: Antibiotics have no effect on the virus itself but may be prescribed to prevent or treat secondary bacterial infections, particularly if there is significant respiratory involvement or skin breakdown from oedema.

Isolation: Acutely infected horses must be isolated immediately to prevent spread to in-contact animals. Strict biosecurity measures including dedicated equipment, protective clothing, and footbaths are essential.

Abortion management: Pregnant mares that abort must be isolated, and all foetal and placental material handled and disposed of safely by your vet to minimise environmental contamination.

Most adult horses recover fully from acute EVA within 1–3 weeks with appropriate supportive care. Stallions that become persistent carriers require ongoing management and must not breed with unvaccinated mares.

Prevention and Vaccination

Vaccination is the cornerstone of EVA prevention in breeding populations. A modified live virus (MLV) vaccine is available in many countries and provides good protection against clinical disease and abortion. In the UK, the vaccine requires a veterinary prescription and is used under specific biosecurity protocols.

Key vaccination considerations:

  • Mares should be vaccinated at least 3 weeks before breeding to a carrier stallion and should not be in foal at the time of vaccination with MLV
  • Stallions should be tested to confirm they are seronegative before vaccination; vaccinating a seropositive stallion without testing can complicate carrier status interpretation
  • Vaccinated horses become seropositive and are indistinguishable serologically from naturally infected horses — full vaccination records must be maintained
  • Vaccination does not eliminate virus shedding in already-carrier stallions

Biosecurity measures:

  • Test all stallions annually for carrier status via semen PCR before the breeding season
  • Only breed carrier stallions to seropositive (immune) mares or vaccinated mares
  • Quarantine and test newly arrived horses before allowing contact with the resident population
  • Isolate horses returning from shows, sales, or race meetings for a minimum of 2–3 weeks
  • Implement strict hygiene protocols around covering and AI (artificial insemination) procedures

The British Equine Veterinary Association and the Horserace Betting Levy Board publish detailed EVA control guidelines for the UK breeding industry.

When to Call the Vet

EVA is a notifiable disease in the UK — contact your vet immediately if you suspect it. Specifically, call your vet urgently if:

  • A horse develops unexplained fever, conjunctivitis, periorbital oedema, and nasal discharge — particularly during the breeding season or after contact with new horses
  • A pregnant mare aborts, especially if other mares on the yard are showing respiratory signs
  • Multiple horses on the same yard develop similar signs simultaneously
  • A stallion shows scrotal oedema or signs of systemic illness during or after the breeding season
  • You receive semen test results suggesting EVA virus shedding in a stallion
  • Any horse returns from an auction, sale, or major show event and subsequently develops fever or respiratory signs

Prognosis

The prognosis for individual adult horses with EVA is generally good — most recover fully within 2–3 weeks with appropriate supportive care. Complications including severe pneumonia or secondary infections are uncommon in adult horses but increase the risk of prolonged illness.

The reproductive prognosis is more guarded. Mares that abort during EVA infection usually return to normal breeding function in subsequent seasons. Stallions that become persistent carriers can continue to have normal reproductive function, but their management is permanently complicated by the need to only breed to immune mares.

The economic prognosis for an outbreak on a stud farm is potentially very serious — abortion storms, temporary export restrictions, and reputational damage can have lasting financial consequences.

Frequently Asked Questions

Is EVA the same as equine herpesvirus (EHV)?

No. EVA and equine herpesvirus (particularly EHV-1 and EHV-4) are entirely different viruses that can both cause respiratory disease and abortion in horses, but they are caused by unrelated pathogens. EHV is a DNA herpesvirus, while EAV is an RNA arterivirus. They require different diagnostic tests and different vaccines. Both are important diseases in the UK breeding population.

Can horses pass EVA to humans?

No. Equine arteritis virus is not known to infect humans. EVA is specific to equids. There is no public health risk to people working with EVA-infected horses, though standard hygiene precautions (washing hands, changing clothes) are always advisable when working with sick animals.

How do I know if a stallion is an EVA carrier?

Carrier stallions show no clinical signs — they appear completely healthy. Carrier status is confirmed by a combination of positive serology (seropositive on virus neutralisation test) and virus detection in semen by PCR. In the UK, the HBLB Codes of Practice require annual testing of stallions before the breeding season. Any stallion that is seropositive on first testing should have semen PCR performed to determine carrier status.

Is EVA vaccination compulsory in the UK?

Vaccination is not universally compulsory but is strongly recommended by BEVA and HBLB for breeding stallions, mares on breeding premises, and horses at high risk of exposure. Some stud farms and AI centres require proof of vaccination or negative carrier status as a condition of entry. Check with your stud farm manager and vet regarding requirements specific to your situation.