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Key takeaways

Equine viral arteritis (EVA) is an uncommon, highly contagious viral disease of horses.

  • EVA is spread through inhalation of viral particles, direct contact with infected horses or contaminated equipment, and from infected semen
  • Symptoms in adult horses include lethargy, decreased appetite, nasal discharge, redness around the eyes, stiff gait, swelling of the limbs and lower abdomen, and hives
  • EVA causes abortions in pregnant mares, and a severe, often fatal disease in foals
  • EVA is primarily diagnosed using blood or nasal swabs, but semen can also be tested
  • Only supportive care is available to treat EVA, and includes anti-inflammatories, diuretics, and stall rest
  • While most horses have a good prognosis following recovery, a high percentage of stallions continue to asymptomatically spread viral particles in their semen
  • Vaccination and sound breeding practices are key for prevention
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A closer look: Equine Viral Arteritis

Outbreaks of Equine viral arteritis (EVA) are uncommon, but it still remains to be a highly contagious disease that is found worldwide. Although most horses recover without significant effects, EVA infection can impact the value of breeding stallions, and may cause abortion in pregnant mares. As such, immediate isolation of horses suspected to be infected with EVA is warranted. As EVA is often a reportable disease (location dependent), veterinary attention to properly diagnose the disease is necessary.

Risk factors

Abortions are common in pregnant mares with EVA. Infected males may also have lower fertility for months following recovery.

Neonatal and young foals can become infected with the virus in utero, and are born infected. These foals are prone to developing serious and potentially fatal pneumonia and diarrhea. These foals often have difficulty breathing, have an increased respiratory rate, and are lethargic to unresponsive.

The severity of disease is often dependent on the strain of the virus horses are infected by. Some viral strains of EVA cause severe disease whereas others only cause mild symptoms.

Possible causes

EVA is a viral infection caused by equine arteritis virus. Following infection, the virus infects the cells lining blood vessels, causing severe damage and inflammation (vasculitis) over subsequent days.

Horses become infected with the virus primarily by inhaling viral particles when in close contact with other infected horses. Other modes of transmission include venereal (following natural breeding or artificial insemination) and from contaminated equipment or handlers. Stallions are important sources of infection as they can continue to shed viral particles in their semen following recovery.

Main symptoms

Some horses may contract the disease and show no symptoms.

Testing and diagnosis

An initial suspicion of EVA is based on the symptoms observed during a physical examination. Other diagnostic tests may include:

  • Blood tests or nasal swabs to identify the virus
  • Bloodwork
  • Semen testing for the virus
  • Examination of aborted fetuses or placentas

Steps to Recovery

There is no specific treatment for EVA; only supportive care to reduce symptoms. Supportive treatments include:

  • Anti-inflammatories
  • IV fluids
  • Stall rest
  • Diuretics to reduce edema
  • Gradual return to breeding

Antibiotics may be warranted if secondary bacterial infections are present.

Foals that are affected are often euthanized due to an extremely poor prognosis.

Most horses recover quickly from the disease with no consequences. However, following recovery, upwards of 70% of stallions carry and shed the virus in their semen, without showing other symptoms. These stallions may be carriers for months, years, or even life-long, and can spread the virus whenever they are bred. These stallions must be managed carefully and only bred to mares who are vaccinated or previously exposed to EVA. Mares that become infected while pregnant have a high rate of abortion, stressing the importance of careful breeding management of infected stallions.

The prognosis for foals with severe infection is very poor. Euthanasia is typically recommended for these foals to prevent further spread of the virus, and due to the extremely poor prognosis.


EVA is highly contagious among equids (horses, donkeys, mules).

EVA is a preventable disease through good vaccination protocols and breeding management. Vaccines are widely available and are effective against disease in adult horses and foals, and they also prevent EVA-related abortions. Sound breeding management entails avoiding contact with infected and non-infected horses, isolating new arrivals and pregnant mares, and identifying male carriers of the virus through semen testing. To minimize viral exposure, male carriers are only bred with positive mares.

The virus that causes EVA readily becomes non-infectious with common disinfectants and detergents. Stalls and equipment in contact with infected horses require thorough sanitization to prevent spread of the virus.

Is Equine Viral Arteritis common?

Although EVA is found worldwide, outbreaks of the disease are uncommon.

Typical Treatment

Supportive care

  • Anti-inflammatories
  • Fluids
  • Stall rest
  • Diuretics
  • Physical support
  • Gradual return to breeding
  • Antibiotics


No Author - Writing for Animal and Plant Health Inspection Service
Peter J. Timoney , MVB, PhD, FRCVS - Writing for Merck Veterinary Manual
- Writing for American Association of Equine Practitioners

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