Equine viral arteritis (EVA) is a serious infectious disease that affects all equine species. EVA is particularly significant within breeding herds, as the disease causes abortion in up to 60% of affected pregnant mares. [1][2]

Many horses with equine viral arteritis do not show any signs of the disease. However, some horses display symptoms including a runny nose, cough, fever, and swelling of the lower limbs. [1][3]

There is no specific treatment for EVA. Horses seriously affected by the disease may need supportive care to control symptoms and prevent further complications. [3][4]

EVA is carried and spread by stallions, with an estimated 30 – 60% of intact adult male horses carrying the disease. [1][2] Testing and vaccinating horses prior to breeding is the best way to prevent the transmission of the disease. [2]

Equine Viral Arteritis (EVA)

Equine viral arteritis (EVA) is a disease that affects horses caused by a virus known as the equine arteritis virus (EAV or Alphaarterivirus equid). [1][4] EAV belongs to the RNA virus family called Arteriviridae. EAV is a horse-only virus that does not affect humans. [1]

The equine arteritis virus was first discovered in a Standardbred breeding farm near Bucyrus, Ohio in 1953. Reports of horses experiencing cold-like symptoms and abortions led to discovery of the virus. [6]

Colts and stallions are responsible for maintaining the virus within the equine population. Approximately 30-60% of stallions are persistently infected with equine viral arteritis. [2]

Unvaccinated pregnant mares are most vulnerable to the effects of EVA, and experience high rates of abortions when infected. [1] Young foals can also develop pneumonia from EVA, which can be fatal. [2]

Mad About Horses
Join Dr. Chris Mortensen, PhD on an exciting adventure into the story of the horse and learn how we can make the world a better place for all equines.
Apple Podcasts Spotify Youtube
Mad Barn Equine Nutrition Consultants


The equine arteritis virus is spread between horses through direct and close contact. Bodily secretions can become aerosolized into tiny airborne particles, leading to the spread the virus. [12][13]

Affected horses with symptoms are most contagious in the first 7 – 14 days after they become sick. [2] Sick horses can spread the virus to healthy horses through their respiratory secretions. [12]

The virus can also be spread through aerosolized urine and other secretions, although this is less common. [12] Aborted fetuses and placental tissues can also carry and transmit the EAV virus. When these tissues are not properly handled, they can potentially infect other horses. [13]

The virus can also be indirectly transmitted to horses if they come into contact with materials contaminated with viral particles, including: [3][4][10][11]

  • Tack and horse accessories
  • Grooming brushes and tools
  • Other barn equipment
  • Artificial vaginas and breeding dummies
  • Barn personnel and clothing
  • Water and feed buckets
  • Hay and bedding materials
  • Stable or barn surfaces

Transmission During Breeding

Many stallions and colts (6 months – 4 years) develop persistent infections with equine viral arteritis, even after they no longer show clinical signs. In persistently infected male horses, the virus is found within the reproductive tract and semen. [2]

Breeding mares to persistently infected horses carrying the virus leads to spread of EAV. Direct contact during live cover breeding and indirect contact through artificial insemination are the most common ways the virus is transmitted. [1][3][14][15]

Infected stallions can shed the virus in their semen for short periods of time after infection, but some stallions may shed the virus for the remainder of their lives. Contagious periods are defined as follows: [13]

  • Short-term: A few weeks after their symptoms subside
  • Intermediate: 3 – 7 months after infection
  • Long-term: Several years after infection or lifelong

EVA is also spread globally due to the international transport of horses for competition and breeding. Imports of infected semen for artificial insemination have also contributed to its international spread. [1][14][15]

It is currently unclear whether donor embryos are capable of transmitting the disease to mares. [2] Broodmares can give birth to an infected foal if they become infected late in gestation. [5]

EVA Outbreaks

The first outbreak of equine viral arteritis in the United States occurred in 1984 at a Standardbred racetrack in Kentucky during breeding season. This major outbreak raised public concerns about the impact of the virus on the equine breeding industry and led to increased efforts in monitoring and prevention. [3][16][19]

Several recent outbreaks of EVA have also been documented throughout Europe and North America. Outbreaks often occur due to the emergence of new strains of the virus. During outbreaks, abortion rates can reach between 10% to 60% in pregnant mares. [2][3][17]

Controlling EVA outbreaks involves strict biosecurity measures, such as isolating infected animals, testing and vaccinating horses, implementing proper breeding protocols, and restricting the movement of horses.

EVA is also considered a reportable disease in many countries, including the United States. This means that veterinarians and horse owners are legally required to report any suspected cases to the relevant authorities, such as state animal health departments or the United States Department of Agriculture (USDA). Reporting is essential for swift containment of outbreaks.

Disease Progression

After a horse is infected by the equine arteritis virus, clinical signs typically arise 2 – 14 days following the initial exposure.

In horses that become infected by inhaling viral particles, the virus invades the upper and lower respiratory tract. It then replicates in the lung and lymph nodes before circulating through the bloodstream. [17]

Once the virus enters the bloodstream, it targets the lining of small blood vessels in several organs throughout the body. [17] The term “arteritis” originates from the inflammation and damage inflicted upon blood vessels.

Abortions caused by EVA arise from compromised placental function due to inflammation, as well as fetal infection. Within the developing fetus, several organs such as the liver, lungs, spleen, and heart can be affected, leading to loss of the pregnancy. [2]

In most cases, mares, colts, and geldings successfully clear the infection and recover fully after symptoms subside. [2]

Clinical Signs

Most cases of EVA are subclinical, meaning that horses do not exhibit signs of the disease. H