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]

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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. However, in some EVA outbreaks, all horses on the farm can have visible symptoms. [2]

Typical symptoms in adult horses include respiratory issues, behavioural changes, vision problems, as well as changes in gait. Individual horses may display other signs, including any of the following: [2][9][20]

Ocular Signs:

  • Eye discharge
  • Redness of the eyes
  • Redness of the tissues around the eye
  • Swelling around the eye (periorbital edema)
  • Shying away from or squinting in bright light (photophobia)
  • Cloudiness to the front of the eye (corneal opacity or edema)

Respiratory Signs:

  • Nasal discharge
  • Coughing
  • Difficulty breathing

Systemic Signs:

  • Swelling (edema) of the limbs
  • Midventral, scrotal, prepuce, or mammary gland swelling
  • Hives
  • Fever (up to 41°C or 105.8°F)
  • Depression
  • Anorexia
  • Yellowing of the skin, eyes, or gums (icterus)
  • Colic
  • Diarrhea
  • Red spots on the gums (petechiae)
  • Enlarged lymph nodes (lymphadenopathy)

Gait-related changes:

  • Uncoordinated gait (ataxia)
  • Stiff gait

Reproductive signs:

  • Abortions in mares
  • Subfertility in stallions

Stallions may have lower fertility due to decreased libido and poor sperm quality for 6 – 7 weeks after infection. [2][18]

If bloodwork is performed by your veterinarian, another common sign of EVA is reduced white blood cell count, also known as leukopenia. [2] Other changes in bloodwork values are often secondary to vascular injuries. [2]

Effects on Pregnancy

One of the most serious consequences of EVA is the high rate of abortions observed in pregnant mares. [1] Infected mares may experience abortion anywhere between 3 to 10 months into gestation. [2][21][22][24]

Aborted fetuses infected late in gestation may have identifiable lesions. A veterinary work-up or post-mortem examination of aborted fetuses may identify: [2][19][20][22]

  • Pinpoint red spots (petechiae, or hemorrhages) on internal organs and body cavity linings
  • Abundant fluid in the lungs, chest cavity, and abdomen, as well as surrounding the heart and under the skin

The placenta of aborted fetuses may also have: [2]

  • Thickening
  • A dull, grey appearance

Foals infected in the later stages of pregnancy may be born alive but are often smaller, weaker, and susceptible to pneumonia. These young horses face a significantly elevated risk of mortality.

Risk Factors

Risk factors for infection with equine arteritis virus include exposure to contagious horses, particularly during breeding activities or horse shows. The prevalence of this virus varies significantly between geographic regions, as well as horse breeds and ages. [2]

The risk of viral arteritis is much higher in unvaccinated horses than vaccinated ones. For example, the prevalence of EAV in unvaccinated Californian horses is 18.6%, but only 1.9% in vaccinated horses.

The highest recorded prevalence of EAV is observed in American Standardbred horses, with reports ranging from 77.5% to 84.3%. In contrast, Thoroughbreds exhibit a lower prevalence of 5.4%. [2][11][25]

Horses at a higher risk of developing severe disease from EVA include the young, the aged, those with compromised immune systems, and those with other underlying health issues. [2] The likelihood of a positive viral test appears to rise with age. [23]


Equine viral arteritis is diagnosed by a combination of clinical signs, laboratory tests, and diagnostic procedures. [2] Consult with your veterinarian if you suspect your horse may be affected by EVA.

Veterinary professionals typically conduct blood tests, nasal swabs, and semen evaluation to detect the presence of the virus and confirm the diagnosis. These tests help in assessing the horse’s infection status and guiding appropriate treatment and control measures.

Additionally, post-mortem examinations may be performed in cases of severe disease or death to confirm EVA as the cause.

Virus isolation

The gold standard test for diagnosing EVA is virus isolation. This test involves collecting samples from the horse to confirm the presence of the virus. Suitable samples for virus isolation include nasal swabs or washes, swabs from the eye’s conjunctiva, semen, and blood samples. [26]

Samples for testing should be collected as soon as symptoms begin to ensure accurate diagnosis. Beyond 28 days, the virus is generally not detectable, unless the animal becomes a carrier for EAV. [2][12] In stallions that are carriers for EAV, the virus can be detected in sperm through virus isolation. [17]

Blood samples must be collected in specialized blood tubes, such as citrated or ethylenediaminetetraacetic acid (EDTA) tubes, which allow for the separation of white blood cells. [25]

After collection, swabs or semen samples are kept in the refrigerator or freezer until testing. Blood samples are refrigerated until testing. [2][24]

If shipment is required, swabs and semen samples are packaged with dry ice, whereas blood is kept cool during shipment. [2][24]

Post-Mortem Examination

Foals and fetuses that have died from EVA may have signs that indicate a potential diagnosis. However, these signs are not unique to EVA and can be found in other conditions, requiring further diagnostic measures to confirm a diagnosis.

In aborted fetuses and young foals, tissues that can be collected for virus isolates include: [2]

  • Placenta
  • Fetal fluids
  • Lung
  • Spleen
  • Lymph nodes

Microscopic examination and special stains (immunohistochemistry) can also be applied to target and identify the virus in these collected tissues. [2]


Serology is a method to detect the number of antibodies against the virus in the horse’s blood. This diagnostic method is also referred to as an antibody or vaccine titre.

A positive test indicates that an animal has been exposed to the virus either by becoming infected or from being vaccinated. [2] For this reason, it is important to know the vaccination history of the horse to accurately interpret serology results.

To detect if a stallion is a carrier, blood samples are taken 21 to 28 days apart. The presence of very high antibody levels (four times higher than normal) indicates that a stallion is a carrier for the virus. [2]

Other Tests

Stallions that have antibodies against the virus, but are unvaccinated, require other tests to confirm if they are carriers. Additional tests include: [1]

  • Collection and testing of two separate semen samples for virus isolation
  • Breeding with two test-negative mares

If the virus is detected in the semen samples or in a mare following breeding, then the stallion is considered a carrier for EVA. [1]

Field-side tests, such as enzyme-linked immunosorbent assay (ELISA) tests, are also available and are appealing due to their ease of use. However, these tests have a high likelihood of producing false-negative results and are not recommended for diagnosis. [2][27]

PCR tests are also available to detect the virus in semen, but are considered less reliable than virus isolation. [28]

Differential Diagnosis

Differential diagnosis is the process of distinguishing between two or more medical conditions that share similar symptoms to accurately identify the specific condition affecting your horse. Before confirming a diagnosis of EAV, your veterinarian will rule out other conditions that could be responsible for their symptoms.

Other viruses or disease that cause similar or overlapping symptoms include: [8]

Disease Notification

Disease notification involves reporting or informing relevant authorities, such as veterinarians or animal health agencies, about the occurrence of specific diseases or health issues in horses.

This is a crucial step in monitoring, controlling, and preventing the spread of contagious or reportable diseases within the equine population.

If a case of EVA is suspected, the state or provincial veterinarian is contacted immediately. The following actions are also recommended: [1][17]

  • Isolate affected horses
  • Cease movement of horses on and off the farm
  • Discontinue breeding

Isolation protocols are typically lifted following 3 – 4 weeks with no evidence of EVA. [1]


There are no specific treatments for horses with EVA. Like many other viral diseases in horses, providing supportive care to address symptoms is the best course of action. [1]

Most horses with EVA do not require any treatment and recover without complications or lasting symptoms. [11] Horses with severe clinical signs may need: [3]

  • Anti-inflammatory drugs
  • Anti-pyretic drugs to control fever
  • Diuretics to eliminate excess fluid and prevent swelling
  • Rest from exercise
  • Nutritional support

In foals that develop respiratory issues due to EAV, your veterinarian may recommend using antibiotics to prevent secondary bacterial infections. Additional supportive care with fluids, medications to relieve symptoms, and nutritional support are also recommended in foals. [3]

The only effective treatment of EVA in persistently infected stallions is surgical castration. Drugs to suppress testosterone may be able to eliminate the infection, but more research is needed to evaluate this treatment. [11]

Other medications, such as gonadotropin-releasing hormone (GnRH) antagonists and vaccines designed to temporarily suppress the virus in semen, have not been fully tested. [29][30]

Some research has explored methods of “cleaning” semen to remove the virus, but more studies are required to validate efficacy. [1]


The best way to prevent the spread of EAV is by vaccinating horses and testing stallions to detect the virus prior to breeding.


EAV vaccines are designed to stimulate the horse’s immune system, enabling it to recognize and combat the virus effectively. These vaccines play a pivotal role in protecting breeding operations and preventing the spread of the virus within the equine population.

All horses should be tested for EAV prior to being vaccinated. [1] This is essential to identify carrier horses and prevent inadvertent spread of the virus.

Both modified live-virus vaccines and killed-virus vaccines are widely available. These vaccines are administered by injection into muscle. Inactivated vaccines are also available in North America and Europe. [1]

Your veterinarian will determine an individualized vaccination plan taking into account your horse’s health history and vaccine availability. Frequency of vaccination may differ depending on the type of vaccine used. [10]

Mares, geldings and foals younger than 6 months that have recovered from EVA are believed to be immune to reinfection. [7] Repeat vaccination may still be recommended for horses with a higher risk for severe infection, as well as those that travel or are bred frequently. [1]

Testing and Vaccination Schedule

While the virus remains viable for only a short time on surfaces, it can survive freezing temperatures for years. This means that the virus can survive within cryopreserved semen for many years, highlighting the importance of testing prior to breeding. [1]

Breeding Stallions:

Stallions should be tested for EAV at least 60 days before breeding and before being vaccinated. [1]

  • Stallions that test negative:
    • Vaccinate and quarantine for 28 days
    • Vaccinated annually thereafter
  • Stallions that test positive:
    • Show proof of vaccination and vaccinated annually thereafter
    • If stallion is a carrier for EVA, they should only be bred to vaccinated or test-positive mares

Breeding mares: [1]

  • Mares that test-negative:
    • Vaccinate and isolate for 21 days before breeding
    • Vaccinate annually thereafter, at least 21 days before breeding
  • Mares that test positive: [1][5]
    • Vaccinate annually
    • Isolate for 24 hours after breeding with a carrier stallion to avoid indirect transmission of virus to other horses

Pregnant Mares:

Vaccines can be administered up to three months prior to foaling. Modified live-virus vaccines are not recommended for pregnant mares, especially in the last two months of gestation. [31]


Foals born to immune mares are protected against EVA, so long as colostrum intake is sufficient at birth. Foals should be vaccinated at six months of age when maternal antibodies have waned. [31]

Modified live-virus vaccines are not recommended in young foals. [31]

Other non-breeding horses: [1]

  • Test for EVA prior to vaccination
  • Vaccinate annually

Biosecurity Measures

Implementing rigorous biosecurity measures is essential to prevent the spread of EVA within the equine population. Quarantine protocols, strict hygiene, prompt reporting and good breeding practices are key to protecting all horses.

Common disinfectants and detergents have proven effectiveness in killing the equine arteritis virus. Barn stalls and equipment should be thoroughly disinfected using products containing phenolic, chlorine, iodine, or quaternary ammonium compounds if a horse is detected with EAV. [1]

Breeding stallions must be regularly tested to identify persistently infected horses. Carrier stallions should not be housed with non-carrier stallions, and should only be bred to EVA-positive mares. [1]

After breeding, mares should be isolated from non-vaccinated and test-negative horses for at least three weeks to prevent indirect infection. [1]


  • Equine viral arteritis (EVA) is a serious infectious disease that leads to cold-like symptoms in affected horses, and can cause abortions in pregnant mares. [1][3]
  • The disease is spread by stallions, who can carry and transmit the virus without showing symptoms. [1][2]
  • The virus is transmitted through direct and indirect contact with respiratory secretions, as well as during breeding and artificial insemination.
  • Testing of nasal swabs, blood samples, and collected semen is used to diagnose EVA. [1][2]
  • No treatment is available for horses with EVA, but supportive care is administered in severe cases. Most horses recover without treatment. [1]
  • Vaccination and testing of stallions prior to breeding is the best way to prevent the spread of disease. It’s also important to implement rigorous biosecurity protocols. [1]

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