Equine Viral Arteritis Back »

Written by Meghan Syrstad and Ian Hauffe under the direction and review of of Dr. Alan Young and Dr. Rebecca Bott.


Equine viral arteritis (EVA) is a viral infection of horses caused by the equine arteritis virus. EVA leads to respiratory illness, inflammation, bleeding, and abortion in pregnant mares, creating significant economic losses to the equine industry. The disease is caused by an enveloped RNA virus, which infects equine species. The virus is environmentally sensitive, meaning that it is generally not able to persist outside of the horse, though it can persist longer in a cold environment.


EVA is spread by: aerosolized respiratory droplets, sexual intercourse, artificial insemination with infected sperm, or by people and equipment that can transfer virus from infected to non-infected horses. Rare cases of transmission by contact with feces or urine have been reported but not substantiated, likely due to the environmental sensitivity of the virus.

This viral pathogen can cause rapid illness or can persist for a long period of time. Horses of all types and some other equine species can be infected by EVA and show signs of this disease. While mortality in younger equids can be an issue, most adult horses will recover completely, though stallions may become asymptomatic carriers, shedding the virus in semen for months post-recovery. In addition, a horse that does not show any signs of infection can shed the EVA virus in the latent period prior to showing symptoms.

Implications of EVA infection

EVA is present in horse populations around the world and is a common cause of economic loss in the equine breeding industry. Due to this, many international restrictions have been placed on the transport of semen from horses that test positive to EVA. While vaccination is available, careful documentation of seronegativity (no EVA-specific antibodies) prior to it is important, as they will test seropositive for EVA post-vaccination despite being free of the disease. Mares infected with EVA while pregnant have a greater likelihood of spontaneous abortion, and foals born to an infected mare that can still spread the virus, are at risk for contracting the disease.

Clinical Signs

Signs of EVA can vary greatly, and in some instances, there are no clinical signs. In other cases, symptoms can range from depression, decreased appetite, and leukopenia (low white blood cell count) to swelling, severe respiratory distress, and abortion (Del Piero, 2000). Natural outbreaks of the disease are characterized by abortion in pregnant mares, severe respiratory disease in newborn foals, and persistent infection in stallions. Abortion in mares usually occurs between 3 and 10 months of gestation and is not preceded by any other clinical signs. Abortion rates in outbreaks can range from 10-50% and the mare can be infected by either natural or artificial insemination. The variation in signs between horses is attributed to age, physical condition, environment, and viral dose. The incubation time for the EVA virus is 3-14 days. Since the virus can be transmitted in many ways including respiratory, venereal, in utero, and through contaminated tack or equipment, early detection and control are very important.

Diagnosis, Treatment, and Prevention

EVA is a manageable disease with many control and prevention strategies. Diagnosis is based on laboratory tests since the clinical signs of EVA overlap with many other equine diseases, including equine herpesvirus and African Horse Sickness. Diagnosis can be based on virus isolation or identification of viral particles through lab testing. If a horse is diagnosed as having EVA it is important to eliminate both direct and indirect contact with susceptible horses. Most horses will clear the infection on their own within 2-3 weeks however, in severe cases, a veterinarian can provide treatment.

There is an effective vaccine against EVA and vaccination has been very successful in the control of this disease. The vaccine has been shown to be effective and safe in non-pregnant mares, fillies, geldings, colts, and stallions. Vaccine guidelines should be carefully followed. It is important to test the infectivity status of all horses prior to vaccination and document this thoroughly; a negative certification for stallions is important if the stallion or its semen is ever used for commercial purposes. The EVA test results of all semen samples should also be determined prior to artificial insemination. It is recommended to wait two weeks after vaccination to begin using a stallion for breeding.


Equine viral arteritis is a contagious and costly viral infection in horses that shows a wide variety of signs, often resulting in abortion in pregnant mares and increased mortality in foals. Stallions can become persistently infected and serve as carriers for the virus, and any non-vaccinated seropositive semen is ineligible for export. Vaccination is the preferred prevention strategy, though proper documentation of seronegative status before vaccination is critical. All horses should be tested for EVA, and horses that test negative for prior exposure to the virus should be vaccinated according to the vaccine guidelines. Horses that have previously been exposed to the virus can also be vaccinated, but the vaccine guidelines are different than non-exposed horses, so it is important to read the vaccine instructions carefully. Accurate diagnosis of the virus is important, as the signs are similar to many other equine diseases; once diagnosed EVA is a manageable disease. It is also important to test any semen before use in artificial insemination. With proper testing, isolation, and vaccination, the risk of spread this disease in breeding populations and its economic impacts can be greatly minimized.


  • AAEP. Equine Viral Arteritis. AAEP Factsheets.
  • Balasuriya, U. B. R., Go, Y. Y., & MacLachlan, N. J. (2013). Equine arteritis virus. Veterinary  Microbiology, 167(1-2), 93-122. doi:10.1016/j.vetmic.2013.06.015
  • Del Piero, F. (2000). Equine viral arteritis. Veterinary pathology, 37(4), 287-296. doi:10.1354/vp.37-4-287
  • Holyoak, G. R., Balasuriya, U. B. R., Broaddus, C. C., & Timoney, P. J. (2008). Equine viral arteritis: current status and prevention. Theriogenology, 70(3), 403-414. doi:10.1016/j.theriogenology.2008.04.020
  • Vetstream. Equine Viral Arteritis (EVA).
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