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Equine Viral Arteritis

About EVA

Much of the controversy surrounding equine viral arteritis (EVA) is based on myth, misinformation and misconception. The disease first came to the public’s attention when a 1984 epidemic on Kentucky Thoroughbred breeding farms created headline news. Because EVA can lead to abortion in mares, it is a major cause for concern among horse breeders. But, control and prevention of EVA is easy, when one first understands some facts about the disease.

What is EVA?

  • Equine Viral Arteritis (EVA) is caused by the equine arteritis virus (EAV).
  • It is an acute, contagious viral disease which affects horses and other equids only.
  • Like rhinopneumonitis, EVA is primarily a viral infection of the respiratory tract.
  • It is not a new disease, having been documented in the latter part of the 19th century.

Signs of EVA

  • Fever.
  • Swelling (edema) mostly of the legs, scrotum, sheath or mammary glands.
  • Loss of appetite (anorexia).
  • Depression.
  • Nasal discharge, initially watery, but becoming mucoid later.
  • Conjunctivitis that may be accompanied by tearing down the face and swelling above or around the eyes.
  • Skin rash (urticria), often localized to the cheeks or sides of the neck, but sometimes generalized over the body.
  • Abortion.
  • Pneumonia or pneumonia with enteritis in very young foals.

Effects of EVA

  • EVA causes contagious respiratory disease in horses and abortion in mares.
  • Most horse that get EVA develop no observable symptoms.
  • EVA seldom results in the death of an infected horse.
  • The presence of the virus in a herd seldom leads to an outbreak.

EVA-caused abortion

  • Abortion occurs late in the acute phase or early in the recovery phase of the infection, not months after the virus exposure has taken place.
  • Abortion has been observed as a sequel to either clinical or asymptomatic infection.
  • Abortion often results when an unprotected or first-time EVA-vaccinated mare has been bred with virus-infected semen and is then commingled with other pregnant mares.

How is EVA spread?

  • The virus is airborne and is spread by close contact between horses.
  • EVA is transmitted by infected mares, through the placenta, to their unborn foals. 
  • EVA is transmitted to breeding partners of acutely infected stallions and mares.
  • The virus can be transmitted via fresh-cooled or frozen semen used in artificial insemination.

How can EVA be prevented?

  • Isolate all new horses and those returning from farms, racetracks, show, sales, etc. for 3 to 4 weeks.
  • Segregate pregnant mares from other horses if possible.
  • Vaccinate EAV antibody-negative mares against EVA at least 3 weeks prior to breeding to a known carrier stallion or with virus-infective semen.
  • Isolate mares vaccinated for the first time against EVA from all but known EAV antibody-positive animals for 3 weeks after they have been bred to a carrier stallion. It is especially important to avoid contact between such mares and other pregnant mares, as they can spread the virus via the respiratory route.
  • Do not house recently vaccinated mares or mares recently bred with virus-infected semen with pregnant mares.

Managing stallions to prevent or control EVA

  • Blood test all new breeding stallions for the presence of antibodies to EAV prior to breeding season.
  • Have semen of any antibody-positive, non-vaccinated stallion laboratory tested to identify carriers.
  • Annually vaccinate all non-carrier stallions at least four weeks prior to the start of each breeding season.
  • Maintain any EAV carrier stallions in physical isolation.
  • Restrict breeding EAV carrier stallions to only vaccinated mares or mares that have previously tested positive for naturally acquired antibodies to the virus.
  • It is strongly recommended that all colts (immature male foals) between 6 and 12 months of age be vaccinated against EVA. This practice will minimize, if not eliminate, the risk of their becoming carriers of the virus later.

What should be done during an outbreak?

  • Promptly isolate all affected horses.
  • Notify your veterinarian immediately.
  • In consultation with your veterinarian, seek laboratory confirmation of a diagnosis.
  • In the case of an abortion or the death of a newborn foal, place the placenta, fetus, or foal in a leak-proof bag, keep cold (not frozen) and dispatch to nearest diagnostic laboratory.
  • Disinfect stall, any equipment or other potentially contaminated facilities using phenolic disinfectant. After treatment with disinfectant, dispose of any bedding by composting in an area away from horses.
  • Using an antiseptic recommended by your veterinarian, wash down the hindquarters and tail of any mare that has aborted and isolate her from other horses for 4 weeks.
  • Restrict movement of horses onto or off the affected premises.
  • Suspend breeding operations until the outbreak is over. Mares can be rebred later in the same breeding season, once they have fully recovered.
  • Vaccinate all at-risk horses.
  • Notify the State Veterinarian or appropriate regulatory agency of the outbreak. 


  • Carrier stallions, whether through artificial insemination or natural cover, are the primary transmitters of EVA, and may have the disease in their system for life.
  • The carrier state has never been confirmed in a stallion vaccinated against EVA.
  • Once a horse has had the disease it is resistant to it.
  • Mares, geldings and sexually immature colts do not become persistently infected or carriers of the EAV virus.
  • Vaccination is the best form of protection against EVA.
  • Consult your veterinarian to develop an EVA vaccination program.