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United States Department of Agriculture
Animal and Plant Health Inspection Service

Equine Viral Arteritis (EVA)

Last Modified: Jun 2, 2020
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Equine Viral Arteritis (EVA) is a contagious viral disease in horses caused by Equine Arteritis Virus (EAV).  Infection can go undetected by horse owners/breeders and in herds that were previously unexposed (naïve) abortion rates in pregnant mares can reach up to 70%.  Most horses infected with EAV will show minimal if any clinical signs and recover without incident although stallions can become carriers and lifelong shedders of the virus.  There is a vaccine available that has been shown to prevent infection with EAV.  Vaccination should be performed at least 21 days prior to the start of breeding season to provide adequate levels of immunity.  EVA has not been shown to be zoonotic.

Many infections are non-clinical. Older, very young or immunocompromised horses may show more severe clinical signs which can include:

  • Fever
  • Depression
  • Anorexia
  • Limb (more frequently hindlimb) edema or edema of prepuce/scrotum/mammary glands/underside of abdomen (accumulation of serous fluid under the skin/between cells)
  • Hives (localized or generalized)
  • Conjunctivitis
  • Abortions and/or stillbirths in pregnant mares
  • Decreased fertility (Stallions)

EVA is spread by acutely infected horses through respiratory secretions in close contact settings (racetracks, shows, sales, etc.).   The virus is also transmitted through breeding (natural service or artificial insemination).  Infected stallions shed the virus in semen and can serve as long term carriers.  Fomites (equipment such as buckets, brushes, shoes and humans) are another source for viral spread.

Treatment is symptomatic focused on reducing swelling and inflammation and removal of edema in patients with severe clinical signs.  Rest and adequate nursing care aids in resolving most cases.

Be familiar with whether EVA is a reportable disease in your state and report findings accordingly.

Isolation of infected horses and carrier stallions will help reduce spread of the virus.  There is a modified live vaccine available for use in the United States.  Mares being bred to known infected or carrier stallions or with semen containing EVA should be vaccinated.  Stallions that are not carriers should be vaccinated prior to the breeding season and records certifying the vaccination status of the stallion should be maintained.  Viral shed post-vaccination is possible, so vaccinated horses should be isolated per the vaccine manufacturer recommendations for a period of time after vaccination.  Use of good hygiene and biosecurity during the breeding season may reduce spread.

  1. Call your veterinarian
  2. Isolate sick horses immediately
  3. Suspend all breeding and semen collection activities


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