Equine Viral Arteritis (EVA) is a contagious disease of equids caused by equine arteritis virus (EAV), an RNA virus that is found in horse populations in many countries.  While typically not life-threatening to otherwise healthy adult horses, EAV can cause abortion in pregnant mares (and uncommonly cause death in young foals) and establish a long-term carrier state in breeding stallions.


The current licensed vaccine in North America is a highly attenuated, modified live virus product. It has been shown to be safe and effective in stallions and non-pregnant mares. Mild post-vaccinal febrile reactions with transient lymphopenia have been observed in a small percentage of first-time vaccinated horses.

Vaccination is an effective means of protecting against EVA. Like most vaccinations, it does not necessarily prevent viral re-infection and limited viral replication when the horse is exposed to a challenge virus.

However, in first-time vaccinates, the frequency, duration, and amount of vaccine virus that is shed via the respiratory tract is significantly less than that observed with natural infection.  Should shedding occur, it takes place during the first week following vaccination.

The occasional stallion may shed very low concentrations of vaccine virus in its semen for one to three days following first time vaccination.

Vaccination Schedules:

In planning a vaccination program against EVA, it is important to consult with state and/or federal animal health officials to ensure that any such program is in compliance with the state’s control program for EVA, if one exists.

It is not possible to differentiate the vaccine-induced antibody response from that due to natural infection. Negative certification is of importance should a vaccinated stallion be considered for export later. It is strongly recommended therefore that prior to vaccination, serum from all first-time vaccinates be tested and confirmed negative for antibodies to EAV by a USDA-approved laboratory using virus neutralization.  Testing should be performed shortly prior to, or preferably at, the time of vaccination. Mares intended for export should be similarly tested.

The indications for vaccination against EVA have been:

  1. To protect stallions against infection and the potential development of the carrier state.
  2. To protect colt foals against infection and the potential to become carriers after reaching sexual maturity.
  3. To prevent outbreaks of abortion by immunizing seronegative mares before being bred to a carrier stallion or with EAV-infective semen.
  4. To curtail outbreaks in non-breeding populations.


  • Breeding stallions, previously vaccinated against EVA:
    o Annual booster every 12 months and not less than 3 to 4 weeks prior to breeding.
  • Breeding stallions, unvaccinated or having unknown vaccine history:
    o All first-time vaccinated stallions should be isolated for 3 weeks following vaccination before being used for breeding.
  • Teaser Stallions:
    o Vaccination against EVA is recommended on an annual basis.            


  • Maiden and non-pregnant mares may be vaccinated at any time but should be vaccinated not less than 3 weeks prior to breeding.
  • Mares of unknown vaccination status to be bred to carrier stallions or to be bred with virus-infective semen: Mares to be tested to determine their serological status for EAV antibodies.

Seropositive mares: Can be bred to a carrier stallion or with infective semen for the first time without the need for prior vaccination against EVA. After breeding, such mares should be physically separated from unvaccinated or unprotected horses for 24 hours to reduce possible risk of respiratory transmission from contact with infectious virus in voided semen. Further reduction of risk possible with enhanced biosecurity measure taken with the breeding stallion and mare.

Seronegative mares: Vaccinated against EVA and isolated from any other seronegative horses for 3 weeks.
– The purpose of the isolation period is twofold:

  1. To enable the vaccinated mare adequate time to develop immunity against the disease before being exposed to EAV infection at time of breeding.
  2. To afford ample opportunity for cessation of possible post-vaccinal viral shedding via the respiratory tract.

First time vaccinated mares: Following insemination must be isolated for an additional 3-week period as they are likely to experience a limited re-infection cycle with the strain of EAV present in the semen.
– Should such mares fail to become pregnant, they can be bred back to a carrier stallion or with infective semen without the need for revaccination or an additional 3-week isolation period post-insemination.
– After being re-bred, such mares should be physically separated from unvaccinated or unprotected horses for 24 hours to avoid possible risk of respiratory transmission from contact with infectious virus in voided semen.

Embryo Transfer Mares: Recommended both donor and recipient mare, if seronegative, be vaccinated against EVA if the donor mare is to be bred with virus infective semen

  • Pregnant mares:

o Mares in foal should not be vaccinated until after foaling and not less than 3 weeks prior to breeding.
o The manufacturer does not recommend use of this vaccine in pregnant mares, especially in the last two months of pregnancy. Under circumstances of high risk of natural exposure to infection, the vaccine has been administered to pregnant mares in order to control outbreaks of the disease and abortion. Based on early experimental studies and field experience in using this vaccine, the last 1 to 2 months of pregnancy represent the greatest risk of a possible adverse effect on pregnancy. This was illustrated in the aftermath of the 2006 multi-state occurrence of EVA when a very limited number of abortions associated with the vaccine virus were confirmed in mares vaccinated within the final 2 months of gestation.

  • Nurse mares

o  Can play a role in the introduction and spread of EAV among resident equine populations and should be vaccinated annually according to recommended protocols.


  • The manufacturer does not recommend use of this vaccine in foals less than 6 weeks of age unless under circumstances of high risk of natural exposure to infection.
  • Colt (male) foals

o  Colt foals, especially in EAV endemic breeds, should be vaccinated between 6 and 12 months of age and thereafter at annual intervals to protect against the risk of becoming carriers later in life.

o Colts should be confirmed seronegative for antibodies to EAV prior to vaccination as described above and kept isolated for 3 weeks following vaccination.
– Because foals of EAV-seropositive mares can carry colostrally-derived antibodies for up to 6 months, testing and vaccination should not be performed prior to 6 months of age.

Other Considerations

  • Outbreak Mitigation:

o  Non-breeding population:  Vaccination is an effective strategy in preventing and controlling outbreaks of EVA, particularly in congregated groups of horses where isolation may be problematic. Serologic testing, as described above, should be performed on intact males and females that may be intended for future breeding purposes and/or export.

o Breeding population:  Vaccination is a component of outbreak management but should be performed only under the direct supervision of a veterinarian and in prior consultation with the State Veterinarian in states in which EVA is reportable. (View AAEP Infectious Disease Control Guidelines)

  • Vaccination and Exporting of Horses:

o In instances where there is uncertainty or concern over whether vaccination against EVA could prevent the export of a horse to a particular country, it is advisable to consult one of the USDA APHIS VS EXPORT SERVICE CENTERS  to determine the specific import requirements of that country. There are several countries that bar entry of any equid that is serologically positive for antibodies to EAV, regardless of vaccination history. Countries that do accept EVA vaccinated horses, regardless of gender, typically require stallions or colts to have a certified vaccination history including confirmation of pre-vaccination negative serological status and annual revaccination at 12-month intervals.

Copyright – AAEP 2020

Resource Type

  • Guideline


  • Vaccinations

Publish Date

April 20, 2024