Skip to main content

Background: 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.  While various horse breeds appear equally susceptible to EAV, the prevalence of infection can vary widely with higher seropositivity rates occurring in Standardbred and Warmblood horses. 

Historically, outbreaks of EVA have been relatively infrequent. However, the number of confirmed occurrences appears to be increasing, likely attributable to increases in the: 

  • Global movement of horses.

  • Accessibility of carrier stallions.

  • Use of shipped cooled or frozen virus-infective semen. 

Transmission most frequently occurs through direct contact with virus-infective respiratory secretions, leading to widespread dissemination of the virus among susceptible horses that are close to each other.  Venereal transmission by infected stallions has a significant role in virus spread on or between breeding farms, and experience has shown that carrier stallions and EAV infective semen are principally responsible for the international spread of EVA. 

Equine arteritis virus can be efficiently spread through artificial insemination (AI) and the use of raw, cooled-transported or frozen semen.  The virus has been shown to remain viable for considerable periods of time in raw (fresh), extended or frozen semen held at temperatures equal to or less than 40C. Indirect transmission, though less significant, can occur through contact with virus-contaminated fomites. 

The majority of primary EAV infections are subclinical or asymptomatic.  EVA can vary in clinical severity both between and within outbreaks.  EVA cannot be diagnosed on clinical signs alone, as case presentation is similar to various other infectious and non-infectious equine diseases.  Laboratory confirmation is required for diagnosis. 

Clinical signs: Clinical signs, if they occur, typically develop 3-13 days post exposure (6-8 days where transmission has occurred by the venereal route).  They may include all or any combination of the following: 

  • Fever

  • Depression (Lethargy)

  • Anorexia

  • Dependent edema (ventral thorax/abdomen, lower limbs, scrotum and prepuce or mammary glands)

  • Localized or generalized urticaria

  • Supra- or peri-orbital edema

  • Conjunctivitis

  • Serous to mucoid nasal discharge 

Abortion is a frequent sequel to infection in pregnant mares and can occur from 2 months gestation to term in the unprotected mare.  Abortion due to EAV only occurs in mares that are already pregnant at time of exposure to the virus.  Young foals exposed to EAV can develop a life-threatening pneumonia or pneumoenteritis. 

A carrier state can develop following EAV infection in the post-pubertal colt or stallion.  The virus can persist in the accessary sex glands (particularly the ampulllae) of the reproductive tract of stallions for many years and may result in lifelong infection.  The carrier stallion is widely accepted as the natural reservoir of EAV and the source of diversity among naturally occurring strains of the virus.

Diagnostic Testing:  Laboratory confirmation is required for diagnosis of EVA.  This is based on virus isolation, RT-PCR testing, and/or serological examination of paired sera. 

Virus detection is accomplished by virus isolation in cell culture and/or RT-PCR testing of whole blood (EDTA or citrate but not heparin), nasal, nasopharyngeal swabs/washings, fetal and placental tissues/fluids.  

Sampling for virus detection should be initiated as early as possible after onset of clinical signs.  EAV is stable at refrigeration or lower temperatures.  With the exception of unclotted blood samples, specimens should be refrigerated or frozen and shipped on frozen freezer packs.  Unclotted blood samples should be kept cold but not frozen in transit to the laboratory. 

Sampling for serologic titers:  Paired (acute and convalescent) sera should be collected over an interval of 2-4 weeks.  Previous vaccination history against EVA should be considered when interpreting positive titers.  Vaccinated individuals may develop a serologic response or rapid rise in titer in response to natural exposure to infection. 

Vaccine: The current licensed vaccine in North America is a highly attenuated, modified live virus (MLV) product.  It has been shown to be safe and effective in stallions and non-pregnant mares.  When used in pregnant mares, recommendations are to vaccinate before 9 months gestation.  Mild post-vaccinal febrile reactions with transient lymphopenia have been observed in a small percentage of first-time vaccinated horses.  Primary vaccination provides clinical protection against EVA but does not prevent re-infection and limited replication of 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 (one ten-thousandth) than that observed with natural infection.  Nasopharyngeal shedding, if it occurs, is frequently intermittent and of less than 1 week duration.  The occasional stallion may shed very low concentrations of vaccine virus in barely detectable quantities in its semen for a limited period. 

Vaccination in the face of an EVA outbreak has been successful in controlling further spread of the virus within 7 to 10 days.  Immunization with the MLV vaccine stimulates a rapid protective response, which in turn restricts development of the cell-associated viremia and viral shedding via the respiratory tract or in semen in horses naturally exposed to infection.  As a consequence, the amount of EAV in circulation is greatly decreased, limiting further spread of the virus.

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.

The indications for vaccination against EVA have been to: 

  • Protect stallions against infection and subsequent development of the carrier state.
  • Immunize seronegative mares before being bred with EAV-infective semen.
  • Curtail outbreaks in non-breeding populations. 

NOTE:  It is not possible to differentiate a vaccine-induced antibody response from that due to natural infection.  It is strongly recommended therefore, that prior to vaccination, serum from all first-time intact males to be vaccinated against EVA be tested and confirmed negative for antibodies to EAV by a USDA/CFIA-approved laboratory.  Mares intended for export should be similarly tested.

Vaccination Schedule for Stallions: Breeding stallions that were previously vaccinated should receive a booster vaccination against EVA every 12 months and not less than three weeks prior to the start of each breeding season. 

For breeding stallions that are first-time vaccinates: 

  • Prior to initial vaccination, all stallions should undergo serologic testing and be confirmed to be negative for antibodies to EAV.
  • Testing should be performed shortly prior to or preferably at the time of vaccination.
  • Certification of seronegative test is of high importance should a vaccinated stallion be considered for export at a later date.
  • All first-time vaccinated stallions should be isolated from any other seronegative horses for not less than three weeks following vaccination and before being used for breeding. 

Teaser stallions can play a role in the introduction and dissemination of EAV within a breeding population.  Vaccination (as described above for breeding stallions) against EVA is highly recommended on an annual basis. 

Vaccination Schedule for Mares: Mares to be bred to carrier stallions or to be bred with virus-infective semen should first be tested to determine their serological status for EAV antibodies.  Seronegative mares should be vaccinated against EVA and isolated from any other seronegative horses for three weeks.  The purpose of the isolation period is twofold: 

  • To enable the vaccinated mare adequate time to develop immunity against the virus before potentially being exposed to EAV infection during natural breeding.  Retesting to confirm seroconversion prior to breeding can be done.
  • To afford ample opportunity for cessation of possible post-vaccinal viral shedding via the respiratory tract.

Following insemination, first-time vaccinated mares must be isolated for an additional three-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 three-week isolation period post insemination. 

Mares testing seropositive for antibodies to EAV 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 avoid possible risk of mechanical transmission of virus from voided semen. 

The manufacturer does not recommend use of the MLV vaccine in pregnant mares, especially in the last two months of pregnancy.  Under circumstances of high risk of natural exposure to infection, however, the vaccine has been administered to pregnant mares in order to restrict spread of the virus and to control outbreaks of the disease.  Based on experimental studies and extensive field experience using this vaccine, the last 1-2 months of pregnancy represent the time of greatest risk for a possible adverse effect on pregnancy.  This was most recently illustrated in the aftermath of the 2006 multi-state occurrence of EVA when a very limited number of abortions caused by the vaccine virus were confirmed in mares vaccinated within the final two months of gestation but not during the first two trimesters of pregnancy. 

Nurse mares can play a role in the introduction and spread of EAV among resident equine populations and should be vaccinated annually according to the above recommended protocols.

Vaccination Schedule for Foals:  The manufacturer does not recommend the use of the MLV vaccine in foals less than six weeks of age unless under circumstances of high risk of natural exposure to infection. 

Male foals (colts), especially in EAV-endemic breeds, should be vaccinated between 6-12 months of age to protect against the risk of them becoming carriers later in life.  Colts should be confirmed seronegative for antibodies to EAV prior to vaccination as described above and kept isolated for three weeks following vaccination.  Because foals of EAV-seropositive mares can carry colostral derived antibodies for up to six months, testing and vaccination should not be performed prior to six months of age.  Colt foals should be vaccinated annually in accordance with the manufacturer’s recommendations. 

Outbreak mitigation: For the non-breeding population, vaccination is an effective strategy in containing outbreaks, particularly in closely 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. 

For breeding populations, outbreaks of EVA can be complex and can have far-reaching implications.  Vaccination is a component of outbreak management but should be performed only after consultation with and under the direct supervision of a veterinarian. 

Equine Infectious Disease Outbreak:  AAEP Control Guidelines

Vaccination and Exporting of Horses: 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 the USDA-APHIS National Import and Export Service Center for the state of origin to determine the specific import requirements of that country.  A number of countries 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 and confirmation of pre-vaccination negative serological status and proof that immunized horses are vaccinated annually in accordance with vaccine manufacturer’s recommendations.