Equine Viral Arteritis in Horses: Respiratory and Breeding Disease Overview

Quick Answer
  • Equine viral arteritis, or EVA, is a contagious viral disease that can cause fever, nasal discharge, swollen eyelids and limbs, and serious breeding problems.
  • Many horses have mild signs or no obvious signs, but pregnant mares can abort and some stallions can become long-term semen shedders after natural infection.
  • Diagnosis requires lab testing. Your vet may collect blood, nasal swabs, semen, or fetal and placental samples depending on the horse's age, sex, and breeding status.
  • Most horses recover with supportive care, but isolation and breeding management are important to limit spread within barns and breeding programs.
  • Vaccination can be part of prevention for at-risk horses, especially breeding stallions, colts before puberty, and mares bred to known carrier stallions.
Estimated cost: $250–$2,500

What Is Equine Viral Arteritis in Horses?

Equine viral arteritis, often called EVA, is a contagious disease caused by equine arteritis virus. It can affect the respiratory tract and blood vessels, which is why horses may develop fever, nasal discharge, red eyes, swelling, and general illness. In many horses the disease is mild, but it matters a great deal in breeding programs because it can also cause abortion in pregnant mares.

EVA spreads in two main ways: through respiratory secretions and through infected semen. That second route is especially important because some stallions become long-term carriers after natural infection and can continue shedding virus in semen even when they look healthy. This carrier state is a major reason the virus persists in horse populations.

For pet parents and breeding managers, the big picture is this: EVA is not always dramatic at first, but it can have major consequences for herd health, breeding plans, and horse movement. If your horse has fever and swelling after exposure to new horses, breeding activity, or semen shipments, your vet may want to rule it out quickly.

Symptoms of Equine Viral Arteritis in Horses

  • Fever
  • Depression or low energy
  • Reduced appetite
  • Clear nasal discharge
  • Cough
  • Red eyes or conjunctivitis
  • Excess tearing
  • Swollen eyelids or tissue around the eyes
  • Swelling of the limbs, sheath, mammary gland, or underside of the belly
  • Hives or skin rash
  • Abortion in pregnant mares
  • Pneumonia or severe illness in young foals

Some horses with EVA look only mildly sick, and some show no obvious signs at all. That can make outbreaks harder to spot, especially in busy barns or breeding facilities. Swelling around the eyes, legs, sheath, or udder can be a useful clue when it appears along with fever and respiratory signs.

See your vet promptly if your horse has fever, eye swelling, limb edema, or nasal discharge after contact with new horses or breeding activity. See your vet immediately if a pregnant mare aborts, if a foal seems weak or develops breathing trouble, or if multiple horses in the barn become ill around the same time.

What Causes Equine Viral Arteritis in Horses?

EVA is caused by equine arteritis virus, an RNA virus found in horse populations around the world. Horses can become infected by breathing in virus from respiratory secretions, especially when horses are housed closely together, traveling, showing, racing, or otherwise mixing with unfamiliar horses.

The other major route is venereal transmission, meaning spread during breeding or through transported semen. This is especially important in stallions. After natural infection, some stallions become persistent carriers and continue shedding virus in semen for long periods, even if they appear healthy. These carrier stallions are central to the long-term spread of EVA in breeding populations.

Pregnant mares are a special concern because infection can lead to abortion. Young foals can also become severely ill. Not every exposed horse gets seriously sick, and not every horse needs the same prevention plan. Your vet can help match testing, isolation, and vaccination decisions to your horse's age, sex, breeding role, and travel risk.

How Is Equine Viral Arteritis in Horses Diagnosed?

EVA cannot be confirmed by symptoms alone. Your vet usually needs laboratory testing because the signs can overlap with other equine infectious diseases. Depending on the case, samples may include blood, nasal secretions, semen, or, in abortion cases, placental and fetal tissues and fluids.

Testing may involve PCR or virus detection methods to look for the virus directly, along with serology to look for antibodies. In breeding stallions, the workup often starts with a blood test. If an unvaccinated stallion has antibodies, your vet may recommend semen testing to determine whether he is a carrier and actively shedding virus.

Timing matters. Samples collected early in illness can improve the chance of finding the virus. If your horse has been vaccinated in the past, that history also matters because it can affect how antibody results are interpreted. Your vet may also coordinate with a state animal health official or breeding registry when EVA is suspected in a breeding horse.

Treatment Options for Equine Viral Arteritis in Horses

Spectrum of Care means you have options. Here are treatment tiers at different price points.

Budget-Conscious Care

$250–$700
Best for: Mild, uncomplicated illness in a stable adult horse when the main goals are comfort, monitoring, and reducing spread while still involving your vet.
  • Farm call or exam
  • Basic supportive care plan from your vet
  • Isolation from other horses
  • Temperature monitoring and rest
  • Anti-inflammatory medication if your vet recommends it
  • Limited diagnostic testing when finances are tight
Expected outcome: Many adult horses with mild disease recover well with supportive care, but this approach may miss important breeding-status information or herd-level risk.
Consider: Lower upfront cost range, but less testing can make it harder to confirm EVA, guide breeding decisions, or identify a carrier stallion.
Most Common

Recommended Standard Treatment

$700–$1,500
Best for: Most suspected EVA cases, especially horses in boarding barns, show barns, or breeding programs where diagnosis affects management decisions.
  • Full veterinary exam and isolation plan
  • CBC/chemistry as indicated
  • PCR and or serology for EVA
  • Nasal swab and blood sampling
  • Supportive medications and fluids as needed
  • Breeding-risk counseling for mares and stallions
  • Follow-up testing or recheck
Expected outcome: Good for most adult horses with uncomplicated disease when supportive care and biosecurity are started early.
Consider: Higher cost range than conservative care, but it gives clearer answers for isolation, breeding safety, and outbreak control.

Advanced / Critical Care

$1,500–$2,500
Best for: Foals, severe systemic illness, abortion cases, valuable breeding animals, or situations where herd, registry, or movement consequences are significant.
  • Hospitalization or intensive on-farm monitoring
  • IV fluids and more aggressive supportive care
  • Expanded lab work and repeat testing
  • Semen testing for carrier status in stallions
  • Abortion workup with fetal and placental testing
  • Foal support for severe respiratory disease
  • Coordination with breeding managers, diagnostic labs, and animal health officials when needed
Expected outcome: Outcome depends on the horse's age, severity, and breeding status. Adult horses often recover, while foals and breeding-related cases can be more complicated.
Consider: Most intensive and highest cost range, but provides the most information for complicated medical and breeding decisions.

Cost estimates as of 2026-03. Actual costs vary by location, clinic, and individual case.

Questions to Ask Your Vet About Equine Viral Arteritis in Horses

Bring these questions to your vet appointment to get the most out of your visit.

  1. Based on my horse's signs and exposure history, how likely is EVA compared with other infectious diseases?
  2. Which samples do you recommend today: blood, nasal swab, semen, or abortion tissues?
  3. Does my horse need strict isolation, and for how long?
  4. If this is a breeding stallion, do we need semen testing to check for carrier status?
  5. If this mare is pregnant or recently aborted, what testing should be done on the mare, fetus, and placenta?
  6. What supportive care can safely be done at home, and what changes would mean I should call you right away?
  7. Should other horses on the property be tested, monitored for fever, or vaccinated?
  8. Could vaccination affect future testing, export paperwork, or breeding plans for this horse?

How to Prevent Equine Viral Arteritis in Horses

Prevention focuses on testing, vaccination, and breeding management. In breeding programs, new stallions should be tested before the breeding season. If an unvaccinated stallion has antibodies, semen testing may be needed to determine whether he is a carrier. Good recordkeeping matters because vaccination history changes how test results are interpreted.

Vaccination is used selectively, not universally, in horses at meaningful risk. Current guidance supports vaccinating noncarrier breeding stallions, colt foals before puberty when appropriate, and seronegative mares before breeding to a known carrier stallion or with infective semen. First-time vaccinated stallions and mares need an isolation period after vaccination, and pregnant mares should not be vaccinated unless your vet specifically advises otherwise under current label and regulatory guidance.

Daily biosecurity also helps. Isolate horses with fever or respiratory signs, avoid sharing equipment between sick and healthy horses, and use careful hygiene during breeding and semen handling. If your horse travels, breeds, or lives in a high-traffic barn, ask your vet to build a prevention plan that fits your horse's real risk rather than using a one-size-fits-all approach.

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