Equine Viral Arteritis

Equine viral arteritis (EVA) is a contagious disease that horse owners, breeders and handlers around the world need to be aware of. EVA can have significant impacts on equine health, particularly in breeding populations where it may cause respiratory illness, abortion in pregnant mares and complications in young foals. Beyond individual horse health concerns, EVA can significantly affect breeding operations, travel and competition management. Because of the condition’s potential to spread quickly and the lack of specific treatment, supportive care and vaccination guided by a veterinarian are essential to help control symptoms and prevent further complications.

Key Takeaways

• Equine viral arteritis is a contagious viral disease that affects horses, impacting the respiratory and reproductive systems. 

• Diagnosis requires veterinary evaluation and laboratory testing, as EVA often resembles other illnesses. 

• There is no specific cure for EVA, so treatment focuses on supportive care, rest and strict isolation to prevent spread.  

• With no direct treatment available, prevention through vaccination, testing and biosecurity is strongly advised. 

What Is Equine Viral Arteritis?

Equine viral arteritis is an infectious viral disease caused by the equine arteritis virus (EAV), an enveloped ribonucleic acid (RNA) virus. The highest infection rate is seen in Standardbreds and warmbloods, with pregnant mares most vulnerable. It primarily affects the respiratory and reproductive systems, and the severity of the disease can vary widely from horse to horse. However, foals, immunocompromised horses (those with a weakened immune system) and debilitated horses are at greater risk of experiencing severe symptoms.

Exposure to the virus (EAV) that causes EVA has been documented in equine populations worldwide, but it was first identified near Bucyrus, Ohio, in 1953. The outbreak involved widespread respiratory disease and high rates of abortion in mares, prompting an investigation. Researchers isolated the virus from the lung tissue of an aborted fetus, identifying it as an etiologically distinct disease.

When horses inhale viral particles, the virus first invades the upper and lower respiratory tracts, replicates in the lungs and lymph nodes and then spreads through the bloodstream. Once in the bloodstream, the virus targets the lining of small blood vessels in various organs throughout the body. Abortions caused by EVA result from both compromised placental function due to inflammation and direct fetal infection. The developing fetus’s organs, such as the liver, lungs, spleen and heart, may be affected, leading to pregnancy loss. In most cases, mares, colts and geldings clear the infection and make a full recovery once symptoms subside. 

Carrier stallions can harbor the virus in their reproductive tracts and shed it in their semen, serving as a reservoir in horse populations. The proportion of infected stallions that develop the persistent carrier state varies considerably – published studies report rates ranging from less than 10% to over 70% depending on the outbreak, breed and individual host factors. Standardbreds have historically shown the highest seroprevalence, with up to 85% of adult Standardbreds frequently seropositive in the United States. 

Transmission can occur by:

• Respiratory secretions (sneezing or coughing)

• Venereal (sexual) transmission, including infected fresh, cooled or frozen semen 

• Indirect spread via contaminated materials, including tack and saddlery, stable surfaces, bedding, grooming tools and show supplies

• Congenital infection in foals born to mares infected late in gestation

Because EVA spreads efficiently through respiratory secretions and breeding activity, rapid containment is critical when infection is suspected.

Expert tip: According to the American Association of Equine Practitioners (AAEP), equine viral arteritis is a reportable disease in many states because outbreaks can cause significant industry disruption, including canceled events, quarantine and trading restrictions. Therefore, suspected cases must be reported to state animal health officials or the United States Department of Agriculture (USDA).  

What Are the Symptoms of Equine Viral Arteritis?

The symptoms of EVA can range from very mild to severe, and in some cases, infected horses may not show any visible signs at all (asymptomatic). The incubation period varies by route of exposure; following respiratory transmission, clinical signs typically appear within 2 to 3 days; following venereal transmission, the incubation period is usually 6 to 8 days but may be longer. 

Common signs of EVA include: 

Ocular Signs

• Swelling around the eye (periorbital edema)

• Eye discharge  

• Red, inflamed eyes and/or redness of the tissues around the eye (conjunctivitis) 

• Squinting (photophobia)

• Cloudiness

Respiratory Signs

• Nasal discharge, either clear or cloudy  

• Coughing 

• Mild respiratory distress (trouble breathing) 

Systemic Signs

• Fever (often one of the earliest signs)

• Generalized swelling (edema) of the limbs, ventrum (lower belly), around the eyes, scrotum/prepuce or mammary glands 

• Enlarged lymph nodes (lymphadenopathy)

• Hives that may also be itchy

• Colic (any type of abdominal pain)

• Changes in bowel movements, including diarrhea  

• Yellowing skin, eyes or gums (icterus)

• Red spots on the gums (petechiae)

• Reduced white blood cell count (leukopenia)

Behavior Changes

• Depression that affects daily routines 

• Reduced interest in surroundings or activity 

• Loss of appetite (anorexia)

Gait-Related Changes

• Stiffness in the limbs

• Uncoordinated walking or movement (ataxia)  

• Reluctance to move 

• Shortened stride due to discomfort or swelling 

Respiratory Signs

• Abortion in pregnant mares (anywhere between 3 to 10 months into gestation) 

• Temporary subfertility in stallions (typically poor sperm quality for 6 to 7 weeks after infection) 

• Birth of weak or compromised foals 

Expert tip: Because EVA can resemble other equine illnesses and is highly contagious, prompt veterinary attention is crucial to confirm the viral disease and limit spread. Always contact your vet if EVA is suspected to receive further guidance – do not try to handle the situation on your own. 

How Do Veterinarians Diagnose Equine Viral Arteritis?

Diagnosing EVA cannot be done solely by observing clinical signs. Veterinarians rely on a combination of history, physical examination and laboratory testing to confirm the presence of the virus. The diagnosis process typically consists of the following:

Clinical Suspicion

The diagnostic process usually begins with the veterinarian assessing the horse’s recent history, including travel, exposure to unfamiliar horses and breeding activity. Because EVA can spread through both respiratory routes and reproductive fluids, this background information is critical in raising suspicion of the disease and ruling out other conditions. A physical examination may be conducted to check for clinical signs of EVA. However, this step is not always used because some cases are subclinical, meaning horses show no obvious signs.  

Laboratory Testing

For accurate diagnosis of EVA, laboratory testing is essential. Common diagnostic testing methods include:

• Virus isolation: The gold standard for diagnosing EVA, this test involves culturing the virus from collected nasal swabs, blood or semen (for carrier stallions). It confirms active infection by growing the live virus in laboratory cell cultures, but samples must be collected as soon as symptoms begin for accurate results. The virus is usually not detectable after 28 days, unless the horse becomes a carrier for EAV. 

• Polymerase chain reaction (PCR) testing: This highly sensitive and rapid diagnostic molecular assay detects the genetic material (RNA) of EAV in samples such as blood, nasal swabs or semen. PCR testing confirms the presence of the live virus, making it essential for identifying carrier stallions. 

• Serologic testing: Blood tests that detect antibodies against EAV help determine whether a horse has been exposed to the virus in the past or has been recently infected. A second sample, taken 21 to 28 days after the initial test, is often needed to confirm the results. 

Postmortem Confirmation

In the event a horse has perished, a postmortem exam (necropsy) is conducted to confirm if EVA was the cause. It is most often used in cases of abortion or unexplained death in young foals. Veterinarians collect samples from the placenta, fetal fluids and tissues (lung, liver, spleen and lymph nodes) of aborted foals to test for the virus. The collected samples are tested using PCR or virus isolation to detect the virus antigen.

Expert tip: If EVA is suspected, isolate the potentially infected horse immediately and avoid moving it until further instructions from your veterinarian. This step can reduce the risk of an outbreak and prevent other horses from also being infected. 

How Is Equine Viral Arteritis Treated?

There is no specific antiviral cure for EVA, so treatment under veterinary guidance primarily focuses on supportive care and managing symptoms while the horse’s immune system clears the infection. Most healthy adult horses fully recover without the need for medications, but some circumstances call for them to prevent lasting symptoms and complications. 

Veterinarians typically tailor treatment based on each individual case of EVA, which may include the following: 

• Rest and isolation: The most crucial element of recovery is rest, as most horses clear the infection on their own. Infected horses should be kept in a stress-free environment, isolated from others to prevent the virus from spreading. 

• Fluid support: Horses that are dehydrated or failing to drink enough water may require intravenous (IV) fluids.

• Nutritional support: Encouraging feed intake or adjusting the horse’s diet, under veterinary guidance, may be necessary to help the horse maintain strength during recovery.

• Anti-inflammatory medications: Non-steroidal anti-inflammatory drugs (NSAIDs), such as Flunixin Meglumine (Banamine®),may be prescribed by a vet to help reduce swelling, fever and discomfort, particularly in affected stallions (uncastrated adult male horses) to protect their fertility. 

• Antibiotics: In the event secondary bacterial infections arise, antibiotics may be prescribed by your vet.

Treating Carrier Stallions

Infected stallions can become long-term, asymptomatic carriers, and there is no known treatment to eliminate the virus from the accessory sex glands. Castration remains the only reliable method of eliminating the carrier state, as it removes the site of persistent infection. Non-surgical strategies, including GnRH antagonists and anti-GnRH vaccines, have been investigated experimentally, but they are not currently validated treatments, and they are not part of standard clinical practice. These approaches may also have significant effects on libido and fertility. Do not pursue these strategies without detailed guidance from a veterinarian experienced in managing carrier stallions. 

Expert tip: Even after clinical signs improve, continue isolation for the full period recommended by your vet (usually 3 to 4 weeks). Horses can still shed the virus and infect others even if they appear healthy and fully recovered.

Can Equine Viral Arteritis Be Prevented?

With no direct treatment available for EVA, prevention becomes critical. Horse owners and handlers can protect their herds through a combination of vaccination, testing, biosecurity and thoughtful breeding. Because the virus spreads both through respiratory secretions and reproductive fluids, effective prevention requires a layered approach, including: 

Vaccination

The primary strategy for reducing the risk of EVA outbreaks is vaccination. The vaccine is meant to stimulate the horse’s immune system, enabling it to recognize and combat the virus effectively upon exposure. Vaccination is especially important for breeding stallions and mares, as well as high-risk horses, such as those that frequently travel. 

According to AAEP guidelines, horses should be vaccinated annually to retain immunity. Initial vaccination typically occurs between 6 and 12 months of age, following a seronegative test (pre-vaccination blood test) to confirm the absence of specific antibodies associated with EVA. Colts intended for breeding are typically vaccinated in this window to prevent them from becoming lifelong carriers. 

Following initial vaccination, horses should be isolated for 21 to 28 days. This period allows immunity to develop before the horse returns to contact with unvaccinated animals. It also accounts for the fact that first-time vaccinates may transiently shed very low concentrations of vaccine virus in respiratory secretions for up to a week, though at levels far below those seen with natural infection. 

Seronegative stallions should be vaccinated at least 28 days prior to breeding or semen collection, with annual boosters thereafter. The EVA vaccine (Arvac®, Zoetis) has been shown to be safe in non-pregnant mares and, when vaccination of pregnant mares is indicated, AAEP guidance recommends it be administered before 9 months of gestation. Vaccination of mares in late gestation is not recommended. Always consult your veterinarian to determine the appropriate timing based on each mare’s reproductive status.  

Hygiene and Biosecurity

Apart from vaccination, strong biosecurity practices form the foundation of EVA prevention. This includes using the following:

• Disinfectant sprays and cleaners for equipment and stable surfaces

• Disposable gloves and protective clothing for handling suspected cases

• Separate grooming supplies and buckets per horse during quarantine 

• Digital thermometer for daily temperature monitoring

Consistent hygiene protocols are especially important in various equine settings, including competitions, breeding farms and sale shows.

Testing and Screening

Routine testing is another key prevention strategy, particularly in breeding populations. Stallions should be screened for carrier status, as some can harbor and shed the virus in semen – even when appearing healthy. Testing before breeding helps identify infected or carrier horses early, allowing owners to make informed decisions and prevent transmission. 

Breeding Management

Careful breeding significantly reduces the threat of EVA outbreaks. Your breeding practices should include using only verified, disease-free semen and ensuring mares are appropriately vaccinated or immune before breeding. Your vet can advise you on how to implement controlled breeding programs to limit exposure between horses of unknown health status, further decreasing risk.

Isolation and Quarantine

New horses entering a facility, as well as those returning from shows or travel, should be quarantined before being reinserted into the general population. This isolation period, which should last 2 to 3 weeks, allows time to observe for signs of illness and reduces the likelihood of introducing the virus to healthy horses. Any horse suspected of EVA or another respiratory illness should also be isolated immediately while waiting for veterinary attention.

Expert tip: Prevention is most effective when strategies are used together, as vaccination alone is not enough. A coordinated plan developed with your veterinarian provides the strongest protection against EVA outbreaks. 

Frequently Asked Questions About Equine Viral Arteritis

Can equine viral arteritis spread to humans, horses or other animals? 

No, equine viral arteritis is species-specific and affects only horses and related equines, such as donkeys, mules and zebras. It poses no risk to humans or other animal species.

Can a vaccinated horse still get EVA?

While vaccination significantly reduces the risk of infection and helps prevent severe illness, it does not offer 100% protection. Vaccinated horses may still be exposed to the virus, though they are less likely to develop serious symptoms and spread EVA, so vet-administered vaccination is still highly recommended. 

What should I do if I suspect EVA in my horse? 

If you suspect EVA, contact your vet immediately, and isolate the horse from others. Avoid moving the horse until testing confirms whether the virus is present. Early action is key to preventing outbreaks and protecting the rest of the herd.

Support Equine Health and Recovery with Tractor Supply’s Horse Pharmacy

Equine viral arteritis spreads quickly and can disrupt routines, but with the right combination of prevention and veterinary support, it can be effectively managed and, in many cases, avoided altogether. Staying proactive through vaccination, testing and biosecurity measures is crucial to protecting both individual horses and the broader herd. Tractor Supply’s horse pharmacy carries a range of equine supplies, including prescriptions and other equine wellness products that support Life Out Here.