Equine Viral Arteritis in Horses: Symptoms, Transmission and Breeding Risks

By Dr Duncan Houston

A practical guide to EVA signs, stallion carrier risk, testing, vaccination and outbreak control.

Equine Viral Arteritis is one of the most important viral diseases for breeding farms to understand because it does not always look dramatic at first.

Many adult horses show mild signs or no obvious signs at all. That is exactly why EVA can become such a problem. A horse may appear only mildly unwell, while the virus spreads through respiratory secretions, contaminated equipment, breeding contact, or semen.

The biggest concern is not usually death in healthy adult horses. The real concern is reproductive loss, abortion in pregnant mares, severe disease in young foals, and stallions becoming long term carriers that can shed the virus in semen. APHIS describes EVA as especially important for breeders, racehorse owners and show horse owners because of its impact on breeding and movement of horses and semen. (APHIS)

Quick Answer

Equine Viral Arteritis, or EVA, is a contagious viral disease of horses caused by equine arteritis virus. It can spread through respiratory secretions, contaminated equipment, natural breeding, artificial insemination, and fresh, cooled or frozen semen. The most important risks are abortion in pregnant mares, severe pneumonia or intestinal disease in young foals, and stallions becoming carriers that shed the virus in semen. (APHIS)

If you breed horses, ship semen, collect semen, manage stallions, or have a pregnant mare exposed to a horse with fever, swelling, respiratory signs, or unknown EVA status, contact your veterinarian before moving horses or continuing breeding.

What Is Equine Viral Arteritis?

Equine Viral Arteritis is a contagious viral disease caused by equine arteritis virus, often shortened to EAV. It affects equids, including horses, ponies, donkeys, mules and zebras. In healthy adult horses, infection is often mild or subclinical, but it can cause respiratory disease, abortion, severe disease in young foals, and carrier status in stallions. (AAEP)

The word “arteritis” refers to inflammation involving blood vessels. That is why EVA can cause swelling of the limbs, swelling under the belly, swelling around the scrotum or mammary glands, conjunctivitis, hives and reproductive complications.

In practice, EVA is best thought of as a breeding biosecurity disease.

It is not just about whether one horse gets a fever. It is about whether a breeding farm can safely continue serving mares, collecting semen, shipping semen, moving horses, exporting horses, and protecting pregnant mares.

Why EVA Matters So Much in Breeding Programs

EVA becomes a major problem because it can sit at the intersection of three things breeders care about deeply:

• Pregnant mares
• Stallion fertility and semen movement
• Foal survival

APHIS notes that infected stallions can become carriers and shed virus in semen, while pregnant mares may abort. EVA can also create economic consequences through abortion losses, disease and death in young foals, reduced commercial value of carrier stallions, reduced demand for carrier stallions, export issues, and disruption to events or racing. (APHIS)

That is why EVA prevention is not a “nice extra” for breeding farms.

It is part of responsible reproductive management.

How Does EVA Spread?

EVA spreads in several important ways.

Respiratory spread

Acutely infected horses can spread the virus through respiratory secretions. This can happen in barns, stables, training facilities, sales, shows, racetracks, and other close contact settings. APHIS lists respiratory secretions from breathing and sneezing as a key route, especially in close contact environments such as barns. (APHIS)

Venereal spread

The most important breeding route is semen.

Infected stallions can shed EAV in semen and transmit it to mares during natural breeding or artificial insemination. AAEP infectious disease guidance notes that the virus may be present in infective fresh, cooled or frozen semen, and that carrier stallions act as reservoirs for EAV.

This is the part that catches people.

A stallion can look normal and still be a major EVA risk through semen.

Contaminated equipment and people

EVA can also spread indirectly through contaminated objects, hands, clothing, buckets, tack, brushes, breeding shed equipment and other fomites. APHIS specifically lists contaminated objects such as buckets, tack, brushes, shoes and clothing as possible sources of spread. (APHIS)

On a breeding farm, this means EVA control is not only about the horse. It is also about people, semen handling, phantoms, gloves, sleeves, wash areas, stocks, buckets, trailers and workflow.

Congenital infection

Foals can be infected if a mare is infected in late pregnancy. AAEP infectious disease guidance lists congenital infection in foals born to mares infected with EAV in late gestation.

How Long After Exposure Do Signs Appear?

The incubation period depends on the route of infection.

AAEP infectious disease guidance states that respiratory spread has an incubation period of about 2 to 3 days, while venereal spread is usually 6 to 8 days and may be up to 14 days in some cases.

That timeline matters for outbreak control.

If a horse becomes febrile a few days after arriving at a farm, respiratory spread becomes more suspicious.

If a mare shows signs after breeding or insemination, venereal exposure and semen status become more important.

What Are the Symptoms of EVA in Horses?

Many horses have mild signs or no obvious signs. When signs occur, they can look similar to other respiratory or systemic diseases.

Common signs include:

• Fever
• Depression
• Reduced appetite
• Nasal discharge
• Coughing
• Conjunctivitis
• Swelling around the eyes
• Swelling of the hind limbs
• Swelling along the underside of the abdomen
• Swelling around the scrotum or mammary glands
• Hives or skin rash
• Stiffness or reluctance to move
• Abortion or stillbirth in pregnant mares
• Reduced fertility in stallions

APHIS lists fever, depression, lack of appetite, respiratory disease, swelling of the hind limbs or underside of the abdomen, hives, abortion, stillbirth and decreased fertility in stallions as signs to watch for. (APHIS)

Merck Veterinary Manual also notes that EAV infection can cause respiratory illness, abortion, carrier status in stallions, and interstitial pneumonia and death in young foals. (Merck Veterinary Manual)

What Happens in Pregnant Mares?

Pregnant mares are one of the highest concern groups.

EVA can cause abortion, stillbirth, or infection of the foal if exposure occurs late in pregnancy. The risk is especially important on breeding farms because one infected stallion, one infected semen shipment, or one infected horse moving through the property can place multiple mares at risk.

The clinical detail that matters most is timing.

A mare exposed during pregnancy may not look severely ill, but abortion can still occur. In a breeding setting, sudden abortion should never be treated as “just one of those things” until infectious causes have been considered.

What Happens in Foals?

Young foals are a major concern because EVA can be severe and sometimes fatal.

Merck Veterinary Manual states that EAV infection can cause interstitial pneumonia and death in young foals, and that there is no effective treatment for EVA related pneumonia or pneumoenteritis in foals. (Merck Veterinary Manual)

Red flags in foals include:

• Fever
• Weakness
• Poor nursing
• Respiratory distress
• Coughing
• Nasal discharge
• Diarrhoea
• Depression
• Collapse

A sick newborn foal on a breeding property is not a “wait and see” situation. It needs urgent veterinary assessment.

What Happens in Stallions?

Stallions are central to EVA control because they can become long term carriers.

A stallion may recover clinically and still shed EAV in semen. APHIS states that infected stallions can become carriers and shed the virus in semen, and Merck notes that carrier stallions are the primary reservoir of EAV. (APHIS)

Carrier status does not always mean the stallion looks sick.

That is the problem.

A stallion can appear healthy, have normal libido, and still create major breeding biosecurity issues if semen contains EAV.

In practice, the stallions that require the most careful evaluation are:

• Unvaccinated stallions
• Stallions with unknown vaccination history
• Stallions with EVA antibodies but no certified vaccination history
• Stallions used for shipped semen
• Stallions entering a breeding farm
• Stallions being exported
• Stallions with fever, scrotal swelling, or reduced semen quality after illness

Merck states that stallions with EAV antibody titres and no certified vaccination history should be regarded as potential carriers until semen testing proves otherwise. (Merck Veterinary Manual)

EVA Risk Framework for Horse Owners and Breeders

Risk level	What it looks like	What it may mean	What to do
Low risk	Healthy nonbreeding horse, no exposure history, no fever, no respiratory signs	Routine disease prevention and biosecurity are the focus	Keep vaccination and movement records current
Moderate risk	New arrival, recent show or sale, mild fever, mild nasal discharge, mild swelling, unknown EVA status	Possible infectious respiratory disease	Isolate the horse and contact your vet for guidance
High risk	Breeding farm with fever, limb swelling, hives, conjunctivitis, abortion, unknown stallion status, or semen of unknown EVA status	EVA is a serious rule out	Stop breeding movement until veterinary advice and testing are arranged
Critical	Abortion cluster, sick newborn foals, confirmed EAV positive semen, suspected carrier stallion, multiple exposed pregnant mares	Potential outbreak or major breeding biosecurity event	Isolate, stop breeding, contact your vet and relevant animal health authorities immediately

The key decision point is simple: EVA becomes much more urgent when breeding, pregnancy, semen movement or foals are involved.

When Is EVA an Emergency?

EVA should be treated as urgent if you see any of the following:

• A pregnant mare aborts unexpectedly
• More than one mare aborts on the same property
• A newborn foal is weak, not nursing, coughing, struggling to breathe, or has diarrhoea
• A stallion develops fever with scrotal or preputial swelling
• A breeding stallion has fever, swelling, reduced fertility, or unknown EVA status
• Multiple horses develop fever, swelling, respiratory signs or hives
• EVA is suspected after shipped semen, artificial insemination or natural breeding
• A horse with signs has recently arrived from another property, sale, racetrack, show or breeding farm

APHIS advises owners who suspect animal disease to contact their veterinarian, and animal health professionals report diagnosed or suspected cases according to applicable regulations. (APHIS)

The practical rule is this:

If the horse is a breeding animal, pregnant mare, newborn foal, stallion, or part of a property with multiple exposed horses, do not sit on it.

How Do Vets Diagnose EVA?

EVA cannot be diagnosed by looking at the horse alone. The signs are too nonspecific and can overlap with many other diseases.

APHIS states that EVA cannot be diagnosed based only on visible clinical signs and requires laboratory testing of appropriate specimens such as nasal secretions, blood, semen, placental fluids, fetal fluids and tissues. (APHIS)

Testing may include:

• Blood testing for antibodies
• Virus neutralisation testing
• PCR testing
• Virus isolation
• Nasal or nasopharyngeal swabs
• Whole blood during acute illness
• Semen testing in stallions
• Placental and fetal tissue testing after abortion
• Paired blood samples to detect rising antibody levels

For stallions, diagnosis of carrier status usually starts with blood testing. If a stallion has antibodies to EAV but no certified vaccination history, he should be treated as a potential carrier until semen testing shows whether virus is present. (Merck Veterinary Manual)

Why Testing Before Vaccination Matters

This is one of the most important EVA points for breeders.

The vaccine can cause antibodies that show up on EVA blood testing. AAEP states that it is not possible to differentiate vaccine induced antibody response from natural infection, so first time vaccinates should be tested and confirmed negative before vaccination. (AAEP)

That means:

• Test first
• Document the result
• Vaccinate according to protocol
• Keep accurate records
• Maintain annual boosters where required
• Protect export and breeding status

This matters especially for stallions, colts intended for breeding, mares intended for export, and any horse whose future movement depends on clean documentation.

A horse with antibodies but no pre vaccination negative test can create a paperwork headache that is about as fun as trying to halter a yearling with opinions.

EVA Vaccination: Who Needs It?

Vaccination is not used the same way as every routine vaccine. EVA vaccination is targeted around risk, breeding status, local rules, movement, export requirements and farm biosecurity.

AAEP lists the main indications for EVA vaccination as protecting stallions against infection and carrier status, protecting colt foals from becoming carriers after sexual maturity, preventing abortion outbreaks by immunising seronegative mares before breeding to carrier stallions or EAV infective semen, and helping control outbreaks. (AAEP)

Breeding stallions

Previously vaccinated breeding stallions are generally boosted annually, with AAEP guidance stating boosters should be every 12 months and not less than 3 to 4 weeks before breeding. First time vaccinated stallions should be isolated for 3 weeks after vaccination before breeding, according to AAEP vaccination guidance. (AAEP)

AAEP transported semen recommendations are even more specific for semen movement, stating that seronegative stallions should be vaccinated at least 28 days before breeding or semen collection, receive annual boosters, and be isolated for 28 days after initial vaccination and annual boosters. (AAEP)

The safest approach is to work with your veterinarian, your breed requirements, semen collection centre, export rules and state or national animal health requirements.

Mares before breeding

Maiden and nonpregnant mares may be vaccinated, but AAEP recommends vaccination not less than 3 weeks before breeding. Seronegative mares being bred to a carrier stallion or EAV infective semen should be vaccinated and isolated according to veterinary protocol. (AAEP)

APHIS similarly recommends vaccinating mares at least 3 weeks before breeding with a known infected carrier stallion or semen from a known carrier. (APHIS)

Pregnant mares

Pregnant mares require careful veterinary guidance.

AAEP notes that mares in foal should not be vaccinated until after foaling and not less than 3 weeks before breeding, and that the vaccine manufacturer does not recommend use in pregnant mares, especially in the last 2 months of pregnancy, except under high risk circumstances where a veterinarian and authorities may be involved. (AAEP)

Do not vaccinate pregnant mares casually.

This is a protocol decision, not a “squeeze it in before lunch” job.

Colts before puberty

Vaccinating colt foals before sexual maturity can reduce the risk that they become carriers later in life. APHIS recommends considering vaccination of colts between 6 and 12 months, especially in breeds where EAV infection is more prevalent, and AAEP recommends testing and vaccination of colt foals between 6 and 12 months in relevant situations. (APHIS)

This is particularly important for colts with breeding potential.

Can Carrier Stallions Be Cured?

There is no simple antiviral treatment that reliably clears carrier status while preserving normal breeding value.

Merck states that there is no specific antiviral treatment for EVA and no treatment to eliminate persistent infection in stallions. It also notes that the carrier state can be permanently eliminated by surgical castration, while hormonal strategies have not been fully proven free of possible adverse effects on reproductive behaviour. (Merck Veterinary Manual)

In real world breeding terms, management is usually the focus.

Carrier stallions may be managed by:

• Confirming shedding status
• Disclosing carrier status to mare owners
• Breeding only to appropriately vaccinated or naturally seropositive mares
• Strict hygiene during semen collection
• Separate management where needed
• Avoiding exposure of unprotected mares
• Following local reporting and movement rules
• Testing semen intended for artificial insemination

Merck recommends that carrier stallions be managed separately and bred only to naturally seropositive mares or mares vaccinated against EVA. (Merck Veterinary Manual)

What Else Can Look Like EVA?

EVA is not the only cause of fever, swelling, respiratory signs, abortion or reduced fertility.

Important rule outs include:

• Equine herpesvirus 1 and 4
• Equine influenza
• Equine rhinitis virus
• Purpura haemorrhagica
• Equine infectious anaemia
• Strangles
• Allergic reactions
• Toxicity
• Placentitis
• Leptospirosis
• Other infectious causes of abortion
• Noninfectious causes of abortion
• Trauma or reproductive tract disease

AAEP infectious disease guidance specifically lists EHV 1 and 4, equine influenza, equine rhinitis virus, purpura haemorrhagica, equine infectious anaemia, allergic reactions and toxicosis from hoary alyssum as differential diagnoses for EVA like signs.

This is why lab testing matters.

The clinical signs can point you toward a problem, but they do not prove EVA.

What Should You Do If EVA Is Suspected?

1. Stop movement immediately

Do not move horses on or off the property until your veterinarian has assessed the risk.

This includes:

• Breeding mares
• Stallions
• Teaser stallions
• Foals
• Horses going to shows
• Horses going to sales
• Semen shipments
• Embryo transfer movement

2. Isolate affected horses

Isolate horses with fever, respiratory signs, swelling, hives, conjunctivitis, abortion exposure or known contact with a suspected EVA case.

APHIS recommends isolating infected horses and carrier stallions, and isolating new or returning horses for 3 to 4 weeks. (APHIS)

3. Call your veterinarian

Your vet will help decide which horses need testing, which samples are needed, whether authorities need to be notified, and whether breeding should stop.

Tell your vet:

• Which horses are sick
• Which horses are pregnant
• Which stallions have bred recently
• Which semen was collected or shipped
• Which mares were inseminated
• Which horses moved on or off the property
• Vaccination records
• Pre vaccination test records
• Any recent abortions or sick foals

4. Suspend breeding activity

Merck recommends suspending breeding activity on breeding farms during a suspected EVA outbreak to minimise further spread. (Merck Veterinary Manual)

This can feel painful commercially, but continuing to breed during an investigation can turn one problem into a farm wide disaster.

5. Trace semen and contacts

Trace:

• Natural breeding contacts
• Artificial insemination records
• Fresh semen shipments
• Cooled semen shipments
• Frozen semen shipments
• Embryo transfer donor and recipient mares
• Shared handlers
• Shared equipment
• Transport links

In EVA outbreaks, paperwork is not boring. Paperwork is outbreak control.

6. Clean and disinfect equipment

EVA can spread through contaminated objects, so strict hygiene matters.

Focus on:

• Breeding phantom
• Stocks
• Buckets
• Halters
• Lead ropes
• Twitches
• Gloves
• Sleeves
• Collection equipment
• Wash areas
• Boots
• Clothing
• Vehicles and trailers

7. Follow veterinary and authority guidance

Merck recommends notifying relevant animal health authorities in suspected outbreaks, isolating affected and in contact horses, restricting movement, collecting specimens early, suspending breeding, disinfecting equipment, and considering vaccination of at risk horses under veterinary guidance. (Merck Veterinary Manual)

Lesson From a Real EVA Outbreak

The 2006 multi state EVA occurrence in the United States showed how quickly the virus can spread through breeding networks.

An APHIS report described widespread spread through semen from a stallion that was acutely and later persistently infected with EAV. The report noted high seroprevalence on many affected farms, virus isolation from aborted fetuses, and confirmed carrier status in multiple stallions. (APHIS)

The lesson is brutally simple:

A stallion does not need to look sick to create a major EVA problem.

That is why testing, semen status, vaccination records and breeding biosecurity are not optional details.

Common Mistakes With EVA

Mistake 1: Vaccinating before testing

This is the big one. Once vaccinated, a horse may test positive for antibodies, and AAEP notes that vaccine induced antibodies cannot be differentiated from natural infection by routine serology. (AAEP)

Test first. Document properly. Then vaccinate.

Mistake 2: Assuming a healthy looking stallion is safe

Carrier stallions can shed virus in semen while appearing normal. Semen testing and certified records matter more than eyeballing the horse.

Mistake 3: Using shipped semen without EVA status

Fresh, cooled and frozen semen can all be important sources of EAV. Merck states that fresh cooled or frozen semen should be tested by a laboratory with appropriate expertise to confirm negative EAV status, especially if imported. (Merck Veterinary Manual)

Mistake 4: Continuing breeding during a suspected outbreak

Breeding activity should stop while EVA is being investigated. Continuing to collect, ship or inseminate can increase spread and multiply the damage.

Mistake 5: Mixing new arrivals too quickly

APHIS recommends isolating new horses and returning horses from farms, sales, events or racetracks for 3 to 4 weeks. (APHIS)

Mistake 6: Forgetting the paperwork

EVA control depends heavily on documentation:

• Pre vaccination test results
• Vaccination dates
• Annual booster records
• Semen test results
• Breeding dates
• Mare exposure records
• Movement records
• Export paperwork

When there is a disease investigation, “I think he was vaccinated” is not a plan.

How To Prevent EVA on a Breeding Farm

A strong EVA prevention plan should include:

• Testing stallions before breeding season
• Semen testing where indicated
• Pre vaccination serology before first EVA vaccination
• Annual boosters for breeding stallions where recommended
• Vaccination of at risk mares before breeding
• Colt vaccination before sexual maturity where appropriate
• Quarantine for new and returning horses
• Separate management of pregnant mares
• Strict hygiene during collection and breeding
• Written semen handling protocols
• Clear disclosure of carrier status
• Movement records
• Immediate isolation of febrile or suspicious horses
• Veterinary involvement before breeding to known carrier stallions

APHIS recommends testing new breeding stallions, semen testing antibody positive nonvaccinated stallions, vaccinating noncarrier breeding stallions before breeding season, vaccinating mares before breeding to known carrier stallions or semen, and using strict hygiene during breeding or semen collection from carrier stallions. (APHIS)

This is not about making a farm paranoid.

It is about making the farm hard for the virus to exploit.

Frequently Asked Questions

Is EVA deadly?

EVA is not usually fatal in healthy adult horses, but it can be serious in breeding populations. It can cause abortion in pregnant mares, severe pneumonia or intestinal disease in young foals, and long term carrier status in stallions. (Merck Veterinary Manual)

Can a stallion with EVA still be used for breeding?

Sometimes, but only under controlled veterinary and biosecurity protocols. Carrier stallions should be identified, managed separately, and bred only to naturally seropositive mares or mares properly vaccinated against EVA. (Merck Veterinary Manual)

Can EVA spread through frozen semen?

Yes. EVA can spread through fresh, cooled and frozen semen. AAEP notes that virus may be present in infective fresh, cooled or frozen semen, and Merck states that fresh cooled or frozen semen can be an important source of EAV.

Should horses be tested before EVA vaccination?

Yes, especially stallions, colts intended for breeding, mares intended for export, and first time vaccinates. AAEP strongly recommends testing and confirming horses are negative before first vaccination because vaccine induced antibodies cannot be distinguished from natural infection on routine serology. (AAEP)

Can humans catch EVA from horses?

There is no evidence that equine arteritis virus is zoonotic or transmissible to humans. (Merck Veterinary Manual)

The Bottom Line

Equine Viral Arteritis is not usually the scariest looking disease in an adult horse, but it can be one of the most disruptive diseases in a breeding program.

The danger is not just fever or nasal discharge.

The danger is a pregnant mare aborting, a young foal becoming severely ill, a stallion becoming a carrier, or semen spreading virus across properties before anyone realises there is a problem.

If you breed horses, manage stallions, ship semen, import semen, handle pregnant mares, or run a breeding facility, EVA control should be built around four things:

• Test before vaccination
• Keep excellent records
• Vaccinate strategically
• Stop movement and breeding quickly if EVA is suspected

That is how you protect horses, foals, bloodlines and the entire breeding operation.

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If you are unsure whether your horse’s signs, breeding history or semen records create an EVA risk, ASK A VET™ can help you organise the key details and decide when veterinary testing or urgent biosecurity advice is needed.