Equine Viral Arteritis in Horses: Signs, Testing and Breeding Risk

By Dr Duncan Houston

Equine viral arteritis can look like an ordinary respiratory infection, yet the consequences may extend far beyond a few days of fever and nasal discharge.

An outbreak can cause abortions, severe illness in newborn foals, temporary infertility in stallions and major restrictions on breeding, semen movement, competition and international trade. Many infected horses show few or no signs, which makes testing and breeding records just as important as recognising a visibly sick horse. (USDA APHIS)

The real concern is not simply whether a horse recovers from the acute illness. It is whether the virus has reached pregnant mares, whether a postpubertal colt or stallion may become a semen-shedding carrier, and how many horses or semen shipments were exposed before the problem was recognised.

Quick Answer

Equine viral arteritis, or EVA, is a contagious viral disease of horses and other equids. It spreads mainly through respiratory secretions during acute infection and through semen from acutely infected or persistently infected stallions.

Most healthy adult horses recover, but EVA can cause abortion, severe disease in young foals and a long-term carrier state in sexually mature intact males. Suspected cases require immediate isolation, veterinary examination, laboratory testing and notification of the relevant animal-health authority.

EVA at a Glance

Question	Practical answer
What causes EVA?	Equine arteritis virus, commonly abbreviated EAV
Which animals can be infected?	Horses, donkeys, mules, zebras and other equids
Are most infections obvious?	No. Many are subclinical or mild
Main respiratory route	Close contact with an acutely infected horse
Main breeding route	Infective semen from an acute or carrier stallion
Can frozen semen transmit EAV?	Yes
Which horses become long-term carriers?	Sexually mature intact males
Can mares become persistent carriers?	No
Does a positive blood test prove a stallion is a carrier?	No. Semen must be tested for virus
Main reproductive consequences	Abortion, stillbirth, congenital infection and temporary stallion subfertility
Is there a specific antiviral treatment?	No. Treatment is supportive
Is vaccination available?	Yes in some regions, but it is risk-based and requires careful documentation
Is EVA dangerous to humans?	No known zoonotic risk

 

What Is Equine Viral Arteritis?

Equine viral arteritis is a contagious systemic and reproductive disease caused by equine arteritis virus.

The term “arteritis” refers to inflammation and injury involving blood vessels. This vascular damage explains many of the recognisable signs, including swelling of the legs, lower abdomen, scrotum, sheath, mammary glands and tissues around the eyes.

Most naturally infected horses do not develop dramatic illness. Others may develop fever, depression, respiratory signs, conjunctivitis, oedema, hives or reproductive complications. Mortality is rare in healthy adult horses but can occur in neonatal or young foals with severe pneumonia or intestinal disease.

Are Some Breeds More Susceptible?

All equids should be considered susceptible.

EAV antibody prevalence is often higher in Standardbreds and some Warmblood populations, which is why targeted vaccination of colt foals is particularly discussed in these breeding sectors. This does not mean Quarter Horses, Thoroughbreds or other breeds are naturally protected. A breed being heavily involved in a previous outbreak does not prove that it is biologically more susceptible. (USDA APHIS)

How Does EVA Spread?

EVA has several transmission routes, but the importance of each route depends on whether the horse has an acute infection or a persistent stallion carrier state.

Respiratory Transmission

Respiratory spread is the main route during acute infection.

Virus may pass through respiratory and ocular secretions when susceptible horses are kept in close contact at:

• Breeding farms

• Sales

• Racetracks

• Shows

• Training yards

• Veterinary hospitals

• Transport facilities

• Shared barns and yards

Some acutely infected horses show minimal signs but remain infectious. Following respiratory exposure, clinical signs may begin within approximately two to three days.

Venereal Transmission

EAV can be spread during natural service or artificial insemination.

Infective semen may be:

• Fresh

• Chilled

• Extended

• Frozen

Freezing does not make infected semen safe. In fact, semen kept at very low temperatures can preserve viral infectivity for long periods.

Carrier stallions continuously shed virus in the sperm-rich fraction of their semen, despite appearing healthy and often maintaining normal breeding performance.

Transmission From Abortion Material

An aborted fetus, placenta, fetal fluids and contaminated bedding can contain large amounts of virus.

Horses may be exposed through:

• Direct contact with fetal tissues

• Placental fluids

• Contaminated bedding

• Hands and clothing

• Buckets and equipment

• Movement of contaminated material around the farm

An equine abortion should therefore never be treated as a simple clean-up job. Isolate the mare, secure the fetus and placenta, restrict access and call the veterinarian before materials are moved or discarded.

Indirect Transmission

EAV can also move through contaminated objects and personnel, including:

• Head collars

• Twitches

• Breeding phantoms

• Collection equipment

• Buckets

• Brushes

• Clothing

• Gloves

• Footwear

• Hands

The risk is greatest when fresh respiratory secretions, semen, vaginal secretions or abortion material contaminate shared equipment.

Congenital Infection

A mare infected late in gestation may deliver a congenitally infected foal.

These foals can shed large amounts of virus through the respiratory tract and may develop rapidly progressive pneumonia, enteritis or combined respiratory and intestinal disease. They can be an intense source of infection for nearby pregnant mares and young foals.

What Are the Signs of EVA?

Clinical signs vary widely. One horse may show nothing more than a short fever, while another develops marked swelling, hives and respiratory illness.

Possible signs include:

• Fever, sometimes approaching 41°C

• Depression

• Reduced appetite

• Nasal discharge

• Conjunctivitis

• Excessive tearing

• Swelling around the eyes

• Swelling of the lower limbs

• Swelling beneath the abdomen

• Scrotal and preputial swelling in males

• Mammary-gland swelling in mares

• Hives or an urticarial rash

• Stiff gait

• Diarrhoea

• Temporary subfertility in stallions

• Abortion

• Stillbirth

• Severe pneumonia or enteritis in foals

Not every horse develops every sign. The combination of high fever, dependent oedema, conjunctivitis or ocular discharge and reproductive disease should move EVA much higher on the differential list.

How Long After Exposure Do Signs Begin?

After respiratory exposure, signs commonly begin within two to three days.

Following venereal exposure, the incubation period is more commonly six to eight days and may extend to approximately 14 days. This timing is useful when tracing recent arrivals, breeding events and semen use.

Why Is EVA So Important in Pregnant Mares?

EVA may cause isolated abortions or an abortion storm involving multiple mares.

Abortion can occur across much of gestation, rather than only during one narrow “high-risk” month. Some mares show fever, oedema, conjunctivitis or respiratory signs before aborting. Others abort without an obvious warning noticed by the owner.

The aborted fetus itself is heavily infected and becomes a major biosecurity hazard. People handling the mare, fetus, placenta or bedding can carry contaminated material to other horses on footwear, clothing, equipment and hands.

Does Breeding to a Carrier Stallion Automatically Cause Abortion?

No.

A seronegative mare bred with infective semen may become infected, which is why she must be protected and managed carefully. However, EVA-associated abortion outbreaks usually involve mares that were already pregnant when they became infected, commonly through respiratory contact with an acutely infected horse or contaminated abortion material.

The risk-management objective is to prevent susceptible pregnant mares from being exposed to actively shedding horses, contaminated reproductive material or newly infected mares. (AAEP)

Why Is EVA Dangerous in Foals?

Young foals are much more vulnerable to severe disease than healthy adult horses.

Congenitally infected or very young infected foals may develop:

• Severe respiratory distress

• Interstitial pneumonia

• Rapid breathing

• Weakness

• Fever

• Diarrhoea

• Enteritis

• Failure to nurse

• Collapse

• Death

A compromised newborn foal can also release large amounts of virus into the environment. Strict isolation from pregnant mares, newborn foals and unprotected horses is essential.

A newborn foal with fever, respiratory distress, diarrhoea or profound weakness is an emergency regardless of whether EVA has already been confirmed.

What Is an EVA Carrier Stallion?

A carrier stallion is a sexually mature intact male that remains persistently infected within the reproductive tract after recovering from acute EAV infection.

The stallion usually appears healthy and sheds virus continuously in semen, not through ordinary respiratory secretions, urine or blood once the acute phase has resolved. The persistent carrier state is testosterone-dependent and does not develop in mares, geldings, fillies or sexually immature colts.

Does a Carrier Stallion Become Infertile?

Not necessarily.

Persistent semen shedding does not usually damage the stallion’s long-term fertility or general health. However, acute EVA with high fever and severe scrotal oedema may temporarily reduce libido and semen quality.

The commercial consequences may still be substantial because carrier status can affect:

• Demand for the stallion

• Mare-management costs

• Semen sales

• Breed-association requirements

• Interstate movement

• International export

• Access to some markets

(USDA APHIS)

Does a Positive Antibody Test Mean the Stallion Is a Carrier?

No.

A positive serology result may reflect:

• Previous natural infection

• Previous vaccination

• Recent infection

Serology cannot reliably distinguish natural exposure from vaccination. An unvaccinated antibody-positive stallion should be treated as a potential carrier until the sperm-rich fraction of his semen has been tested for EAV by an appropriate laboratory method. (WOAH)

Documented vaccination history is therefore extremely valuable. A stallion shown to be antibody-negative immediately before his first vaccination can later remain seropositive without automatically being treated as naturally infected.

Can the Carrier State Be Eliminated?

Castration remains the only reliable way to permanently eliminate the testosterone-dependent carrier state.

Hormonal methods involving suppression of testosterone may help some stallions clear infection, but these approaches are not fully validated and may affect libido or semen production. They should not be used to conceal or bypass proper carrier-status reporting.

A carrier stallion can sometimes remain in a breeding programme, but only under strict veterinary, regulatory and biosecurity management.

How Worried Should You Be?

Lower Risk

The horse:

• Has no clinical signs

• Has no known EVA exposure

• Is not part of a breeding population

• Has not recently attended a high-contact event

• Has appropriate vaccination and movement records

What this means: Immediate EVA risk appears low.

What to do: Maintain normal biosecurity and review vaccination only if the horse’s breeding, travel or competition risk changes.

Moderate Concern

The horse has:

• Fever

• Mild nasal or ocular discharge

• Depression

• Recent travel, breeding or show exposure

• Contact with a recently imported horse

• No reproductive signs

What this means: EVA is one of several possible contagious diseases.

What to do: Isolate the horse immediately and arrange veterinary examination and diagnostic sampling. Do not continue training, transport or breeding while waiting.

High Concern

The horse has:

• High fever

• Limb, ventral, scrotal or periorbital oedema

• Conjunctivitis

• Hives

• Temporary stallion infertility

• Known contact with a carrier stallion or infective semen

• A pregnant mare aborting after recent exposure

What this means: EVA requires urgent investigation.

What to do: Stop horse and semen movements, isolate contacts, contact the veterinarian and notify the relevant animal-health authority.

Critical

The situation includes:

• Multiple abortions

• A newborn foal with severe respiratory or intestinal disease

• Several horses developing fever and oedema

• Rapid spread through a breeding or competition facility

• Suspected transmission through widely distributed semen

• A collapsing or severely dyspnoeic foal

What this means: A major outbreak or life-threatening neonatal infection is possible.

What to do: Treat this as an immediate farm-level emergency involving the attending veterinarian, diagnostic laboratory and regulatory authority.

What Else Can Look Like EVA?

EVA cannot be diagnosed from visible signs alone.

Important differential diagnoses include:

• Equine herpesvirus types 1 and 4

• Equine influenza

• Equine rhinitis virus infections

• Purpura haemorrhagica

• Equine infectious anaemia

• Allergic urticaria

• Hoary alyssum toxicosis

• Streptococcal respiratory disease

Depending on the country and travel history, foreign or emerging diseases may also need to be considered.

EVA Versus Equine Herpesvirus Abortion

Both EAV and EHV-1 can cause abortion outbreaks.

A mare aborting after obvious fever, swelling or respiratory signs raises concern for EVA, but this distinction is not reliable enough to make the diagnosis. Placental and fetal tissues must be tested for the relevant pathogens. No abortion should be labelled EVA or herpesvirus from appearance alone. (WOAH)

When Is This an Emergency?

Contact a veterinarian immediately when a horse has:

• High fever with limb or ventral swelling

• Scrotal or preputial swelling

• Swelling around the eyes

• Hives accompanied by fever

• Respiratory signs after recent travel or breeding

• Sudden abortion

• Several mares aborting

• A weak or breathless newborn foal

• Known exposure to infective semen

• Several horses becoming ill within a few days

An EVA emergency is not limited to a horse that is physically collapsing. A mare aborting on a large breeding farm or a carrier stallion whose semen has been widely distributed may create a much larger epidemiological emergency than one visibly ill adult horse.

What To Do Right Now

1. Isolate the Suspect Horse

Use a separate stable, paddock or isolation area.

Prevent:

• Nose-to-nose contact

• Shared water

• Shared equipment

• Common handlers where possible

• Breeding

• Semen collection

• Movement on or off the property

Assign dedicated buckets, halters, thermometers and grooming equipment.

2. Contact the Veterinarian

Provide:

• Horse identification

• Sex and reproductive status

• Vaccination history

• Recent temperatures

• Date signs began

• Recent movements

• Show or race attendance

• Breeding dates

• Semen source

• Contact with pregnant mares

• Number of additional exposed horses

3. Notify the Appropriate Authority

EVA reporting requirements differ between jurisdictions.

In the United States, EVA is listed as a reportable programme disease for animal-health professionals, and State regulations may impose additional requirements. Owners should contact their veterinarian rather than attempting to interpret the reporting rules alone. (USDA APHIS)

4. Stop Breeding and Semen Movement

Suspend:

• Live cover

• Artificial insemination

• Semen collection

• Semen shipment

• Embryo-transfer procedures involving exposed animals

Preserve complete records of semen batches, recipient mares, transport dates and premises involved.

5. Protect Pregnant Mares

Keep pregnant mares in small, stable groups away from:

• New arrivals

• Sick horses

• breeding-shed traffic

• Abortion material

• Recently infected mares

• Newly vaccinated animals where isolation is required

APHIS recommends maintaining pregnant mares separately where practical and quarantining new or returning horses for three to four weeks. (USDA APHIS)

6. Manage an Abortion Safely

Do not allow horses, dogs, wildlife or unnecessary people near the fetus or placenta.

Use gloves and protective clothing. Secure the fetus, placenta and contaminated bedding according to veterinary instructions. Do not hose contaminated fluid through communal yards or drains.

The veterinarian should arrange appropriate fetal, placental and maternal samples before disposal.

7. Begin Contact Tracing

Identify:

• Horses sharing the barn

• Across-the-fence contacts

• Recent arrivals and departures

• Mares bred to suspect stallions

• Semen shipments

• Shared handlers

• Shared veterinary or breeding equipment

• Events attended during the likely infectious period

How Is EVA Diagnosed?

Laboratory confirmation is essential because the clinical signs are nonspecific and many infections are subclinical.

Testing an Acutely Ill Horse

Useful samples include:

• Nasopharyngeal or deep nasal swabs

• Conjunctival swabs where directed

• EDTA or citrated whole blood

• Acute serum

• Convalescent serum collected two to four weeks later

Samples for virus detection should be collected as early as possible after the onset of fever or other signs.

Testing may involve:

• RT-PCR

• Virus isolation

• Virus-neutralisation serology

• Validated ELISA

• Demonstration of viral antigen in tissues

A single positive antibody result cannot determine whether antibodies came from vaccination or natural infection. (WOAH)

Testing an Abortion

Appropriate samples may include:

• Placenta and fetal membranes

• Fetal lung

• Fetal liver

• Kidney

• Spleen

• Fetal or placental fluids

• Maternal blood

The fetus may show few or no obvious lesions, so the absence of a dramatic post-mortem appearance does not rule out EVA.

Testing a Stallion for Carrier Status

The process generally begins with serology.

An unvaccinated seropositive stallion is considered a potential carrier. Confirmation requires detection of virus in semen, preferably from the sperm-rich fraction, through RT-PCR or virus isolation. Test breeding to seronegative mares is an alternative method under specialist supervision. (WOAH)

How Is EVA Treated?

There is no specific antiviral treatment for EVA.

Most healthy adult horses recover with:

• Rest

• Good nursing care

• Hydration

• Monitoring

• Veterinarian-selected anti-inflammatory treatment

• Management of fever and oedema

• Gradual return to exercise or breeding

Moderate and severe cases require closer veterinary supervision, especially stallions with prolonged fever and scrotal swelling.

What Is the Prognosis?

The prognosis is generally good for healthy adult horses.

Most recover completely, although acutely affected stallions may experience temporary reductions in libido and semen quality. Young foals with fulminating pneumonia or intestinal disease have a much poorer prognosis. Pregnant mares may recover physically but still lose the pregnancy.

A recovered intact stallion still requires assessment for possible persistent semen shedding.

Should Horses Be Vaccinated Against EVA?

EVA vaccination is risk-based, not a routine core vaccination for every horse.

Important reasons to vaccinate include:

• Protecting breeding stallions from infection and carrier-state development

• Protecting at-risk colt foals before sexual maturity

• Protecting seronegative mares before breeding to a carrier stallion

• Protecting mares before insemination with infective semen

• Helping control an outbreak in an exposed population

(AAEP)

Why Must Horses Be Tested Before Their First Vaccine?

Vaccination produces EAV antibodies, and current routine serology cannot reliably distinguish vaccine antibodies from antibodies caused by natural infection.

Before first vaccination, intact males and horses that may later be exported should have a negative baseline serum result documented by an approved laboratory. Testing should be performed shortly before or at the time of vaccination. (AAEP)

Without that documentation, a future positive titre may create uncertainty about natural exposure, carrier risk and export eligibility.

Vaccination of Breeding Stallions

Previously vaccinated breeding stallions generally receive an annual booster, timed approximately three to four weeks before the breeding season.

A stallion receiving the vaccine for the first time should be seronegative beforehand and isolated after vaccination according to the current veterinary, product, State and semen-market protocol. AAEP’s general guidance uses a three-week isolation period, while transported-semen programmes may impose a 28-day interval. (AAEP)

Vaccination of Mares

Open, nonpregnant mares may be vaccinated when indicated.

A seronegative mare being bred to a known carrier stallion or inseminated with infective semen should be vaccinated at least three weeks before breeding and isolated according to the breeding protocol. (AAEP)

A naturally seropositive mare may already possess protective immunity, but her status and breeding plan still require veterinary review.

Vaccination of Pregnant Mares

Routine vaccination of pregnant mares is not recommended.

Current AAEP guidance states that mares in foal should generally wait until after foaling and then be vaccinated at least three weeks before the next breeding. The manufacturer particularly advises against use in the final two months of pregnancy. Emergency outbreak vaccination of pregnant mares is a specialist regulatory and veterinary decision, not a routine farm protocol. (AAEP)

Vaccination of Colt Foals

At-risk male foals may be vaccinated between six and twelve months of age to reduce the chance of becoming persistent carriers after later exposure.

They should be shown to be seronegative before vaccination and isolated afterwards according to the relevant protocol. Vaccination is generally delayed until at least six months because maternal antibodies may persist in foals born to seropositive mares. (AAEP)

Can Carrier Stallions Still Be Used for Breeding?

Yes, in some jurisdictions and breeding programmes, but only under tightly controlled conditions.

A carrier stallion should:

• Be clearly identified and documented

• Be managed separately

• Have carrier status disclosed to mare owners

• Be bred only to naturally seropositive or appropriately vaccinated mares

• Have dedicated breeding and semen-collection equipment

• Follow all regulatory and breed-association requirements

• Have semen labelled and handled as potentially infective

• Avoid contact with unprotected horses

(AAEP)

Carrier semen should never be presented as safe merely because it is chilled, frozen or collected from a clinically normal stallion.

Is EVA Reportable?

Reporting requirements differ internationally.

In the United States, EVA appears on the APHIS list of reportable programme diseases for animal-health professionals. Suspected or diagnosed cases are reported to the APHIS Area Veterinarian in Charge and the relevant State animal-health official, with State requirements potentially adding further obligations. (USDA APHIS)

Other countries may classify EVA as notifiable, monitored or subject to breeding and import controls. Owners moving horses, semen or embryos internationally should confirm the destination country’s requirements before testing or vaccination.

Vaccinating without considering future export can create a positive antibody result that affects access to countries that restrict seropositive horses, even when vaccination records exist. (AAEP)

Common EVA Mistakes

Assuming an Antibody-Positive Stallion Is Definitely a Carrier

Serology identifies exposure or vaccination. Semen testing determines shedding.

Vaccinating Before Collecting Baseline Blood

This can make future natural infection difficult to distinguish from vaccination and may complicate export.

Assuming Frozen Semen Cannot Transmit EVA

Frozen semen from a carrier stallion can remain infective.

Vaccinating a Pregnant Mare Routinely

The current modified-live vaccine is not routinely recommended during pregnancy, particularly late pregnancy.

Treating an Abortion as an Isolated Reproductive Failure

The fetus, placenta and fluids may create a major farm-wide infectious risk.

Allowing a Mildly Affected Horse To Continue Travelling

Many EAV infections are mild or subclinical, yet the horse may still spread virus during the acute phase.

Forgetting Stallion Follow-Up

A stallion recovering clinically from EVA still needs carrier-status evaluation.

Testing Only the Stallion’s Blood

An unvaccinated seropositive result identifies a potential carrier. It does not replace semen testing.

How Can EVA Be Prevented?

Quarantine New and Returning Horses

Isolate new arrivals and horses returning from sales, races, shows or other farms for approximately three to four weeks.

Monitor:

• Rectal temperature

• Nasal discharge

• Eye discharge

• Limb swelling

• Appetite

• Exposure history

(USDA APHIS)

Test Breeding Stallions Correctly

Before breeding season:

• Review vaccination records

• Test unvaccinated or undocumented stallions for EAV antibodies

• Test semen from unvaccinated seropositive stallions

• Confirm regulatory and breed-association requirements

• Maintain annual vaccination for documented noncarrier stallions where indicated

Verify Semen Sources

Before using transported semen, confirm:

• Stallion identity

• Vaccination history

• Serological status

• Carrier or shedding status

• Laboratory certification

• Import requirements

• Mare protection requirements

Unknown semen status should not be treated as negative status.

Protect Pregnant Mares

Keep pregnant mares away from:

• New arrivals

• Horses with fever

• Recently exposed horses

• Breeding-shed traffic

• Abortion material

• Carrier-stallion operations unless strict separation exists

Maintain Breeding-Shed Hygiene

Use dedicated equipment for carrier stallions and disinfect:

• Phantoms

• Collection equipment

• Breeding rolls

• Restraint equipment

• Buckets

• Boots

• Hands and protective clothing

Disposable gloves and sleeves should be changed between horses.

Maintain Complete Records

Record:

• Horse identification

• Serology results

• Vaccination dates

• Vaccine lot numbers

• Stallion shedding status

• Semen batches

• Mares inseminated

• Breeding dates

• Isolation periods

• Horse movements

• Abortions and foal losses

These records turn a chaotic outbreak investigation into an organised tracing exercise rather than a very expensive detective novel.

Frequently Asked Questions

Can a Vaccinated Horse Test Positive for EVA?

Yes. Vaccination produces antibodies, and routine serology cannot reliably distinguish vaccination from natural infection. This is why a documented negative blood test before the first vaccination is so important. (AAEP)

Does a Positive Blood Test Mean a Stallion Is Shedding Virus?

No. An unvaccinated seropositive stallion is considered a potential carrier, but semen must be tested to establish whether EAV is being shed. (USDA APHIS)

Can EVA Be Spread in Frozen Semen?

Yes. Fresh, chilled and frozen semen from a shedding stallion can transmit EAV.

Is EVA Usually Fatal?

No. Mortality is very rare in healthy adult horses, and most recover. Severe and fatal disease is more likely in neonatal or young foals.

Can a Carrier Stallion Still Breed?

Potentially, depending on local rules. Carrier stallions must be disclosed, separately managed and bred only to naturally seropositive or properly vaccinated mares under strict veterinary and regulatory protocols. (AAEP)

Final Thoughts

EVA is often mild in an individual adult horse, but its breeding and population consequences can be substantial.

The most important points are:

1. EVA is caused by equine arteritis virus.

2. Many infections are mild or subclinical.

3. Acute horses primarily spread virus through respiratory secretions.

4. Carrier stallions continuously shed virus in semen.

5. Fresh, chilled and frozen semen can all be infective.

6. Only sexually mature intact males develop the persistent carrier state.

7. Mares, geldings and sexually immature colts do not become long-term carriers.

8. A positive antibody result does not prove semen shedding.

9. Abortion material may contain a large amount of virus.

10. Pregnant mares and newborn foals require the strongest biosecurity protection.

11. Young foals can develop severe and fatal respiratory or intestinal disease.

12. Clinical signs cannot confirm EVA without laboratory testing.

13. Diagnostic samples should be collected early.

14. There is no specific antiviral treatment.

15. Most adult horses recover with supportive care.

16. Breeding stallions require carrier-status follow-up after natural infection.

17. Vaccination is risk-based and should begin with documented pre-vaccination serology.

18. Pregnant mares should not ordinarily receive the current modified-live vaccine.

19. Carrier stallions may sometimes remain in breeding programmes under strict controls.

20. Suspected EVA requires immediate isolation, veterinary involvement and regulatory guidance.

The mistake that causes the greatest damage is focusing only on the visibly sick horse. With EVA, the more important questions may be which mares are pregnant, which stallions are intact, where semen has been shipped and how many apparently healthy horses were exposed.

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ASK A VET™ can help organise fever records, photographs, breeding dates, semen information, vaccination history and recent horse movements while you contact your attending veterinarian and relevant animal-health authority. A pregnant mare that aborts, a newborn foal with respiratory distress or a horse with high fever and rapidly developing oedema still requires immediate hands-on veterinary care.