Equine Herpesvirus in Horses: Symptoms, EHM, Abortion and Outbreak Control

By Dr Duncan Houston

Equine herpesvirus is common, contagious and often underestimated until fever, abortion or neurologic signs appear.

Equine herpesvirus is one of the most important infectious disease risks in modern horse management because it can look mild at first, then become a serious property-wide problem.

Some horses only develop fever, nasal discharge and a cough. Some pregnant mares abort with little warning. A smaller number of horses develop the neurologic form, called equine herpesvirus myeloencephalopathy, or EHM, which can cause hindlimb weakness, urine dribbling, recumbency and death.

The difficult part is that EHV is not controlled by vaccination alone. It needs fast isolation, temperature monitoring, movement control, hygiene, testing, and a clear plan before one sick horse turns into a whole-barn problem.

Quick Answer

Equine herpesvirus, or EHV, is a group of herpesviruses that infect horses. The most important types are EHV-1 and EHV-4. EHV-4 mostly causes respiratory disease, while EHV-1 can cause respiratory disease, abortion, fatal neonatal illness and neurologic disease called EHM. EHV spreads through nasal secretions, close contact, contaminated hands, tack, buckets, stalls, trailers and aborted fetal material. (Equine Disease Communication Center)

If your horse has fever, nasal discharge, coughing, sudden abortion, hindlimb weakness, wobbliness, urine dribbling, difficulty standing or rapid deterioration, isolate the horse and call your veterinarian immediately.

What Is Equine Herpesvirus?

Equine herpesvirus is a family of viruses that infect horses. The most clinically important types for most horse owners are:

Virus type	Main concern
EHV-1	Respiratory disease, abortion, neonatal death, neurologic disease
EHV-4	Mostly respiratory disease, especially in young horses
EHV-3	Coital exanthema, a venereal disease affecting the external genitalia
EHV-2 and EHV-5	Less clearly defined roles, sometimes associated with respiratory or ocular disease

EHV-1 and EHV-4 infect the respiratory tract and can cause fever, lethargy, reduced appetite, nasal discharge, cough and swollen lymph nodes. EHV-1 is the type most associated with abortion, neonatal death and neurologic disease. (AAEP)

One of the reasons EHV is so hard to control is latency. Once infected, a horse may carry the virus silently, and stress can trigger reactivation and shedding later. This means the virus can reappear in a population even when there is no obvious new sick horse introduced. (Merck Veterinary Manual)

In plain terms: EHV is not just a respiratory bug. It is a management problem, a biosecurity problem, and sometimes a reproductive or neurologic emergency.

How Does EHV Spread?

EHV spreads mainly through respiratory secretions and contaminated materials.

Common spread routes include:

• Nose-to-nose contact
• Coughing and respiratory droplets
• Nasal discharge
• Shared water buckets
• Shared feed tubs
• Tack and grooming tools
• Halters, lead ropes and rugs
• Stalls, trailers and wash bays
• Human hands and clothing
• Aborted fetuses, placenta and fetal fluids in EHV-1 abortion cases

Merck Veterinary Manual notes that EHV transmission occurs through direct or indirect contact with infectious nasal secretions, aborted fetuses, placentas or placental fluids. The virus can also remain viable on common surfaces for around 48 hours under realistic environmental conditions. (Merck Veterinary Manual)

This is why one infected horse can cause a major outbreak in:

• Show barns
• Racing stables
• Breeding farms
• Agistment properties
• Sales yards
• Training centres
• Events and competitions
• Any property with frequent horse movement

The dangerous mistake is thinking only visibly sick horses matter. Fever can appear before obvious nasal discharge, and exposed horses can spread infection before owners realise there is a problem.

Why Latency Makes EHV Different

EHV behaves differently from many short-term respiratory infections because herpesviruses can become latent.

Latency means the virus can remain dormant inside the horse after the initial infection. The horse may look completely healthy, then later shed virus again if the immune system is stressed. Stressors may include transport, competition, weaning, mixing with new horses, illness, pregnancy, heavy training or other management changes.

Merck describes latency as a hallmark of EHV infection, with stress able to trigger viral reactivation, shedding and new infections. (Merck Veterinary Manual)

That does not mean every horse with past EHV exposure is actively dangerous.

It does mean EHV prevention cannot rely only on “none of the horses look sick today.”

Good management matters all year.

The Main Clinical Forms of EHV

1. Respiratory EHV

Respiratory disease is the most common form, especially in foals, weanlings, yearlings and young horses entering training.

Signs can include:

• Fever
• Lethargy
• Reduced appetite
• Clear or mucous nasal discharge
• Coughing
• Conjunctivitis
• Swollen lymph nodes around the throat
• Mild lower limb swelling in some cases

Merck lists fever, nasal discharge, malaise, pharyngitis and cough as key EHV respiratory signs. The incubation period is typically 2 to 10 days. (Merck Veterinary Manual)

In many horses, respiratory EHV is mild and self-limiting. The bigger concern is spread, because a mild case in one horse can expose pregnant mares, young horses, performance horses or horses that later develop neurologic signs.

2. EHV-1 abortion

EHV-1 can cause abortion, often in late pregnancy.

This is one of the most devastating presentations because the mare may appear well before suddenly aborting. Merck notes that EHV-1 abortions often occur between months 7 and 11 of gestation, and mares may not show warning signs beforehand. Queensland Government guidance notes that abortion may occur from 10 days to 12 weeks after infection. (Merck Veterinary Manual)

Important signs and risks include:

• Sudden abortion
• Abortion without obvious respiratory signs
• Fetus sometimes expelled still covered by placenta
• Weak live foals if infection occurs late in pregnancy
• Neonatal foals that may become depressed, weak and develop respiratory signs
• High risk of environmental contamination from fetal fluids and placenta

Aborted fetal material should be treated as infectious until your veterinarian advises otherwise.

3. Neonatal EHV

Foals infected late in pregnancy may be born alive but very unwell. These foals can develop severe viral pneumonia and often have a poor prognosis.

Signs may include:

• Weakness
• Depression
• Poor nursing
• Fever or low temperature
• Respiratory distress
• Rapid decline
• Death within hours to days in severe cases

This is an emergency.

4. Neurologic EHV, or EHM

EHM stands for equine herpesvirus myeloencephalopathy.

It is the neurologic form most commonly associated with EHV-1. EHM damages the central nervous system, especially the spinal cord, through inflammation and vascular injury.

Signs can include:

• Fever before neurologic signs
• Hindlimb weakness
• Wobbliness
• Ataxia
• Stumbling
• Dog-sitting posture
• Reduced tail tone
• Urine dribbling
• Difficulty urinating or defecating
• Recumbency
• Inability to rise
• Rarely, severe depression or coma-like signs

Queensland Government guidance lists fever, incoordination, weakness, trouble standing, difficulty urinating or defecating and reduced tail tone as neurologic signs of EHV-1. (Business Queensland)

In practice, the most concerning pattern is fever followed by hindlimb weakness, wobbliness or urine dribbling.

That is not a “monitor for a few days” situation.

Is There Really a Neurologic Strain?

This needs careful explanation.

Some EHV-1 genetic variants have been associated with neurologic outbreaks, but EHM is not as simple as “neurologic strain equals neurologic disease.” The 2024 ACVIM consensus update notes that it is not clear that any specific EHV-1 strain is always more likely to cause neurologic disease, and AAEP guidance states that all recognised EHV-1 DNA polymerase variants are capable of causing EHM outbreaks. (PMC)

That means:

• Some variants may raise concern
• Genotyping can be useful in outbreak understanding
• But any suspected EHV-1 neurologic case should be treated seriously
• Strict biosecurity is required regardless of strain label

The practical message is simple: do not relax because someone says it is “not the neurologic strain.”

If horses are febrile, exposed or neurologic, manage the situation aggressively.

EHV Risk Framework

Risk level	What it looks like	What it may mean	What to do
Low risk	Healthy horse, no fever, no recent travel, no known exposure	Routine prevention and vaccination planning	Maintain normal hygiene and biosecurity
Moderate risk	Recent travel, show exposure, new arrival, mild fever, mild nasal discharge	Possible early respiratory infection	Isolate and call your vet for advice
High risk	Fever plus cough, nasal discharge, multiple horses affected, pregnant mares exposed	EHV or another contagious disease may be spreading	Stop movement, isolate, begin temperature monitoring and test
Critical	Abortion, neurologic signs, urine dribbling, recumbency, severe weakness, rapidly worsening signs	EHV-1 abortion or EHM possible	Urgent veterinary care and strict biosecurity immediately

The key decision point: fever alone matters during an EHV risk period. Fever plus neurologic signs is an emergency.

When Is EHV an Emergency?

Call your veterinarian immediately if your horse has:

• Fever after known exposure
• Fever during an outbreak
• Hindlimb weakness
• Wobbliness or ataxia
• Urine dribbling
• Difficulty urinating
• Loss of tail tone
• Difficulty standing
• Recumbency
• Sudden abortion
• A weak newborn foal
• Severe depression
• Rapid deterioration
• Multiple horses with fever
• Respiratory distress

EHV-1 causing neurologic disease or abortion may also be reportable depending on your location. For example, Queensland Government states that abortogenic and neurologic EHV-1 are restricted matter and must be reported in Queensland, while respiratory EHV-1 alone is not reportable there. Rules vary by region, so follow your veterinarian and local animal health authority. (Business Queensland)

Why Temperature Monitoring Is So Important

Fever is often the first useful warning sign.

A horse may spike a temperature before nasal discharge appears or before neurologic signs develop. During a suspected outbreak or after exposure, twice daily rectal temperatures are one of the most practical tools you have.

Record:

• Morning temperature
• Evening temperature
• Appetite
• Coughing
• Nasal discharge
• Attitude
• Limb swelling
• Urination pattern
• Any wobbliness or weakness

A fever threshold commonly used in EHV outbreak monitoring is around 38.6°C or 101.5°F, but your veterinarian may set a specific threshold for your property based on the case situation.

If one horse spikes a fever, isolate first and ask questions second. The virus does not politely wait for the stable group chat to agree.

How Do Vets Diagnose EHV?

EHV cannot be diagnosed reliably from symptoms alone because it can look like several other diseases.

Testing may include:

• Nasal swab PCR
• Nasopharyngeal swab PCR
• Whole blood or buffy coat PCR
• Virus isolation
• Paired blood samples for antibody titre changes
• Testing of aborted fetal tissues and placenta
• Necropsy in fatal neurologic or neonatal cases

EDCC states that EHV-1 and EHV-4 can be diagnosed through PCR testing of nasal swabs, buffy coat samples, virus isolation from blood, or significant antibody titre rises in paired serum samples. (Equine Disease Communication Center)

Merck notes that diagnosis is most often made by PCR testing from nasopharyngeal swab and citrated blood collected early in infection. Nasal swab PCR is useful for assessing shedding, but intermittent shedding can produce false negative results. (Merck Veterinary Manual)

That is why your vet may repeat testing if the timing does not fit the clinical picture.

A single negative early test is helpful, but it is not always the end of the story.

What Else Can Look Like EHV?

A horse with fever, nasal discharge, cough, abortion or neurologic signs does not automatically have EHV.

Important rule-outs include:

• Equine influenza
• Strangles
• Equine viral arteritis
• Equine rhinitis virus
• Equine coronavirus
• Bacterial pneumonia
• Pleuropneumonia
• Guttural pouch disease
• West Nile virus
• Equine protozoal myeloencephalitis
• Botulism
• Trauma
• Cervical spinal cord disease
• Tetanus
• Rabies risk where relevant
• Toxicity
• Placentitis
• Other causes of abortion
• Colic causing weakness or reluctance to move

Merck specifically notes that equine viral rhinopneumonitis can be difficult to distinguish clinically from equine influenza, equine viral arteritis and other respiratory infections without testing. (Merck Veterinary Manual)

This is the veterinary reasoning that matters: EHV may be the concern, but the horse still needs a full differential list.

Can EHV Be Treated?

There is no simple cure that kills EHV and instantly stops disease.

Treatment is usually supportive and depends on the form of disease.

Treatment may include:

• Rest
• Isolation
• Anti-inflammatory medication for fever or discomfort
• Fluids if needed
• Nursing support
• Monitoring hydration, appetite, urination and manure output
• Antibiotics only if secondary bacterial infection is suspected
• Antiviral treatment in selected high-risk or neurologic cases
• Intensive nursing for EHM horses
• Bladder management for horses with urine retention
• Sling or recumbency care in selected cases

Merck states there is no specific treatment for EHV infection and that rest, supportive care and NSAIDs may be used to minimise secondary complications and treat fever. Antimicrobials are used only when secondary bacterial infection is suspected. (Merck Veterinary Manual)

For EHM, prognosis depends heavily on severity and whether the horse remains standing. Horses that become recumbent for prolonged periods have a more guarded prognosis.

What Can EHV Vaccines Do?

EHV vaccines are useful, but they have limits.

Vaccination can help reduce:

• Severity of respiratory disease
• Duration of respiratory disease
• Nasal shedding
• Viremia in some cases
• EHV-1 abortion risk when the correct vaccine is used in pregnant mares

AAEP states that EHV-1 and EHV-4 vaccine indications include prevention of EHV-1 abortion and reduction in severity and duration of respiratory disease. EDCC states vaccines are available to help prevent respiratory and abortive forms, but they do not completely prevent disease. (AAEP)

The important limit: there is currently no vaccine labelled to prevent the neurologic form, EHM. EDCC and AAEP both state this clearly. (Equine Disease Communication Center)

Vaccination is one layer of protection.

It is not a force field.

EHV Vaccination Basics

Your veterinarian should tailor the schedule based on age, pregnancy status, travel, exposure risk and local requirements.

Common principles include:

• Foals usually start EHV vaccination around 4 to 6 months, depending on product and maternal antibody considerations
• Young horses, performance horses, racehorses and frequently travelling horses may need boosters every 6 months
• Pregnant mares are commonly vaccinated during pregnancy using an EHV-1 vaccine labelled for abortion prevention
• Horses at high-movement facilities may need more frequent risk-based planning
• Event rules may require vaccination within a defined time window

Merck notes that vaccination usually begins when foals are 4 to 6 months old, with additional primary doses and boosters depending on age and risk. It also notes that high-risk horses, including young horses, breeding farm horses, performance horses and horses at facilities with frequent movement, may be revaccinated at 6-month intervals. (Merck Veterinary Manual)

For pregnant mares, Merck describes use of a high-antigen inactivated EHV-1 vaccine to help prevent abortion, commonly administered during pregnancy. Exact protocols should follow product label and veterinary advice. (Merck Veterinary Manual)

Should You Vaccinate During an Outbreak?

Do not make this decision casually.

Vaccination during an outbreak is a risk-based veterinary decision. It is not a treatment for sick horses and it does not replace quarantine.

In general:

• Do not vaccinate a febrile or sick horse without veterinary direction
• Do not assume vaccination will stop an active outbreak
• Do not move horses because they were just boosted
• Do not use vaccination instead of isolation and testing
• Ask your vet whether unexposed or previously vaccinated horses should be boosted

EDCC states testing and quarantine of affected horses in isolation is the primary way to control EHV, and vaccines do not completely prevent disease. (Equine Disease Communication Center)

The boring answer is the correct answer: biosecurity first.

What To Do During a Suspected EHV Outbreak

1. Isolate the sick horse immediately

Separate any horse with fever, nasal discharge, cough, abortion, neurologic signs or known exposure.

Use a physical barrier and distance where possible.

2. Stop horse movement

No shows. No lessons off property. No new arrivals. No casual trips to the next stable. No “just one float ride.”

Movement control is how outbreaks stop.

3. Take temperatures twice daily

Check every horse on the property, especially exposed horses.

Record results clearly so trends are not missed.

4. Separate horses into groups

Use groups such as:

• Clean group
• Exposed group
• Sick group

Handle clean horses first, exposed horses second and sick horses last.

5. Use dedicated equipment

Do not share:

• Buckets
• Feed tubs
• Halters
• Lead ropes
• Tack
• Grooming gear
• Thermometers
• Rugs
• Wheelbarrows
• Stable tools

6. Improve hygiene

Use gloves, handwashing, dedicated clothing, footbaths where appropriate and proper disinfection.

Disinfect high-contact surfaces, stalls, trailers, wash bays and shared areas.

7. Call your veterinarian early

Your vet will advise on testing, isolation length, movement restrictions, treatment, reporting obligations and when horses can safely return to work or travel.

8. Communicate clearly

Tell owners, riders, staff, farriers, dentists, bodyworkers and trainers what is happening. A half-hidden outbreak spreads faster than a fully managed one.

How Long Should Horses Be Isolated?

Isolation length depends on the clinical form, test results, local regulations and whether new cases appear.

Merck recommends isolating new horses or returning horses for 21 days before mixing with residents, especially pregnant mares. In outbreak control, it recommends isolation of affected and in-contact horses for 28 days after recovery of the last clinical case. (Merck Veterinary Manual)

Queensland Government recommends isolating visiting and newly introduced horses for at least 14 days as part of prevention guidance. (Business Queensland)

Because recommendations vary by situation and region, your vet or animal health authority should guide the final release plan.

Common Mistakes Owners Make With EHV

Mistake 1: Waiting for nasal discharge before isolating

Fever may be the first sign. If you wait for snot, you may already be late.

Mistake 2: Thinking vaccination prevents EHM

No current vaccine is labelled to prevent neurologic EHV-1 disease. Vaccinated horses can still develop fever, shed virus or develop EHM.

Mistake 3: Moving horses too soon

Horse movement is one of the easiest ways to turn a property problem into a regional problem.

Mistake 4: Sharing equipment between groups

Buckets, halters, tack and grooming tools can become disease buses with handles.

Mistake 5: Ignoring a sudden abortion

EHV-1 abortion can occur without obvious warning signs. Fetal material, placenta and fluids should be treated as infectious until proven otherwise.

Mistake 6: Assuming a negative test always ends quarantine

Timing matters. Horses can shed intermittently, and your veterinarian may recommend repeat testing depending on signs and exposure.

Mistake 7: Forgetting people spread the virus too

Hands, boots, clothing and shared stable equipment matter.

How To Reduce EHV Risk Year-Round

A strong EHV prevention plan includes:

• Risk-based vaccination
• Quarantine for new and returning horses
• Separate pregnant mares from high-movement horses
• Twice daily temperature monitoring during high-risk periods
• No shared buckets at events
• No nose-to-nose contact with unfamiliar horses
• Clean and disinfect floats after travel
• Keep sick horses home
• Avoid mixing young horses from multiple sources without planning
• Maintain good ventilation in barns
• Reduce avoidable stress during high-risk periods
• Keep accurate movement, vaccination and illness records
• Have an outbreak plan before you need one

EDCC emphasises vaccination, biosecurity, correct protocols when travelling and quarantine of affected horses as major control tools. (Equine Disease Communication Center)

In practice, the best EHV plan is built before the outbreak starts. During an outbreak, everyone suddenly becomes interested in biosecurity. By then, the virus has usually already RSVP’d.

Myth vs Reality

Myth	Reality
“EHV is just a respiratory virus.”	EHV-1 can also cause abortion, neonatal death and neurologic disease.
“Vaccinated horses cannot get EHV.”	Vaccines reduce risk and severity, but do not fully prevent infection or shedding.
“Only visibly sick horses spread EHV.”	Horses may shed before obvious signs, and latent virus can reactivate under stress.
“EHM only happens with the neurologic strain.”	Multiple EHV-1 variants can cause neurologic disease, so all suspected EHM needs strict control.
“A coughing horse at a show can wait until Monday.”	Fever, cough or nasal discharge after travel should be taken seriously and isolated early.
“Biosecurity is only for big barns.”	Small properties can also have outbreaks, especially after travel, new arrivals or shared equipment.

Frequently Asked Questions

Can a horse recover from EHV?

Yes. Many horses with uncomplicated respiratory EHV recover with rest, isolation and supportive care. Recovery is more guarded if the horse develops EHM, becomes recumbent, aborts, or if a newborn foal is affected. Merck notes that prognosis for neurologic disease depends on severity and duration of recumbency. (Merck Veterinary Manual)

Can vaccinated horses still get EHV?

Yes. Vaccination can reduce disease severity, duration and shedding, but it does not fully prevent infection, and no vaccine is labelled to prevent EHM. (Equine Disease Communication Center)

What is the first sign of EHV?

Fever is often the first useful warning sign. A horse may develop fever before nasal discharge, cough or neurologic signs. This is why twice daily temperature checks are so important after exposure or during an outbreak.

Is EHV contagious to humans?

EHV-1 is not considered zoonotic. NSW DPI states that EHV-1 does not affect humans. The concern with people is that they can mechanically spread virus between horses on hands, clothes, footwear or equipment. (NSW DPI)

How long should a new horse be isolated?

Isolation recommendations vary by region and facility risk. Merck recommends isolating new or returning horses for 21 days before mixing with residents, especially pregnant mares, while Queensland Government recommends at least 14 days for visiting and newly introduced horses. Your veterinarian can tailor this to your property. (Merck Veterinary Manual)

The Bottom Line

Equine herpesvirus is common, but that does not make it harmless.

The respiratory form can spread quickly. EHV-1 abortion can happen suddenly. EHM can progress from fever to neurologic disease and recumbency. Vaccination helps reduce some risks, but it does not eliminate disease and it does not replace biosecurity.

The strongest EHV plan is simple, but it requires discipline:

• Vaccinate based on risk
• Isolate new and returning horses
• Take fever seriously
• Stop movement during suspected outbreaks
• Separate clean, exposed and sick horses
• Test appropriately
• Treat neurologic signs and abortion as urgent
• Follow your veterinarian and local reporting rules

If you remember one thing, make it this:

Fever during an EHV risk period is not just a temperature. It is an early warning system.

Act early, isolate fast, and do not wait for the whole barn to prove the point.

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If you are unsure whether your horse’s fever, travel history, abortion risk, neurologic signs or exposure pattern could be EHV, ASK A VET™ can help you organise the timeline, track temperatures and decide when veterinary care should not wait.