Equine Herpesvirus in Horses: Respiratory Signs, Neurologic Disease and Outbreak Control

By Dr Duncan Houston

Equine herpesvirus can start as a fever and mild respiratory illness, but in some horses it can become a neurologic emergency.

Equine herpesvirus, especially EHV-1, is one of the most important infectious disease risks in horse populations because it can cause several very different problems.

One horse may develop a mild fever and nasal discharge.
A pregnant mare may abort with little warning.
Another horse may become weak, wobbly, unable to urinate, or unable to stand.

That severe neurologic form is called equine herpesvirus myeloencephalopathy, or EHM. It is the form owners fear most, and rightly so. The difficult part is that early EHV can look vague. Fever may be the first sign. Respiratory signs may be mild. Neurologic signs may come later, and not every case follows the textbook version.

That is why the best EHV response is built around early temperature monitoring, fast isolation, strict biosecurity, targeted testing, and realistic vaccination expectations.

Quick Answer

Equine herpesvirus is a group of common equine viruses. The most important types are EHV-1 and EHV-4. EHV-4 mainly causes respiratory disease, while EHV-1 can cause respiratory disease, abortion, fatal neonatal disease, and neurologic disease called EHM. EHV spreads through nasal secretions, respiratory droplets, aborted fetal material, contaminated hands, clothing, tack, buckets, stalls, trailers, and shared equipment.

A horse with fever, nasal discharge, coughing, sudden abortion, hindlimb weakness, wobbliness, urine dribbling, difficulty rising, or recumbency should be isolated and assessed by a veterinarian urgently.

What Is Equine Herpesvirus?

Equine herpesvirus refers to a family of viruses that infect horses. The two most clinically important types are EHV-1 and EHV-4.

EHV-1 can cause:

• Respiratory disease
• Abortion
• Fatal neonatal disease
• Neurologic disease, called EHM

EHV-4 usually causes respiratory disease, especially in young horses, and only rarely causes abortion or neurologic disease. AAEP describes EHV-1 and EHV-4 as common equine DNA viruses found worldwide, with EHV-1 responsible for respiratory disease, abortion, and neurologic disease, and EHV-4 mainly causing respiratory disease.

One of the reasons EHV is so hard to control is latency. After infection, herpesviruses can remain dormant in the horse. Later, stress can trigger reactivation, replication, and shedding. This is why EHV outbreaks can sometimes occur even in horse populations that appear closed or stable.

In plain English: EHV is not just a simple respiratory virus. It is a virus that can hide, reactivate, spread quietly, and occasionally cause devastating complications.

Respiratory EHV vs Neurologic EHV: What Is the Difference?

The respiratory form and neurologic form are linked, but they are not the same clinical problem.

Feature	Respiratory EHV	Neurologic EHV, or EHM
Main system affected	Upper respiratory tract	Brain and spinal cord, especially spinal cord
Common signs	Fever, nasal discharge, cough, lethargy, poor appetite	Weakness, ataxia, urine dribbling, recumbency
Main concern	Contagion and spread through the barn	Paralysis, inability to stand, severe outbreak risk
Horses affected	Often foals, weanlings, yearlings and young horses	Often mature horses, but any horse can be affected
Vaccine protection	Vaccines can reduce respiratory disease severity and shedding	No vaccine is labelled to prevent EHM
Urgency	Isolate and call vet, especially with fever	Emergency veterinary care immediately

AAEP explains that EHM occurs when cell-associated viremia leads to blood vessel inflammation, thrombosis, and focal infarction in the central nervous system, resulting in spinal cord damage and neurologic disease.

The practical difference for owners is simple:

Respiratory EHV is a contagious disease concern. Neurologic EHV is a contagious disease concern plus a neurologic emergency.

How Does EHV Move Through the Horse’s Body?

EHV usually begins in the upper respiratory tract.

The general progression can look like this:

1. The horse inhales or contacts infected nasal secretions

2. The virus replicates in the respiratory tract and nearby lymph nodes

3. Fever develops, sometimes before obvious respiratory signs

4. In some horses, EHV-1 enters white blood cells and causes cell-associated viremia

5. The virus can affect blood vessel lining in the placenta, lungs, or central nervous system

6. In EHM, blood vessel inflammation and clots damage the spinal cord or brain

Merck Veterinary Manual describes EHV replication in the upper respiratory tract followed by cell-associated viremia, while EHV-1 can affect vascular endothelium in tissues including the placenta and central nervous system. (Merck Veterinary Manual)

This is why fever monitoring matters. Fever may be the warning sign before the obvious outbreak signs appear.

Why Fever Is So Important

Fever is often the first useful clue.

AAEP states that fever in EHV can be biphasic and transient. The first fever peak may occur 1 to 2 days after exposure and can occur before respiratory signs. A second fever phase around 6 to 7 days after exposure may precede systemic viremia. AAEP recommends twice daily temperature monitoring in at-risk horses to reduce the chance of missing fever.

A commonly used fever threshold is:

Above 101.5°F or 38.6°C

A horse with a fever during an EHV risk period should not be ignored, even if there is no dramatic snot, cough, or wobbliness yet.

The mistake is waiting for the horse to look obviously sick. EHV does not always start with theatre. Sometimes it starts with a number on a thermometer.

Respiratory EHV Signs

Respiratory EHV can be mild or more obvious.

Signs may include:

• Fever
• Lethargy
• Reduced appetite
• Nasal discharge
• Coughing
• Enlarged lymph nodes under the jaw or throatlatch
• Conjunctivitis or eye inflammation
• Lower limb swelling in some cases

AAEP lists fever, coughing, nasal discharge, lymph node enlargement, lethargy, anorexia, conjunctivitis, ocular disease, and lower limb swelling as possible respiratory EHV signs.

Merck notes that susceptible horses may develop fever, nasal discharge, malaise, pharyngitis, cough, inappetence, and lymph node enlargement, while infection may be mild or inapparent in horses with previous exposure or vaccination. (Merck Veterinary Manual)

The key point: a mild case can still matter if the horse is around other horses.

Neurologic EHV Signs, or EHM

EHM is the neurologic form of EHV-1.

Signs may include:

• Hindlimb weakness
• Wobbliness
• Ataxia
• Stumbling
• Urine dribbling
• Urine retention
• Bladder atony
• Reduced tail tone
• Recumbency
• Inability to rise
• Occasionally cranial nerve signs, seizures, or brainstem signs

AAEP warns that many EHM cases do not fit the classic description of hindlimb ataxia and urinary incontinence, so owners should not wait for the perfect textbook case before calling a vet.

In practice, the most concerning pattern is:

Fever followed by hindlimb weakness, wobbliness, urine dribbling, or difficulty standing.

That is an emergency.

Why Do Some Horses Develop EHM?

This is still an important research question.

EHM risk appears to be influenced by multiple factors, not one simple cause. Viral strain, immune response, age, pregnancy status, stress, viremia, exposure dose, and host factors may all play roles.

AAEP notes that risk factors for EHM are not clearly defined, but factors associated with EHM in one or more outbreak reports include exposure to biosecurity risks at equine events, female sex, increasing age, and prior EHV-1 vaccination. AAEP also states that all recognised EHV-1 DNA polymerase variants are capable of causing EHM outbreaks, so the idea of one simple “neurologic strain” is too simplistic.

That does not mean vaccination causes EHM. AAEP vaccination guidance states that an association between EHV vaccination and EHM has been reported in multiple outbreak investigations, but the mechanism is unknown and controlled challenge studies are lacking, so current recommendations have not been revised on that basis.

The practical takeaway:

Do not assume EHM only happens to unvaccinated horses. Do not assume vaccination caused it either. Treat fever and neurologic signs seriously regardless of vaccine history.

EHV Risk Framework

Risk level	What it looks like	What it may mean	What to do
Low risk	Healthy horse, normal temperature, no recent travel, no known exposure	Routine prevention is the focus	Maintain vaccination, hygiene, and sensible travel biosecurity
Moderate risk	Recent travel, new arrival, exposure to unknown horses, mild fever, mild nasal discharge	Early EHV or another infectious disease is possible	Separate from the herd, check temperature twice daily, call your vet
High risk	Fever plus cough, nasal discharge, multiple horses affected, pregnant mares exposed, recent show exposure	EHV outbreak risk is significant	Isolate, stop movement, test as advised by your vet
Critical	Hindlimb weakness, wobbliness, urine dribbling, inability to stand, abortion, sick newborn foal	EHM, EHV abortion, or severe EHV disease possible	Urgent veterinary care and strict biosecurity immediately

The decision point is simple:

Fever means isolate and investigate. Fever plus neurologic signs means emergency.

When Is This an Emergency?

Call a veterinarian immediately if your horse has:

• Fever after known EHV exposure
• Fever during an outbreak
• Hindlimb weakness
• Wobbliness or ataxia
• Urine dribbling
• Difficulty urinating
• Reduced tail tone
• Difficulty rising
• Recumbency
• Sudden abortion
• A weak or sick newborn foal
• Severe depression
• Rapid deterioration
• Multiple horses with fever on the property
• Respiratory distress

AAEP recommends testing horses showing clinical signs consistent with EHV infection, especially when febrile or showing neurologic signs. EHV and EHM are reportable in many jurisdictions, and neurologic EHV-1 positive horses should be reported to the relevant animal health authority where required.

Do not keep riding, transporting, or mixing a horse with possible EHV signs.

The first move is not to “see how they go.”

The first move is isolation.

How Does EHV Spread?

EHV spreads through direct and indirect contact.

Main spread routes include:

• Nasal secretions
• Respiratory droplets from coughing or snorting
• Nose-to-nose contact
• Shared buckets
• Shared feed or water sources
• Tack and grooming tools
• Human hands, clothing, and footwear
• Stalls, wash racks, tie-up points, and trailers
• Aborted fetuses, placenta, fetal fluids, and fetal membranes

AAEP lists respiratory transmission through droplets and contact with nasal secretions, direct transmission through aborted fetal material, and indirect transmission through contaminated clothing, footwear, grooming gear, tack, feed or water sources, stalls, wash racks, and tie points.

Merck also notes that transmission occurs through direct or indirect contact with infectious nasal secretions, aborted fetuses, placentas, or placental fluids. (Merck Veterinary Manual)

The horse may be the source, but the human often becomes the delivery service.

Hands, jackets, boots, lead ropes, buckets, and “just borrowing this for a second” can all matter.

How Long Can EHV Survive and Shed?

AAEP states that respiratory shedding typically lasts 7 to 10 days, but it may be longer in some cases. It also estimates EHV-1 environmental persistence at no more than 35 days under ideal conditions and probably less than 7 days in most practical field situations.

Recovered horses often develop latent infections and can shed virus again after reactivation, especially under stress.

This is why EHV control is not just about cleaning one sick horse’s stable once.

It is about:

• Movement control
• Temperature monitoring
• Isolation
• Disinfection
• Testing
• Managing exposed horses
• Following veterinary and authority guidance

How Do Vets Diagnose EHV?

Diagnosis usually relies on PCR testing, clinical signs, exposure history, and outbreak context.

Your vet may collect:

• Nasal swab
• Nasopharyngeal swab
• Whole blood or buffy coat sample
• Paired serum samples collected 14 to 21 days apart
• Aborted fetal tissues and placenta if abortion occurs
• Postmortem samples in fatal neurologic or neonatal cases

AAEP recommends testing nasal or nasopharyngeal swabs and EDTA or citrated blood together. Quantitative PCR is more sensitive and faster than virus isolation and is considered the rapid test of choice during outbreaks.

EDCC also lists PCR testing of nasal swabs, buffy coat samples, virus isolation from blood, or significant antibody titer rise in paired serum samples as diagnostic approaches for EHV-1 and EHV-4. (Equine Disease Communication Center)

Important detail: if a febrile horse with suspected exposure tests negative early, AAEP recommends retesting blood and nasal swab PCR in 24 to 72 hours because fever can precede detectable nasal shedding and viremia.

A single negative early test does not always end the conversation.

What Else Can Look Like EHV?

EHV can look like many other diseases, especially early.

Important rule-outs include:

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

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

This is why “looks like EHV” should lead to veterinary testing, not guessing.

How Is EHV Treated?

There is no simple cure that clears EHV instantly.

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

Supportive care may include:

• Rest
• Isolation
• NSAIDs for fever, pain, and inflammation when appropriate
• Fluids if needed
• Nursing care
• Bladder management in EHM cases
• Sling support in selected recumbent horses
• Monitoring hydration, appetite, manure, urination, and neurologic progression
• Antimicrobials only if secondary bacterial infection is suspected
• Antivirals in selected cases under veterinary direction

EDCC states that supportive care and rest are the chief treatments, with NSAIDs used to control fever, pain, and inflammation, and antivirals or heparin used in some cases. (Equine Disease Communication Center)

Merck similarly states that treatment is largely supportive, although antiviral use has been described for EHM. (Merck Veterinary Manual)

Do not start random anti-inflammatories, antibiotics, or antivirals without veterinary direction. Fever patterns, testing, hydration, kidney risk, secondary infection, and outbreak control all matter.

Can Horses Recover From EHV?

Many horses with respiratory EHV recover well with rest and supportive care.

The prognosis becomes more guarded when:

• EHM develops
• The horse becomes recumbent
• The horse cannot urinate properly
• Neurologic signs are severe
• The horse develops complications from prolonged recumbency
• A pregnant mare aborts
• A neonatal foal is infected

Merck notes that prognosis for EHM depends on the severity of signs and the duration of recumbency. (Merck Veterinary Manual)

AAEP also notes that horses with EHM may have residual neurologic deficits for weeks to months after viral shedding has stopped.

This is the hard truth: respiratory EHV is often manageable. EHM can be life changing.

What Can EHV Vaccines Do?

EHV vaccination is important, but it has limits.

EHV vaccines can help:

• Reduce respiratory disease severity
• Reduce respiratory disease duration
• Reduce nasal viral shedding
• Reduce viremia in some cases
• Reduce abortion risk when appropriate EHV-1 abortion vaccines are used in pregnant mares

AAEP states that EHV-1 and EHV-4 vaccines are indicated for prevention of EHV-1 abortion and reduction in severity and duration of respiratory disease. The same guidance states that vaccines can reduce nasal shedding and viremia, with the goal of reducing spread and reducing disease severity.

But this is the key limitation:

There is currently no licensed vaccine labelled to prevent neurologic EHV-1 disease, or EHM.

Vaccination is a layer of protection.

It is not a force field.

Which Horses Need EHV Vaccination?

EHV vaccination should be risk-based.

AAEP recommends six-monthly revaccination for several higher-risk groups, including:

• Horses under 5 years of age
• Horses on breeding farms
• Horses in contact with pregnant mares
• Horses at facilities with frequent movement on and off the premises
• Performance horses or show horses in high-risk settings such as racetracks or show grounds

Pregnant mares are commonly vaccinated during the fifth, seventh, and ninth months of gestation using an inactivated EHV-1 vaccine licensed for abortion prevention. Foals generally start a primary series at 4 to 6 months of age.

The right vaccine plan depends on:

• Age
• Pregnancy status
• Travel
• Show exposure
• Breeding farm risk
• Local disease alerts
• Prior vaccine history
• Product label
• Regional requirements

Ask your veterinarian to build the schedule around your horse’s actual risk, not a generic calendar.

Should You Vaccinate During an EHV Outbreak?

Do not decide this casually.

AAEP states that controlled peer-reviewed research does not currently exist to show whether vaccinating horses during an EHV outbreak reduces transmission. It also states that strict biosecurity, isolation of infected horses, quarantine of affected premises, and monitoring of at-risk populations are currently more effective at controlling outbreaks than any vaccination protocol.

In practical terms:

• Do not vaccinate a sick or febrile horse without veterinary direction
• Do not use vaccination instead of isolation
• Do not move horses because they were “just boosted”
• Do not assume outbreak vaccination will stop spread
• Ask your veterinarian and animal health authority what is appropriate

During an outbreak, the boring tools matter most:

Isolation.
Temperature logs.
No movement.
No shared equipment.
Testing.
Clear communication.

The virus hates boring systems. Excellent.

What To Do Right Now if You Suspect EHV

1. Isolate the horse immediately

If the horse has fever, nasal discharge, cough, abortion, weakness, wobbliness, or urine dribbling, separate them from other horses.

Do not wait for lab results to start basic isolation.

2. Stop horse movement

Do not move horses on or off the property until your veterinarian advises it is safe.

This includes:

• Shows
• Lessons
• Breeding visits
• Sales
• Trail rides
• Farrier visits at other properties
• Transport to non-isolation facilities

3. Check temperatures twice daily

Check every exposed horse morning and evening.

Record:

• Time
• Temperature
• Appetite
• Nasal discharge
• Coughing
• Manure
• Urination
• Attitude
• Any weakness or wobbliness

AAEP specifically recommends twice daily temperature monitoring in at-risk horses because fever can be transient or missed.

4. Call your veterinarian

Have this information ready:

• When signs started
• Highest temperature recorded
• Vaccination history
• Recent travel or shows
• Any new arrivals
• Any pregnant mares on site
• Any abortions
• Any neurologic signs
• Number of horses affected
• Whether other barns are involved

5. Use separate equipment

Use separate:

• Buckets
• Feed tubs
• Thermometers
• Halters
• Lead ropes
• Grooming gear
• Rugs
• Muck forks
• Wheelbarrows
• Gloves and protective clothing where appropriate

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

6. Clean and disinfect properly

Remove organic material first.

Disinfect:

• Stalls
• Buckets
• Feed tubs
• Waterers
• Tack
• Grooming gear
• Trailers
• Wash bays
• Tie-up areas
• Door latches
• Shared surfaces

AAEP notes that indirect pathways through environmental and fomite contamination may play a more important role than previously thought, especially in outbreak situations.

7. Follow quarantine guidance

Confirmed EHM cases require strict isolation and quarantine under relevant animal health authority guidance. AAEP states confirmed EHM cases should be isolated immediately and the premises quarantined for a minimum of 28 days, with all horses monitored at least twice daily for fever or signs of EHV or EHM. New fever, abortion, or neurologic disease can restart the quarantine clock.

Your veterinarian and local authority rules should guide the final quarantine and release plan.

Common Mistakes With EHV

Mistake 1: Waiting for neurologic signs before acting

Fever may be the first sign. Waiting for wobbliness can mean missing the best isolation window.

Mistake 2: Assuming vaccinated horses are safe

Vaccinated horses can still become infected or develop EHM. Vaccination reduces risk and severity, but it does not eliminate disease.

Mistake 3: Treating respiratory EHV as harmless

Respiratory signs may be mild in one horse but create risk for pregnant mares, young horses, older horses, and exposed horses that may develop more serious disease.

Mistake 4: Moving horses too soon

Horse movement during a suspected outbreak can spread disease beyond one property.

Mistake 5: Not checking temperatures twice daily

Fever can be transient. One daily check can miss it.

Mistake 6: Sharing buckets, tack, or staff between groups

EHV can spread through contaminated surfaces, equipment, clothing, and footwear.

Mistake 7: Assuming “not the neurologic strain” means safe

AAEP notes that all recognised EHV-1 strains are capable of causing EHM outbreaks. Manage the risk based on signs and exposure, not a casual strain label.

Prevention: How To Reduce EHV Risk

You cannot eliminate EHV from the horse world, but you can reduce the risk of spread and severe outbreaks.

Practical prevention includes:

• Keep vaccination current based on risk
• Quarantine new arrivals
• Separate returning show horses where practical
• Avoid nose-to-nose contact at events
• Use your own buckets and gear
• Do not share water sources
• Take temperatures during high-risk periods
• Do not travel sick horses
• Separate pregnant mares from high-movement horses
• Clean and disinfect trailers after travel
• Monitor horses after shows, clinics, sales, and transport
• Isolate fever cases early
• Train staff to report fever, nasal discharge, cough, abortion, or neurologic signs immediately

EDCC emphasises testing and quarantining affected horses in isolation as the primary way to control EHV, and recommends correct biosecurity when bringing new horses onto a property or travelling. (Equine Disease Communication Center)

The strongest EHV prevention plan is not one dramatic action.

It is consistent, boring, slightly annoying discipline. Which is basically all good biosecurity.

Myth vs Reality

Myth	Reality
“EHV is just a respiratory virus.”	EHV-1 can also cause abortion, fatal neonatal disease, and neurologic disease.
“Vaccinated horses cannot get EHM.”	No vaccine is labelled to prevent EHM. Vaccinated horses can still be affected.
“Only horses with nasal discharge spread EHV.”	Fever can occur before obvious respiratory signs, and latent virus can reactivate under stress.
“The neurologic form only happens with one strain.”	All recognised EHV-1 strains can cause EHM outbreaks.
“One negative early PCR test clears the horse.”	If exposure and fever fit, retesting 24 to 72 hours later may be needed.
“Vaccination controls outbreaks.”	Biosecurity, isolation, quarantine, and monitoring are more effective outbreak control tools.

Frequently Asked Questions

Can a vaccinated horse still get neurologic EHV?

Yes. Current EHV vaccines can help reduce respiratory disease, shedding, viremia, and abortion risk, but there is no licensed vaccine labelled to prevent neurologic EHV-1 disease, or EHM.

What is the first sign of EHV in horses?

Fever is often the first useful sign. It may occur before nasal discharge, coughing, or neurologic signs. During an outbreak or after exposure, twice daily temperature checks are recommended.

How is EHV diagnosed?

Vets usually use PCR testing on nasal or nasopharyngeal swabs plus whole blood, supported by clinical signs and exposure history. Paired serology or testing of abortion material may be used in selected cases.

How long should horses be isolated after EHV exposure?

Isolation length depends on the situation, test results, clinical signs, and local animal health rules. AAEP notes respiratory shedding often lasts 7 to 10 days but may be longer, and movement restrictions may need 14 to 28 days after clinical signs resolve. Confirmed EHM premises may require at least 28 days of quarantine under authority guidance.

Is EHV dangerous to humans?

No known zoonotic potential has been identified for EHV. The risk is to horses, not people, but people can mechanically spread virus between horses on hands, clothes, footwear, and equipment.

The Bottom Line

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

The respiratory form can look mild, especially at first. The neurologic form, EHM, can be devastating. The link between the two is why fever matters so much. Fever can be the early warning before respiratory signs, abortion, or neurologic disease become obvious.

The best EHV plan is layered:

• Vaccinate based on real risk
• Monitor temperatures during high-risk periods
• Isolate fever cases early
• Stop movement during suspected outbreaks
• Test appropriately
• Separate clean, exposed, and sick horses
• Treat neurologic signs as urgent
• Follow veterinary and animal health authority guidance

If your horse is bright, normal temperature, eating well, and has no exposure risk, keep your routine strong.

If your horse has fever after travel, nasal discharge, coughing, exposure to sick horses, sudden abortion, weakness, wobbliness, urine dribbling, or difficulty standing, do not wait.

With EHV, early action protects more than one horse. It protects the entire property.

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