Equine Herpesvirus in Horses: Respiratory EHV vs Neurological EHM

By Dr Duncan Houston

Equine herpesvirus is one of those horse diseases that can look mild at first, then suddenly become very serious.

A horse may start with a fever, nasal discharge, coughing, dullness or reduced appetite. That can look like a fairly standard respiratory infection. The problem is that some EHV-1 infections can progress into the neurological form, called equine herpesvirus myeloencephalopathy, or EHM.

That is where this disease becomes dangerous.

The key for horse owners is knowing the difference between respiratory EHV, neurological EHM, and the point where isolation and urgent veterinary care are needed.

Quick Answer: What Is the Difference Between Respiratory EHV and Neurological EHM?

Respiratory EHV usually causes fever, nasal discharge, cough, lethargy and reduced appetite. Many horses recover with rest, monitoring and supportive care.

Neurological EHV, called EHM, is much more serious. It can cause hindlimb weakness, wobbliness, toe dragging, urine dribbling, loss of tail tone, difficulty standing, recumbency and death.

The simple rule is this: fever plus any neurological sign should be treated as an emergency.

EHV-1 can cause respiratory disease, abortion, neonatal disease and neurological disease. EHV-4 mainly causes respiratory disease and only occasionally causes abortion or neurological disease. EHV-3 causes coital exanthema, a venereal disease, and EHV does not pose a risk to people. (APHIS)

When Is EHV an Emergency?

Call a vet urgently if your horse has a fever and any sign of neurological change.

Emergency red flags include:

• Hindlimb weakness

• Wobbling or incoordination

• Stumbling

• Toe dragging

• Dog-sitting posture

• Loss of tail tone

• Urine dribbling

• Difficulty urinating

• Difficulty passing manure

• Sudden collapse

• A horse that is down and unable to rise

• Severe depression or altered mentation

UC Davis notes that horses with EHM may become uncoordinated and weak, have trouble standing, develop difficulty urinating or defecating, show dog-sitting or toe-dragging, and in severe cases become extremely lethargic or coma-like. (Center for Equine Health)

Do not wait to “see how it looks tomorrow” if neurological signs are present. With EHM, tomorrow can be a very expensive, very horrible word.

What Is Equine Herpesvirus?

Equine herpesvirus, or EHV, is a family of contagious viruses affecting horses and other equids. The most important types for most horse owners are:

• EHV-1: respiratory disease, abortion, neonatal disease and neurological disease

• EHV-4: mainly respiratory disease, with abortion and neurological disease less common

• EHV-3: coital exanthema, a venereal disease affecting the external genitalia

EHV-1 and EHV-4 are widespread in horse populations. Horses can also carry EHV without obvious clinical signs and may shed virus later, especially around stress, movement, competition or illness. (Equine Disease Communication Center)

You may still hear people call EHV “rhino” or “rhinopneumonitis.” That term is still used in some veterinary references, but for owners, EHV is clearer because it separates herpesvirus disease from true rhinovirus confusion.

The Respiratory Form of EHV

Respiratory EHV is the more common presentation, especially in young horses, recently transported horses, competition horses, horses in boarding facilities, and horses mixing with new groups.

Signs may include:

• Fever

• Nasal discharge

• Coughing

• Dullness or lethargy

• Reduced appetite

• Malaise

• Sore throat or pharyngitis

• Swollen lymph nodes around the throat area

Merck Veterinary Manual lists fever, nasal discharge, malaise, pharyngitis and cough as key signs of equine herpesvirus infection, with EHV-1 also capable of causing late-term abortion and neurological disease. (Merck Veterinary Manual)

In many horses, respiratory EHV is mild and self-limiting. That does not mean it is harmless. The individual horse may recover well, but the virus can still spread through a property, showground, breeding facility or boarding stable.

That is the sneaky part. One horse looks “a bit off,” then suddenly half the barn owns a thermometer and nobody is going anywhere.

The Neurological Form: EHM

The neurological form of EHV-1 is called equine herpesvirus myeloencephalopathy, or EHM.

EHM occurs when EHV-1 infection is associated with damage to blood vessels in the central nervous system. This can lead to inflammation, clotting, reduced blood flow, spinal cord injury and neurological signs. Merck describes EHV-1 neurological lesions as vasculitis with endothelial damage, thrombus formation, haemorrhage and areas of malacia within the brain or spinal cord. (Merck Veterinary Manual)

Signs of EHM may include:

• Hindlimb ataxia

• Weakness behind

• Stumbling

• Toe dragging

• Loss of tail tone

• Urinary incontinence

• Difficulty urinating

• Difficulty defecating

• Recumbency

• Severe depression or altered mentation

Once a horse becomes recumbent, prognosis becomes much more guarded. UC Davis reports that EHM mortality can be high, and that horses able to remain standing generally have a better prognosis than those that become recumbent. (Center for Equine Health)

Respiratory EHV vs Neurological EHM

The practical point is simple: respiratory EHV can still be an outbreak problem, but EHM is a neurological emergency.

Severity Framework: How Worried Should You Be?

Low Risk

This is a horse with no fever, no known exposure, no respiratory signs and no neurological signs.

Action: keep normal monitoring in place, especially after travel, competitions or new arrivals.

Moderate Risk

This is a horse with fever, mild nasal discharge, cough, lethargy or reduced appetite, but no neurological signs.

Action: isolate the horse, take rectal temperatures twice daily, stop sharing equipment and call your vet for advice. This may be respiratory EHV, influenza, strangles, equine viral arteritis or another infectious disease.

High Risk

This is a horse with fever after travel, a show, a known exposure, a boarding facility outbreak, or contact with sick horses.

Action: isolate immediately, stop horse movement and contact your vet. Do not wait for neurological signs before taking EHV seriously.

Critical Risk

This is a horse with fever plus neurological signs, or any sudden hindlimb weakness, ataxia, urine dribbling, loss of tail tone, collapse or inability to rise.

Action: call a vet urgently. Do not move the horse unless directed by your vet or animal health authorities.

Fever Patterns: Why Temperature Monitoring Matters

Temperature monitoring is one of the most useful early tools during an EHV scare.

Fever may be the first or only early sign. UC Davis notes that fever of 102°F or greater may be the initial abnormality, and neurological signs, when they occur, are typically seen around 8 to 12 days after the primary infection involving fever. (Center for Equine Health)

The practical takeaway:

Do not wait for wobbliness before acting.

If a horse has a fever after travel, competition, exposure to sick horses or a known EHV contact, isolate the horse and speak to your vet. A temperature log can be the difference between catching a problem early and discovering it after the virus has already wandered through the barn like it owns the place.

How Does EHV Spread?

EHV spreads through direct and indirect contact.

Common routes include:

• Nose-to-nose contact

• Respiratory droplets

• Nasal discharge

• Shared water buckets

• Shared feed tubs

• Shared tack

• Shared grooming equipment

• Contaminated hands, clothing or footwear

• Contaminated stalls, trailers and stable equipment

• Aborted fetal tissue or placenta in abortion cases

APHIS states that EHV-1 and EHV-4 spread through nose-to-nose contact, contaminated equipment, respiratory secretions, and in abortion cases, aborted fetuses and afterbirth. APHIS also recommends isolating new or returning horses for at least 21 days and avoiding shared tack or equipment at shows, events and boarding facilities. (APHIS)

This is why one sick horse at a show, racetrack, breeding property or boarding stable can quickly become everyone’s problem.

Why Do Some Horses Develop EHM?

This is one of the most frustrating parts of EHV.

Not every horse with EHV develops neurological disease. Some horses only develop mild respiratory signs. Some may show fever and then recover. Others develop EHM.

EHM is multifactorial. Risk appears to depend on the virus, the horse, immune response, age, stress, pregnancy status, exposure pressure, management, travel, housing and outbreak conditions.

Risk factors associated with EHV-1 or EHM in outbreak studies include:

• Increasing age

• High-density horse environments

• Competition settings

• Shared barns

• Horses from multiple home properties being housed together

• Stress and transport

• Delayed isolation

• Shared equipment and poor biosecurity

A 2026 California outbreak study found that greater age, main activity, and sharing a barn with horses from different home barns were associated with higher odds of becoming an EHV-1 or EHM case during a horse show outbreak. (MDPI)

The real-world lesson is not that one single horse “causes” the disaster. It is usually the setup: travel, stress, shared spaces, shared gear, delayed isolation and a virus that is extremely good at being annoying.

What Else Can Look Like EHV or EHM?

EHV is important, but it is not the only cause of fever, respiratory signs or neurological signs in horses.

Respiratory signs may also be caused by:

• Equine influenza

• Strangles

• Equine viral arteritis

• Other viral respiratory infections

• Bacterial pneumonia

• Allergic or inflammatory airway disease

Neurological signs may also be caused by:

• Trauma

• Cervical vertebral compressive myelopathy

• Equine protozoal myeloencephalitis in regions where it occurs

• West Nile virus

• Botulism

• Toxicity

• Severe metabolic disease

• Tick paralysis in some regions

• Other infectious or inflammatory neurological conditions

Merck notes that equine viral rhinopneumonitis can be difficult to clinically distinguish from equine influenza, equine viral arteritis or other equine respiratory infections based on signs alone, which is why laboratory testing matters. (Merck Veterinary Manual)

This is also why the goal is not to “diagnose EHV by vibes.” Very scientific phrase, obviously. The goal is to isolate early, protect other horses, and let testing and veterinary assessment guide the next steps.

How Is EHV Diagnosed?

EHV is usually diagnosed with laboratory testing.

Common tests include:

• Nasal swab PCR

• Whole blood or buffy coat PCR

• Virus isolation in some cases

• Paired blood testing in selected situations

• Fetal and placental testing after suspected EHV abortion

The Equine Disease Communication Center lists PCR testing from nasal swab and buffy coat sample as standard diagnostic options for EHV-1 and EHV-4. Merck also notes that nasal swab PCR is useful for assessing viral shedding, although intermittent shedding can produce false negative results. (Equine Disease Communication Center)

Testing should be guided by your vet. Timing matters. A single negative test does not always rule everything out if the horse is early in disease, intermittently shedding, or sampled at the wrong time.

Can Vaccination Prevent Neurological EHV?

This needs careful wording.

EHV vaccines are useful, but they are not magic armour.

Vaccines are available to help reduce respiratory disease, abortion risk, disease severity and viral shedding. However, there is currently no vaccine labelled to prevent the neurological form, EHM. EDCC states that vaccines are available for respiratory and abortive forms of EHV-1 and EHV-4, but there is no vaccine labelled for prevention of neurological EHV-1 disease. (Equine Disease Communication Center)

The 2024 ACVIM consensus statement also concluded that evidence for successful vaccination against, or effective treatment of, EHV-1 infection remains limited. (PubMed)

So the practical message is:

Vaccination helps reduce risk and outbreak impact, but it does not replace biosecurity.

A vaccinated horse can still become infected, shed virus, or develop EHM. That does not mean vaccination is pointless. It means vaccination is one layer of protection, not the whole fortress.

Treatment for EHV and EHM

There is no simple cure for EHV.

Treatment is mainly supportive and may include:

• Rest

• Fever control

• Anti-inflammatory medication

• Fluids if needed

• Careful monitoring

• Nursing support

• Bladder management in neurological cases

• Sling support in selected recumbent horses

• Antivirals in some cases, depending on timing and veterinary judgement

EDCC lists supportive care, rest and non-steroidal anti-inflammatory medications as key treatments, with antivirals and heparin used in some cases. Merck states there is no specific treatment for EHV infection, and that treatment is largely supportive, although antivirals have been described for EHM. (Equine Disease Communication Center)

For EHM, nursing care can be intense. A horse that cannot urinate properly may need bladder management. A horse that cannot stand may need specialist care, sling support and careful welfare assessment. This is not a “give it a day and see” situation.

What Should You Do if You Suspect EHV?

If your horse has fever, respiratory signs, neurological signs or possible EHV exposure, act early.

Step 1: Isolate the horse immediately

Move the horse away from other horses if this can be done safely and without increasing stress or spreading contamination.

Step 2: Stop horse movement

Do not move horses on or off the property until your vet gives advice. If this is a showground, boarding stable or training facility, notify the manager or organiser.

Step 3: Call your vet

Your vet can advise whether testing is needed, what samples to collect, how to monitor exposed horses, and whether animal health authorities need to be notified.

Step 4: Take temperatures twice daily

Record rectal temperatures. Fever may appear before respiratory or neurological signs.

Step 5: Stop sharing equipment

Do not share buckets, feed tubs, rugs, tack, grooming gear, lead ropes or stable tools.

Step 6: Handle healthy horses first

Care for healthy horses before exposed or sick horses. Change clothing, wash hands and disinfect boots and equipment.

Step 7: Monitor exposed horses closely

Watch for fever, nasal discharge, cough, dullness, reduced appetite, hindlimb weakness, toe dragging, urine dribbling or difficulty standing.

UC Davis recommends isolating horses with fever, respiratory signs or neurological signs, notifying a veterinarian immediately, and ceasing horse movement in the immediate area until a diagnosis is confirmed. (Center for Equine Health)

How to Reduce the Risk of EHV Outbreaks

You cannot eliminate EHV risk completely, but you can reduce it.

Good prevention includes:

• Keeping horses appropriately vaccinated

• Isolating new arrivals

• Isolating horses returning from shows or travel

• Monitoring temperatures after competition or transport

• Avoiding shared water buckets and feed tubs

• Using separate grooming kits

• Cleaning and disinfecting tack, stalls, trailers and equipment

• Keeping pregnant mares away from high-traffic horses

• Reducing unnecessary horse movement during outbreaks

• Having a written outbreak plan before you need one

APHIS recommends vaccination, 21-day isolation for new or returning horses, monitoring for disease signs, and regular cleaning and disinfection of feed buckets, water buckets, tack and other equipment. (APHIS)

The boring biosecurity stuff is usually what saves the day. Not glamorous. Not sparkly. Very much not a hero shot for Instagram. But wildly useful.

Common Mistakes Horse Owners Make

Mistake 1: Waiting to see if the fever passes

A fever after travel, competition or known exposure should not be ignored. Temperature logs are simple, cheap and extremely useful.

Mistake 2: Only isolating once neurological signs appear

By the time neurological signs appear, virus spread may already have occurred. Isolate at the fever stage.

Mistake 3: Assuming vaccinated horses cannot get EHM

Vaccination helps, but no current vaccine is labelled to prevent EHM. Biosecurity still matters. (Equine Disease Communication Center)

Mistake 4: Sharing buckets and equipment

Shared gear is an outbreak amplifier. Buckets, tack, rugs, grooming tools and stable equipment should be separated during a suspected infectious disease event.

Mistake 5: Moving horses too soon

Horse movement during a suspected outbreak can spread disease to new properties, events and regions. Do not move horses until your vet or relevant authority advises it is safe.

Mistake 6: Testing randomly without a plan

Testing clinically normal, non-exposed horses is not always useful. UC Davis notes that PCR testing in asymptomatic, unexposed horses can be difficult to interpret and is not recommended as random screening in that context. (Center for Equine Health)

FAQs About EHV and EHM in Horses

Is equine herpesvirus contagious?

Yes. EHV is highly contagious between horses and can spread through direct contact, respiratory secretions, contaminated equipment, clothing, hands, stalls and stable tools.

Can humans catch EHV from horses?

No. EHV does not pose a risk to people. It is a horse and equid disease concern. (APHIS)

Is EHV the same as EHM?

No. EHV refers to the virus. EHM refers to the neurological disease that can occur, most commonly after EHV-1 infection.

Can a horse have EHV without coughing?

Yes. Fever may be the only early sign. Some horses show mild or nonspecific signs, while others may shed virus without obvious illness.

What temperature is concerning in a horse?

A fever around 102°F or 38.9°C, especially after travel, competition or exposure to sick horses, is concerning. UC Davis notes that fever of 102°F or greater may be an initial sign of EHV-1 infection. (Center for Equine Health)

How long after infection can neurological signs appear?

Neurological signs may appear days after the first fever. UC Davis notes that when neurological disease occurs, it is typically around 8 to 12 days after the primary infection involving fever. (Center for Equine Health)

Can EHV cause abortion?

Yes. EHV-1 can cause abortion, stillbirth and neonatal disease. This is why biosecurity around pregnant mares is so important. (APHIS)

Should I vaccinate my horse for EHV?

Most horses benefit from an appropriate EHV vaccination plan, especially breeding mares, young horses, competition horses and horses that travel. Your vet can recommend the right schedule based on your horse’s risk.

Does vaccination stop neurological EHV?

No current vaccine is labelled to prevent EHM. Vaccination may reduce respiratory disease, abortion risk, disease severity and viral shedding, but it does not guarantee protection from neurological disease. (APHIS)

What should I do if my horse has been exposed to EHV?

Isolate the horse, monitor rectal temperature twice daily, stop unnecessary horse movement, avoid shared equipment and contact your vet for testing and quarantine advice.

Final Thoughts

Respiratory EHV and neurological EHM are linked, but they are not the same level of danger.

A horse with respiratory EHV may recover well with supportive care. A horse developing EHM can become weak, unable to urinate, unable to stand and may not survive. The difference comes down to early detection, isolation, testing and rapid veterinary support.

The big rule is simple:

Fever plus neurological signs is an emergency.

Monitor temperatures. Isolate early. Do not share equipment. Do not move horses during a suspected outbreak. Call your vet before the problem spreads through the barn.

Because with EHV, the cheapest outbreak plan is the one you had before the outbreak.

Need help deciding whether your horse’s signs are respiratory, neurological or something else? ASK A VET™ can help you review symptoms, prepare questions for your local vet, and understand when a horse needs urgent hands-on care.