Equine Herpesvirus: Symptoms, Risks, and Prevention

What Is the Equine Herpesvirus?

Equine herpesvirus (EHV) refers to a group of contagious viruses that affect horses, with varying degrees of severity. The most significant strains are equid herpesvirus 1 (EHV-1) and equine herpesvirus 4 (EHV-4), both of which are alpha-herpesviruses known for causing respiratory illness. Less commonly, equine herpesvirus 5 (EHV-5) and equine herpesvirus 2 (EHV-2) are associated with chronic respiratory issues or other complications.

EHV-1 is especially concerning due to its ability to trigger neurological disorders—specifically equine herpesvirus myeloencephalopathy (EHM)—and equine herpesvirus abortion in pregnant mares. The virus establishes latency, meaning a horse can carry it for life and begin shedding again under stress, even without showing symptoms.

Once infected, horses may become carriers, periodically reactivating and spreading the virus, contributing to horse herpes outbreaks in equine populations.

Curious about equine herpesvirus?

‍In this video, you'll learn everything you need to know about the different types of EHV, including EHV-1 and EHV-4. Discover the most common symptoms, such as respiratory infections, abortions, and neurological complications and understand how the virus spreads. Find out why it's so contagious and what horse owners can do to prevent outbreaks.

Clinical Signs and Complications

What are the symptoms of EHV in horses?
The symptoms of equine herpesvirus (EHV) can vary significantly in both severity and form, depending on the specific virus strain—most notably EHV-1 and EHV-4—as well as the horse’s age, immune status, pregnancy, and stress levels. While some infections remain mild or subclinical, others can escalate into serious health threats affecting the respiratory system, nervous system, or reproductive tract.

Variable Onset and Presentation

In its early stages, EHV infection often mimics more common equine illnesses, making it difficult to detect without testing. Common initial signs include:

• Mild fever
• Reduced appetite
• Lethargy
• Mild nasal discharge

These general symptoms can easily be overlooked or attributed to routine viral exposure, which increases the risk of transmission before an outbreak is recognized.

Multisystemic Impact

EHV infections are categorized into three primary clinical forms:

• Respiratory disease, which is most common and often referred to as equine viral rhinopneumonitis
• Neurological complications, known as equine herpesvirus myeloencephalopathy (EHM), which can lead to paralysis or even death
• Reproductive failure, most notably equine herpesvirus abortion, affecting pregnant mares in late gestation

Each form presents with distinct clinical signs, though some overlap may occur—especially in EHV-1 infections, which can progress from mild respiratory symptoms to severe neurological or reproductive outcomes.

Silent Spread and Risk of Outbreak

One of the greatest challenges with EHV is its ability to spread silently. Horses can shed the virus even without showing symptoms, and those with latent infections can become contagious again under stress. This makes early recognition and proactive isolation critical to controlling disease spread.

In the following sections, we’ll take a closer look at each clinical manifestation—starting with respiratory disease—detailing how they present, how they progress, and what signs to watch for.

Equine Rhinopneumonitis

What is equine rhinopneumonitis, and how does it affect horses?
Equine rhinopneumonitis is the respiratory manifestation of equine herpesvirus infection, caused primarily by EHV-1 and EHV-4. The term refers to inflammation of the upper respiratory tract, specifically the nasal passages, pharynx, and trachea and it is one of the most common clinical forms of EHV, especially in foals, weanlings, and performance horses under stress.

The disease spreads rapidly in environments where horses are housed closely together, such as stables, training centers, or competition venues. Because it often presents with mild symptoms in adults, it can be overlooked, contributing to the silent spread of the virus.

What Are the Symptoms of Equine Rhinopneumonitis?

Respiratory signs generally develop 2–10 days after exposure to the virus. Typical symptoms include:

• Sudden onset of fever (often biphasic, especially in EHV-1 infections)
• Clear nasal discharge that may become mucopurulent with secondary infection
• Dry or moist cough
• Swollen submandibular lymph nodes
• Lethargy and reduced appetite

In some cases, these signs may be mistaken for less serious viral illnesses, such as equine influenza or mild allergic reactions. However, unlike those conditions, EHV can progress to more serious forms and pose a risk to pregnant mares or trigger neurological complications.

How Is Rhinopneumonitis Diagnosed?

Diagnosis is typically confirmed using a PCR test on a nasopharyngeal swab or blood sample. Since early clinical signs overlap with other respiratory infections, laboratory confirmation is essential for accurate diagnosis and appropriate isolation measures.

Equine Herpesvirus Myeloencephalopathy (EHM)

What is equine herpesvirus myeloencephalopathy (EHM)?
EHM is the neurological form of equid herpesvirus 1 (EHV-1) infection. It occurs when the virus invades the central nervous system, leading to inflammation of blood vessels in the brain and spinal cord. This condition is a serious complication of EHV-1 and can result in long-term disability or death.

While not all EHV-1 infections progress to EHM, outbreaks have become increasingly frequent and severe, particularly in competitive equine environments. Importantly, the neurological disease itself is not contagious, but the underlying EHV-1 infection is, which means outbreaks must be managed aggressively.

What Are the Symptoms of EHM in Horses?

What are the neurological symptoms of EHV-1?
Clinical signs of EHM can vary, but they usually appear suddenly and may progress quickly. Common symptoms include:

• Ataxia (uncoordinated movements, especially in the hind limbs)
• Weakness or paralysis of the hindquarters
• Urinary retention or incontinence
• Tail weakness or flaccidity
• Recumbency (inability to stand in severe cases)
• Occasionally: loss of skin sensation in the perineal area

These signs may follow a respiratory phase or emerge without any warning, making EHM particularly difficult to predict.

How Do You Treat Equine Herpesvirus Myeloencephalopathy?

Is there a vaccine for EHM in horses?
Currently, no vaccine offers full protection against EHM. While EHV-1 vaccines may reduce viral shedding and help prevent abortion, they do not prevent the development of neurological symptoms.

How do you treat EHM in horses?
Treatment is primarily supportive. Veterinarians may administer:

• Anti-inflammatory drugs (e.g., NSAIDs or corticosteroids)
• Antiviral medications, such as valacyclovir (oral) or acyclovir (IV), although their efficacy remains under study
• Fluids and nutritional support for horses that cannot stand or eat
• Bladder catheterization and turning schedules for recumbent horses

What Is the Prognosis for Equine Herpesvirus Myeloencephalopathy?

The prognosis for EHM depends heavily on the severity and duration of neurological symptoms. Horses that remain standing and maintain bladder control within the first 48–72 hours often recover with minimal deficits. However, recumbent horses or those with progressive paralysis have a poor to guarded prognosis, even with intensive care.

Prompt detection and intervention are critical. If EHM is suspected, the horse should be isolated immediately, and the facility should implement strict biosecurity to prevent a widespread horse herpes outbreak.

‍Reproductive Risks: Equine Herpesvirus Abortion

How does equine herpesvirus affect pregnancy?
One of the most serious reproductive risks posed by equid herpesvirus 1 (EHV-1) is equine herpesvirus abortion—a condition that can affect pregnant mares, typically in the final third of gestation. EHV-1 infection can lead to "abortion storms", where multiple mares in the same facility abort within a short period.

This form of EHV poses no lasting damage to the mare’s reproductive system, but it often results in the loss of a well-developed fetus. Transmission to other horses through placental fluids or fetal tissues is a major risk factor for barn-wide outbreaks.

When Does EHV-1 Abortion Occur?

What are the symptoms of EHV abortion?
Most abortions occur between months 7 and 11 of gestation and without prior warning signs. Infected mares typically do not show any clinical symptoms before aborting. This silent course of infection makes regular monitoring and vaccination programs essential.

Aborted fetuses are generally well-preserved and expelled suddenly. The placenta is often passed shortly after, and there is no evidence of uterine or reproductive damage in the mare.

Can Foals Survive EHV-1 Infection?

Occasionally, a mare may deliver a live foal after EHV-1 exposure late in pregnancy. Unfortunately, these foals often suffer from fulminant viral pneumonia, are severely immunocompromised, and typically die within hours or days, despite intensive care. These rare survivors require immediate isolation and veterinary support.

How Is EHV Abortion Diagnosed?

Definitive diagnosis is made via:

• PCR testing of fetal tissues (lung, liver, adrenal glands)
• Virus isolation
• Histological examination for intranuclear inclusion bodies

Serological testing of the mare post-abortion is generally unreliable due to widespread EHV-1 exposure in the general equine population.

Prevention and Risk Management

Preventing EHV abortion focuses on vaccination, isolation, and biosecurity:

• Pregnant mares should be vaccinated against EHV-1 at months 5, 7, and 9 of gestation using a high-antigen inactivated vaccine.
• Pregnant mares must be segregated from weanlings, performance horses, or horses returning from events.
• Strict hygiene and handling protocols should be in place during foaling season.

Prompt isolation of any mare that aborts—and careful disposal of fetal tissues and placental material—are crucial steps in preventing herd-wide transmission.

Transmission and Risk Factors

Understanding how equine herpesvirus spreads is essential for effective prevention and management. The virus is highly contagious and spreads easily through direct and indirect contact, making horse herpes outbreaks difficult to contain without strict biosecurity.

Modes of Transmission

EHV-1 and EHV-4 are primarily transmitted through:

• Nasal secretions from coughing, sneezing, or snorting
• Direct contact between horses (e.g., shared feed buckets, grooming tools)
• Aerosolized droplets in enclosed or poorly ventilated spaces
• Fomites, such as contaminated hands, clothing, or equipment
• Contact with aborted fetuses, placental tissues, or fluid in cases of equine herpesvirus abortion

The viruses replicate in the upper respiratory tract and may spread systemically through cell-associated viremia, reaching the uterus or central nervous system in susceptible horses.

Latency and Reactivation

A major challenge with equid herpesvirus 1 is its ability to establish latency. Infected horses may carry the virus lifelong in nerve tissue or lymphocytes without symptoms. Stressful events—such as transport, illness, or competition—can trigger viral reactivation and shedding, even in seemingly healthy horses.

As a result, outbreaks can emerge in herds without any recent outside exposure, making carrier horses an important but often overlooked source of infection.

Viral Shedding and Contagious Period

Infected horses may begin shedding EHV-1 or EHV-4 as early as 24 hours after exposure. Shedding typically lasts 7–14 days, but in some cases can continue up to 28 days. Importantly, asymptomatic horses can shed the virus and infect others, especially during early or latent reactivation phases.

Routine PCR testing may help identify silent shedders during outbreaks or before reintroducing horses to a herd.

Environmental Stability

How long can EHV-1 live on surfaces?
Under typical environmental conditions, EHV-1 can remain viable on surfaces for up to 48 hours, though survival time varies with material, humidity, and temperature. Metal and plastic tend to preserve viability longer than porous surfaces like wood or fabric.

Effective disinfectants include:

• 1:10 diluted bleach
• Accelerated hydrogen peroxide
• Quaternary ammonium compounds

Routine cleaning of stalls, trailers, grooming tools, and hands is essential—especially after contact with new, traveling, or symptomatic horses.

Additional Risk Factors

Certain groups are at higher risk of infection or severe disease:

• Young horses (due to limited immune experience)
• Pregnant mares (risk of abortion from EHV-1)
• Show and racehorses (frequent travel and mixing)
• Immunocompromised horses (due to illness, medication, or age)

Treatment and Management

▶️ Video Tip: How to Manage and Treat Equine Herpesvirus

‍Learn how to support horses with an EHV infection—from using anti-inflammatory medications and fluid therapy to caring for horses with neurological symptoms. This video also explains how proper quarantine and hygiene protocols can help contain the virus and protect your entire stable.

How do you treat equine herpesvirus?
Treatment for equine herpesvirus (EHV) is primarily supportive, as no cure currently exists. The goal is to manage clinical symptoms, reduce viral spread, and support recovery. Because EHV can present in multiple forms—respiratory, neurological (EHM), and reproductive—treatment protocols vary by presentation and severity.

Supportive Care for Respiratory Cases

In uncomplicated respiratory infections, horses often recover with basic supportive measures:

• Rest and isolation to prevent transmission
• Anti-inflammatory drugs (e.g., NSAIDs) to reduce fever and discomfort
• Fluids and electrolytes, especially in febrile or anorexic horses
• Monitoring for signs of secondary bacterial infections, which may require antibiotics

Horses with respiratory EHV are typically contagious for up to 14 days, so quarantine and hygiene protocols must be maintained throughout recovery.

How Do You Treat EHM in Horses?

What is the treatment for equine herpesvirus myeloencephalopathy (EHM)?
EHM is far more serious and may require intensive, around-the-clock veterinary care. Treatment strategies include:

• Corticosteroids or NSAIDs to reduce inflammation and spinal cord swelling
• Antiviral drugs like valacyclovir (oral) or acyclovir (IV), though their effectiveness is still under investigation
• Bladder catheterization, manual repositioning, and other nursing care for recumbent horses
• Nutritional support and prevention of pressure sores, pneumonia, or colic

Recovery can take weeks to months. Horses that remain ambulatory generally have a good prognosis, while those that become recumbent face a much more uncertain outcome.

Is There a Vaccine for EHM in Horses?

At present, no vaccine specifically prevents EHM. However, existing EHV-1 vaccines may reduce the risk of neurological complications by decreasing viral load and shedding. Their use is recommended in at-risk populations, even though protection is not absolute.

Treatment for Pregnant Mares and Foals

There is no treatment for EHV-induced abortion, as fetal loss occurs quickly and without warning. The focus is on:

• Vaccination protocols (months 5, 7, 9 of gestation)
• Herd management and separation of pregnant mares
• Immediate isolation and careful cleanup after an abortion event

Live foals born from EHV-exposed pregnancies often suffer from viral pneumonia and require intensive care, though survival rates remain low.

Biosecurity and Long-Term Management

Whether dealing with an individual case or a full outbreak, effective management includes:

• Immediate quarantine of affected horses
• Thorough disinfection of stalls, equipment, and shared areas
• Restricting movement of horses on and off the property
• Notifying relevant authorities if EHM is diagnosed—EHV-1 is a reportable disease in many jurisdictions

Ongoing risk reduction relies on routine vaccination, stress minimization, and strict isolation procedures for new or returning horses.

Prevention and Biosecurity

Preventing the spread of equine herpesvirus (EHV) requires a proactive combination of vaccination, hygiene, and herd management. Because the virus can spread silently—even from apparently healthy horses—biosecurity must be part of daily routines, not just crisis response.

Why Is Prevention So Critical?

EHV is highly contagious, and outbreaks can have devastating consequences: from respiratory disease and missed training to abortion storms or neurological complications. Infected horses may shed the virus before clinical signs appear, and latent carriers can reactivate the virus without warning. This makes routine prevention essential, especially in breeding farms, training centers, or competition stables.

Vaccination Strategy

Is there a vaccine for EHV?
Yes—but with important limitations. Vaccines are available for EHV-1 and EHV-4, and while they do not prevent infection entirely, they can:

• Reduce the severity and duration of symptoms
• Lower viral shedding, slowing transmission
• Provide some protection against abortion (with high-antigen EHV-1 vaccines)

Vaccination guidelines:

• Foals: Begin at 4–6 months, with boosters every 4–6 weeks
• Performance horses and horses in high-risk environments: Every 6 months
• Pregnant mares: At months 5, 7, and 9 of gestation with inactivated EHV-1 vaccine

Note: Current vaccines do not protect against EHM, although reduced shedding may help prevent outbreaks.

Quarantine and Isolation Practices

New arrivals, horses returning from shows, or those exposed to outside animals should be:

• Isolated for 21–28 days
• Monitored daily for fever, nasal discharge, and neurological signs
• Kept separate from pregnant mares and foals

If an outbreak occurs:

• Isolate infected horses immediately
• Suspend all movement of horses on and off the premises
• Notify your veterinarian and, where required, state animal health officials

Hygiene and Disinfection

How long can EHV-1 live on surfaces?
Up to 48 hours under normal conditions. To reduce indirect spread:

• Disinfect feed and water buckets, tack, and grooming tools regularly
• Use effective agents like diluted bleach, accelerated hydrogen peroxide, or quaternary ammonium disinfectants
• Wash hands and change clothes after handling sick or isolated horses

High-traffic areas such as trailers, wash racks, and cross ties should be cleaned routinely—even in the absence of known cases.

Stress Management and Herd Health

Because stress can trigger viral reactivation in latent carriers:

• Avoid unnecessary transportation, overtraining, or mixing of groups
• Maintain consistent routines and allow recovery time after travel
• Ensure horses have adequate nutrition, ventilation, and low-stress handling

A well-managed herd is more resilient against outbreaks and recovers more quickly when exposed.

Summary: What Horse Owners Should Know

Equine herpesvirus (EHV) is a complex and widespread threat in the equine world—but it is also preventable and manageable with the right knowledge and preparation. Whether causing mild respiratory infections or leading to abortion and severe neurological disease, EHV can affect horses of all ages and disciplines. Because the virus can spread silently—even from horses without visible symptoms—proactive management is essential.

The key to protecting your horses lies in vigilance, prevention, and early response. Don’t wait for signs to become obvious—by then, transmission may already have occurred.

Here’s what every horse owner, breeder, and stable manager should prioritize:

• Vaccinate regularly, especially vulnerable groups like foals, broodmares, and horses in frequent contact with others.
• Quarantine all new or returning horses for at least 21 days to avoid introducing the virus into your herd.
• Monitor daily for early warning signs, such as fever, nasal discharge, coughing, or subtle behavioral changes.
• Maintain strict hygiene and biosecurity protocols, including routine disinfection of tack, tools, and high-contact surfaces.
• Avoid stressors—such as overcrowding, frequent transport, or sudden environmental changes—that may trigger viral reactivation.
• Respond immediately if symptoms appear: isolate the horse, contact a veterinarian, and notify others who may be affected.

Outbreaks often start quietly, but the consequences can be far-reaching. With a structured approach, you can minimize risk, contain infections quickly, and keep your horses safe and thriving.

Horse Herpes Virus (EHV): Key Facts and Answers

What is equine herpesvirus?

Equine herpesvirus (EHV) is a group of contagious viruses that affect horses. The two most significant strains, EHV-1 and EHV-4, can cause respiratory illness, abortions in pregnant mares, and neurological disease.

What are the symptoms of EHV in horses?

Symptoms vary depending on the strain and the horse’s health status but commonly include:

• Fever
• Nasal discharge
• Coughing
• Loss of appetite
• Weakness or unsteady gait in neurological cases

What are the symptoms of EHM in horses?

Equine herpesvirus myeloencephalopathy (EHM) presents with:

• Hind limb incoordination (ataxia)
• Urinary incontinence
• Tail paralysis
• Difficulty standing or recumbency

How long can EHV-1 live on surfaces?

EHV-1 can survive on surfaces like metal, plastic, or grooming tools for up to 48 hours under typical environmental conditions, making disinfection essential.

How do you treat equine herpesvirus?

Treatment is supportive and may include:

• Anti-inflammatories (e.g., NSAIDs)
• Isolation and rest
• Fluids and nutritional support
• Antivirals in EHM cases, although their efficacy is still under study

Is there a vaccine for EHM in horses?

There is no specific vaccine for EHM, but EHV-1 vaccines can reduce the risk by lowering viral shedding and the likelihood of complications.

Can humans get equine herpesvirus?

No—equine herpesvirus is not transmissible to humans. However, people can carry the virus on their hands, clothes, or equipment and unknowingly spread it between horses.