Everything You Need To Know About EHV-1

Learn about this highly contagious equine virus and how to curb its spread

By Christa Lesté-Lasserre, MA

What Is EHV-1?

Viruses have been infecting living cells since they first evolved hundreds of millions of years ago. Over time the microscopic agents diversified into categories depending, in part, on how they multiply, a process known as replication, using the host’s cellular machinery.

To date, scientists have discovered at least 115 species of herpesviruses affecting humans and animals. In humans, herpesviruses cause skin blisters, mononucleosis, shingles, Epstein-Barr disease, and certain kinds of lymphomas. The herpesviruses known to affect horses and donkeys primarily cause respiratory disease, says Dr. Lutz Goehring, Warren Wright Sr.-Lucille Wright Markey Endowed Chair in Equine Infectious Diseases at the University of Kentucky’s Gluck Equine Research Center.

EHV-1—and, more rarely, EHV-4—can also cause abortion or neurologic disorders in horses. EHV-3 causes genital blisters.

EHV-1 outbreaks can wreak havoc when they pop up at equestrian events where many horses commingle. | Flickr CC via trailers of the east coast

Arguably, the herpesvirus that causes the most concern in the equine community is EHV-1, Goehring says. Outbreaks—albeit rare—usually start when a horse returns from an event and spreads the virus within his home farm. Far less frequently the pathogen can pop up at equestrian events, causing disease within the competition barns. If not contained, the highly contagious virus infects horses, catching a ride home with them to various parts of the country or globe.

Since the virus was first discovered in 1966, scientists have found only three variants: D752, N752, and, most recently, in a 2018 study in Europe, H752. In 2021 a team led by Dr. Nicola Pusterla, professor of equine medicine at the University of California, Davis, School of Veterinary Medicine, discovered all three variants were circulating in North America as well.

While EHV-1 is technically the name of the virus itself, the term “EHV-1” is also used to describe the disease the virus causes.

Lutz Goehring, DVM, MS, PhD, Dipl. ACVIM

Goehring is the Warren Wright Sr.-Lucille Wright Markey Endowed Chair in Equine Infectious Diseases at the University of Kentucky’s Gluck Equine Research Center, in Lexington. His Lutz Goehring Research Group focuses on equine herpesvirus type 1 and its interaction with its exclusive host, the horse.

Nicola Pusterla, DVM, PhD, Dipl. ACVIM

Pusterla is professor in equine internal medicine and chief of large animal medicine services at the University of California, Davis, William R. Pritchard Veterinary Medical Teaching Hospital. He graduated from the School of Veterinary Medicine at the University of Zurich, Switzerland, in 1991 and also earned his PhD there with an emphasis on vector-borne diseases.

What Is EHM?

In a small percentage of EHV-1 cases, horses develop neurologic signs referred to as equine herpesvirus myeloencephalopathy (EHM). All three of the known EHV-1 variants can provoke EHM, although it’s less common with N752, Pusterla says.

Scientists are still trying to understand why some horses develop EHM, says Goehring. Tall breeds—such as Thoroughbreds and Warmbloods—appear to be more susceptible, and older horses and mares seem to develop more severe signs. Researchers suspect this might have to do with a weakening immune system or medications used to manipulate estrus in sport horse mares, respectively.

Researchers have found EHM is more likely to occur when the virus gets transported away from the respiratory tract and lymph nodes, through the bloodstream, and into the spinal cord, says Goehring.

Perhaps that’s why EHM generally appears later after infection than respiratory disease does. Gisela Hussey, DVM, MS, PhD, associate professor of pathobiology and diagnostic investigation at the Michigan State University College of Veterinary Medicine, in East Lansing, has been studying the disease and discovered horses with EHM developed fever later than those with the respiratory form of EHV-1. Horses with EHM also started showing signs of disease about nine to 13 days after infection, whereas horses with respiratory disease became sick after only four days, she says.

Plus, the D752 variant is the one that usually causes more cases of EHM, says Goehring. Its presence in the bloodstream—known as viremia—lasts longer, compared to the other variants, he says.

Therefore, EHM might arise from a different or delayed immune response than the respiratory form. Why horses would have that different response remains to be determined.

Horses with EHM started showing signs of disease about

9 to 13 days

after infection, whereas horses with respiratory disease became sick after only

4 days

What Are the Clinical Signs of EHV-1?

The Respiratory Form: EHV-1 can cause very subtle clinical signs that could easily go unnoticed, our sources say. Often, horses might have just a slight fever—and because they otherwise act normal and have no nasal drainage, nobody considers taking their temperature.

If they do develop obvious clinical signs, these might include a runny nose, a loss of appetite, or lethargy. Respiratory signs most commonly occur in young horses, usually younger than 4 years old.

Some horses—especially Warmbloods and draft breeds—might develop swelling in all four limbs, Goehring says. “Very often this type of vasculitis goes along with viremia (and fever),” he explains.

The Abortion Form: Pregnant mares might spontaneously abort within weeks to months of acquiring EHV-1, our sources say. Abortion generally occurs during the last three months of pregnancy. Sometimes, foals are born infected, and they succumb to severe respiratory disease.

The Neurologic Form: Signs of EHM can appear following a fever, usually on the last day of fever associated with viremia, explains Goehring. From there, it quickly evolves into an attack on the nervous system. Horses can become so weak or ataxic (incoordinated) that they drag their hind limbs or can’t coordinate their leg movements.

In addition, horses might retain feces, dribble urine, or both, says Pusterla. Many have reduced tail tone. In severe cases horses sit down like dogs or lie down and can’t get back up again—known as recumbency.

Diagnosing and Testing for EHV-1

The current gold standard for EHV-1 testing is for veterinarians to use a swab to take a sample from the nasal tract, says Pusterla. They then send the swab to a laboratory for quantitative polymerase chain reaction (qPCR) testing.

Suspected EHV-1 cases and exposed horses should be quarantined immediately. | Courtesy Dr. Katie Flynn

These tests detect the presence of even small amounts of viral DNA, Goehring says. And while they don’t distinguish between dead and live viruses, they give a very good indication of whether the horse is shedding and contaminating the environment. That makes the test fast, easy, and reliable, he says.

Even so, the reliability depends on what the test is looking for, Pusterla adds. Most laboratories are currently using tests that check for either the D752 variant, the N752 variant, or both. But they’re not checking for new variants—like H752—that might be popping up. For this reason, truly accurate testing requires using a test that targets a universal EHV-1 gene. False negative EHV-1 tests that missed the H752 variant might be responsible for undiagnosed EHV-1 outbreaks in the U.S. and abroad, he says.

Laboratories must also be careful about overclassifying the results as being either an “abortion strain” or a “neurologic strain,” because all three known variants can cause both forms of the disease, Goehring warns. “Many labs started reporting the N-variant as the nonneuropathogenic variant, and that has been a mistake,” he says.

Swabbing the horse’s muzzle picks up about

77%

of the EHV-1 cases deep nasal swabbing would have detected.

Researchers have recently developed home PCR tests for EHV-1 that work like home COVID-19 tests, says Goehring. Such rapid testing could help prevent and manage outbreaks efficiently, but the tests are currently undergoing the final stages of validation in laboratory settings.

Because repetitive deep-nose swabbing can be uncomfortable for horses, Pusterla’s team has investigated possibilities for less invasive testing. Results from their recent study revealed swabbing the horse’s muzzle effectively picks up about 77% of the EHV-1 cases that deep nasal swabbing would have detected. This offers a promising alternative, he says, even if suspicious negatives might need to be confirmed through conventional deep-nose swabbing.

How To Treat Horses With EHV-1

Equine herpesvirus type 1 has no cure. In fact, the virus can never be eliminated from a horse’s body entirely. It stays nestled in lymphocytes (white blood cells responsible for mounting immune responses) and/or the central nervous system for the horse’s life, usually in a dormant form, Goehring says.

However, during acute infection horses can benefit from supportive care and antiviral therapy, says Pusterla. Valacyclovir appears to have a strong effect against the circulation of the virus in the bloodstream, with viremia rates usually dropping to zero within a week, he says.

“It is important to control the inflammatory mediators that also cause a fever with (non-steroidal) anti-inflammatory drugs,” Goehring adds.

Flunixin meglumine (Banamine) can help control fever, and sodium heparin can treat thromboembolism from vasculitis, says Pusterla.

“However, it is important to start early, following the infection, with these drugs,” Goehring says.

Horses with EHM can also be supported mechanically with slings to keep them on their feet, they say.

New treatments for EHV-1-related abortion are on the horizon.

How Does EHV-1 Spread?

Most horses have acquired the EHV-1 virus by the time they reach adulthood, our sources say. And again, the virus can lie dormant in the lymphatic or neural tissues, remaining inactive for years. Outbreaks begin when a single horse—known as the index horse—suddenly experiences virus reactivation, and the animal starts shedding millions of virus particles through respiratory secretions.

This index horse often has no fever or other clinical signs when shedding begins, allowing the virus to spread rapidly to other horses before anyone becomes aware of it. Horses nearby inhale the particles, which also land on bedding, people, equipment, and walls, where they can survive as what’s known as fomites for several hours to a few days.

Some horses are more significant shedders than others, says Goehring. Recent outbreaks have shown the existence of what he calls “Typhoid Marys”—individuals that, for whatever reason, shed much more virus than others do.

This can mean rapid viral spread to large numbers of horses, especially at competition venues where hundreds of horses are stalled in close proximity, often in indoor barns with stagnant air, and multiple horses use the same stalls and aisles over the course of several weeks, he says.

Once a horse inhales the virus, it attaches to the respiratory tract lining, where it replicates in the epithelial cells, producing “huge amounts” of virus, says Goehring. The virus gets expelled by the millions through the nostrils and also destroys cells within the horse’s respiratory tract, causing swelling, discomfort, rare coughing, and possibly secondary bacterial infections.

Meanwhile, immune cells pick up the virus as they circulate routinely through the respiratory tract and bring it into the lymph nodes. There the virus starts replicating again, this time in the lymphoid tissue. At that point it might enter the bloodstream, triggering viremia.

The bloodstream filters out EHV-1 at one of two primary sites of very specific small blood vessels: in the endometrium of the pregnant uterus or in the spinal cord. At both sites the virus causes an inflammatory response within those small blood vessels, provoking tissue destruction and sometimes even bleeding. The result is either abortion or the classic neurologic signs of EHM.

Reporting Disease Outbreaks

Equine herpesvirus outbreaks are better controlled when information is shared about their spread, which is why many states and countries have declared EHV-1 a notifiable disease, says Goehring. However, some regions dictate reporting requirements based on whether the outbreak involved EHM cases. For example, a 2016 survey in the U.S. revealed EHM to be reportable in 49 states, whereas the respiratory form of EHV-1 was only reportable in 26 states.

An added benefit of obligatory reporting is the power given to government authorities to oversee management by, for example, locking down horse movement (transport), he adds.

When it’s not obligatory, owners and veterinarians can contribute to better outbreak management through voluntary reporting. In the U.S. veterinarians are encouraged to report to the Equine Disease Communication Center, Goehring says. In Europe and elsewhere, they can contact the International Collating Centre.

Pathogenesis of EHV-1 in the Horse

Scientists have made great progress in understanding the pathological pathways of EHV-1 and how it spreads among horses, Goehring says. But one major mystery remains: What makes the long-low-lying virus suddenly wake up and start replicating?

EHV-1 can linger in a state of “hibernation,” so to speak, in the lymph nodes or the trigeminal ganglion—the sensory part of facial nerves near the brain—leading to “a very quiet phase of viral presence,” Goehring says.

He likens this state to a veritable bomb waiting to explode. “You can import that time bomb onto your premises,” he says. “And that’s the very dangerous and tricky part, because we don’t clearly understand what triggers this reactivation of the virus.”

A leading theory is stress reactivates EHV-1, he says. In particular, it might be related to travel stress, as many outbreaks have started after horses either return or pass through a farm.

Even so, reactivation remains a very rare occurrence, Goehring says.

How To Prevent EHV-1

Arguably, the best way to prevent EHV-1 infection is through vaccination, which increases horses’ immunity and reduces viremia when administered at regular intervals, says Goehring.

But while vaccination helps horses fight the disease better and experience milder clinical signs, it won’t stop disease altogether, he adds. It also can’t prevent EHM.

This might be, in part, because of the vaccine’s age, as well as the “sneakiness of the virus,” he says. Veterinarians are still inoculating horses with the same EHV-1 vaccine that was developed 40 years ago.

Overhauling the EHV-1 vaccine, however, isn’t easy. The virus not only affects multiple body systems but also modulates the immune system itself, thereby evading immunity. Plus, EHM is relatively rare and, therefore, difficult to study. Most vaccine research is carried out in young horses that have never been exposed to the virus, which doesn’t reflect real-world scenarios.

Even so, several groups are working on new vaccine candidates, says Goehring. “There’s this energy to produce something more,” he says. In fact, efforts are currently underway to develop, test, and launch new RNA vaccines designed to protect horses from EHM.

Taking horses’ rectal temperatures once or twice daily can red-flag outbreaks early. | The Horse Staff

Meanwhile, owners should aim to “achieve the highest possible herd immunity,” Goehring says. “It’s always better if everyone’s vaccinated, compared to only a certain percentage.”

In addition to vaccination, owners can isolate incoming horses for at least 21 days and keep show horses away from other horses and their equipment/spaces during events, says Pusterla. That doesn’t mean just sheltering horses from animals that look sick, he adds, because many actively shedding horses appear perfectly normal.

Importantly, taking horses’ rectal temperatures once or twice a day can red-flag outbreaks early and allow for rapid veterinary intervention and good biosecurity, he explains. Embedding microchips containing thermometers that automatically alert owners about temperature spikes might be critical tools going forward, he adds.

“Everybody wants to look for the magic bullet—this or that test,” he says. “But taking rectal temperature is still a very powerful tool.”

Keeping Shows Safe

Equine herpesvirus outbreaks at competitive events have led to dozens of horse deaths and hundreds of additional animals exposed to the virus, many of which have fallen ill with disease. These statistics include horses that had never been to showgrounds, because the virus travels back to home farms with returning show horses. Such outbreaks have led to multiple event cancellations and stringent movement restrictions affecting the entire horse industry in affected regions.

Governing bodies that oversee competitions have responded by ramping up biosecurity and prevention requirements at shows, says Pusterla. “The industry is realizing that EHM outbreaks are very bad for business and horse well-being,” he says. “They’ve gone from doing nothing to various biosecurity protocols, intake (or admission) exams, daily monitoring, separating/isolating sick horses, point-of-care testing of sick horses at shows, and more.”

The United States Equestrian Federation (USEF), for example, now requires competition organizers to plan for quarantine stabling, onsite treatment and supplies, and biosecurity protocols. Horses entering USEF venues must have proof of EHV-1 vaccination within the previous six months.

In 2022 the Fédération Equestre Internationale (FEI) moved to implement stringent measures to prevent sick horses from entering FEI venues, detect and isolate suspected cases rapidly, prevent viral spread, and ensure veterinary care of sick horses. Such measures include regular temperature checks—especially before entering the venue—improved stable designs for better social distancing, and rapid PCR tests (once they’ve been scientifically validated). Starting in 2025, vaccination might become mandatory in horses from the 45 member nations where a licensed vaccine is readily available.

Both federations require negative PCR tests within specific windows of time to enter a competition following any EHV-1 outbreak in the region.

“It’s the combined elements of strong preventive biosecurity measures and the early detection and isolation of horses that are at risk of shedding (that matter in risk reduction), and vaccination may add limited protection by reducing viral shedding,” says Goran Akerstrom, DVM, FEI veterinary director .

Isolate incoming horses for at least

21 days

Biosecurity Recommendations

Because EHV-1 is such a “tough and silent disease at the beginning,” biosecurity is critical, says Goehring. During an active outbreak, property owners should establish a rapid, rigid quarantine of the premises and isolate all shedding and/or febrile horses from other horses.

EHV-1’s main route of transmission is horse to horse—and especially nose to nose, Goehring says. So owners should keep horses from all physical contact with each other during an outbreak, including the ones that had negative nasal swabs the day before, as they might be in early phases of infection. “Shedding horses should be in a separate building from the—hopefully—uninfected population,” he says.

Physical distance between horses is important, mainly because of the EHV-1’s physical characteristics, he adds. “This is not an influenza virus,” he says. “It’s not one that carries from one corner of the barn to the other within seconds or minutes. EHV-1 is a slower, heavier virus, and it travels in droplets. So, distance definitely is our friend.”

Smart Biosecurity Steps

Don’t share water sources at events or during outbreaks.

Avoid nose-to-nose contact.

Isolate incoming and sick horses.

Wear protective gear, disinfect boots, and wash hands after handling sick horses.

Designate equipment for use on sick horses only.

Post-outbreak, disinfect surfaces and equipment.

EHV-1 is less likely to be transmitted via people and their clothes than it is horse to horse, Goehring says. But it certainly can travel on humans and items, so using separate equipment for sick vs. healthy horses is important, as is changing clothes and washing hands after handling each group.

“For those who like to comply with biosecurity, that involves the phone that you pick up; that involves a stethoscope, your ultrasound machine, grooming equipment,” Pusterla explains. “And it’s on the person that’s in that room. It’s on the gloves; it’s on the lab coat; it’s on the boots. It even goes as far as to the (humans’) nostrils.”

EHV-1 can remain infectious in water, our sources say, so horses shouldn’t share water sources or buckets at events or during an outbreak.

Critically, owners should never panic and try to get their horses out of quarantined premises, Goehring says. This is likely to let the virus get out along with the horse, spreading EHV-1 wherever that horse goes, with potentially disastrous consequences.

Take-Home Message

Unchecked, EHV-1 can swoop through equestrian communities, causing financial and emotional losses and compromising equine welfare. But by staying up to date on scientific knowledge about the virus and the diseases it causes, and following recommended biosecurity protocols for both preventing EHV-1 and stopping its spread, owners and veterinarians can collaborate to keep transmission—and its consequences—to a minimum.

Sponsor’s Message

An important part of an overall equine health care plan is a vaccination protocol. While a “standard” plan doesn’t exist for all horses, the American Association of Equine Practitioners (AAEP) recommends horses be vaccinated for the following diseases: Eastern and Western equine encephalomyelitis, rabies, tetanus, and West Nile virus.

Horses should also be evaluated to determine whether or not they should be vaccinated for other diseases, such as equine influenza, equine herpes, strangles, and Potomac horse fever, depending on their potential exposure to the disease, age, breed, use, geography, travel, and consequences of the disease, also known as a risk-based analysis.

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Ask your veterinarian about a vaccination protocol that’s designed specifically for your horse’s needs.

Learn more about Boehringer Ingelheim vaccines at: https://bi-animalhealth.com/equine/vaccines.

¹Data on file at Boehringer Ingelheim

Credits:

Christa Lesté-Lasserre, MA

Passionate about horses and science from the time she was riding her first Shetland Pony in Texas, Christa Lesté-Lasserre writes about scientific research that contributes to a better understanding of all equids. After undergrad studies in science, journalism, and literature, she received a master’s degree in creative writing. Now based in France, she aims to present the most fascinating aspect of equine science: the story it creates. Follow Lesté-Lasserre on Twitter @christalestelas.

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