EHV-1: What Are We Learning?

There’s a life-threatening disease horses can harbor in their bodies without showing any signs of illness. But under stress—even inapparent stress—the horse can disperse the virus with every cough or sneeze, exposing nearby equids to the pathogen. All of this can happen undetected until, perhaps, a horse in the same barn turns up with a fever or another begins showing neurologic signs.

This nightmarish scenario can mark the start of an equine herpesvirus-1 (EHV-1) outbreak, which most frequently occurs where horses congregate, such as at horse shows, trail rides, or barns with transient populations.

In the past 15 years veterinarians have seen the number of EHV-1 cases and outbreaks in the United States rise. These scenarios have caused widespread worry and confusion among horse owners and brought equine transport and events to a screeching halt. The end result, however, has been increased awareness—particularly of the potentially fatal neurologic form, equine herpesvirus myeloencephalopathy (EHM)—and research funding to discover ways to prevent and treat the disease.

In this article our sources will describe just how far we’ve come and how much we’ve learned.

What is EHV-1?

There are several equine herpesvirus types, some of which can cause more damage than others. The type that has dominated headlines recently, EHV-1, can cause respiratory illness, abortion in pregnant mares, congenital infection (foals are born with it, displaying weakness), and sometimes the more severe EHM. It’s also more common than many people might think; most horses are latent carriers (not showing signs of active disease) that were infected early in life.

“Equine herpesvirus-1 is specific to horses,” explains Gillian Perkins, DVM, Dipl. ACVIM, senior lecturer in Large Animal Internal Medicine at Cornell University. “It likes to live inside the horse in latent form, which is a good thing in terms of biosecurity issues (because the latent form does not persist in the environment and is not infective). It may recrudesce and become active during periods of stress or other times, but we don’t always know why it comes out of latency.”

Why then doesn’t the horse’s immune system just attack the virus? Because EHV hides in the animal’s white blood cells and lymph nodes, says Josie Traub-Dargatz, DVM, MS, Dipl. ACVIM, professor of equine medicine at Colorado State University and an equine commodity specialist for USDA-Animal and Plant Health Inspection Service Veterinary Services (APHIS-VS) Center for Epidemiology and Animal Health. The body doesn’t recognize the virus as foreign, and so the pathogen remains untouched.

Horses carrying the latent virus generally appear perfectly normal and healthy. When the virus becomes active, however, the horse can shed it readily through nasal secretions. Another route of virus spread is via fomites (objects such as clothing, tack, etc. contaminated by an infected horse) shared among horses.

“The virus-shedding horse may or may not develop clinical signs (e.g., fever, disinterest in food, hind-limb incoordination) of the disease,” Traub-Dargatz says. “It is very difficult to control this disease when we don’t have a vaccine or biosecurity methods that can prevent the initial infection in foals or to prevent reactivation and shedding of the virus.

“We need to be proactive in detecting sick horses, because every outbreak starts with a first case,” she continues. “If people are taking their horses’ temperatures before they go to an event, while they are at the event, and when they come home, they could detect a fever early on. If they then contact a veterinarian to examine the febrile horse and collect diagnostic samples, we’ll determine whether the cause of disease is EHV-1 or not.”

What Have we Learned From Outbreaks?

Horse owners are much more aware of EHV-1 than they used to be, thanks to new regulations and improved communication resulting from recent disease outbreaks. In 2007 USDA-APHIS labeled EHV-1 as an emerging disease and, in response, many states made it a reportable disease, Perkins says. This allows officials to track the disease and close or quarantine barns and facilities to halt its spread.

“Now if our diagnostic lab at Cornell University gets a sample that tests positive for equine herpesvirus, the lab informs the state,” she explains. “In addition, there are news items and e-mail alerts. Horse owners can immediately become aware of where and when the disease shows up.”

In the wake of the 2011 Ogden, Utah, National Cutting Horse Association Western National Championship, where a multistate outbreak is thought to have originated, “many horse show facilities developed more comprehensive biosecurity measures and plans to deal with this disease if something like this happens again,” Perkins says.

Lessons learned following the Ogden event and others, says Traub-Dargatz, include the importance of containing the disease quickly, establishing a plan for isolating and caring for sick horses, limiting horse and human traffic, and communicating infection-control strategies effectively.

“It comes down to basic biosecurity,” says Nicola Pusterla, DVM, PhD, Dipl. ACVIM, associate professor of equine internal medicine at the University of California, Davis, School of Veterinary Medicine.

Despite all we have learned about EHV-1 and smart biosecurity practices from a variety of outbreaks over the years, people still become complacent over time until the next disease situation arises.

Realistically, “Outbreaks like the one at Ogden, Utah, cannot be prevented,” Pusterla says. “When you bring more than 400 horses under the same roof from all over the country, and combine this with minimal biosecurity protocols, it is just a matter of when and where this disease will strike. The events are getting bigger, and more people are bringing horses from various regions.”

We have also learned that the available vaccines are not optimal. “We need to work at new technology and vaccines,” he says. “So far, there is no vaccine labeled for preventing neurological disease.”

Research Efforts

With increased awareness and concern about EHV-1 comes progression of research into the disease. The following are some current areas of study:

Antiviral medications Researchers at Oklahoma State University as well as at Ghent University, in Belgium, are studying these medications, which practitioners frequently administer to affected horses. These might help reduce disease severity and virus shedding, but they are not proven to be effective once a horse is neurologic.

“Primary treatment is still supportive,” Perkins says. “This involves rest, keeping the horse hydrated, and administering Banamine to reduce pain, fever, and inflammation. Research investigating various antiviral drugs and anti-thrombotic (blood clotting, as the neurologic disease is basically caused by a stroke-type lesion or blood clot) agents may help guide us in future treatment.”

Virus types Different strains of EHV-1 exist in nature. Researchers have shown in a number of studies that neurologic disease is more frequently associated with EHV-1 strains of a certain mutant genotype rather than the “wild” type (or “normal” strain, which is less likely to cause neurologic disease), but in one survey scientists determined that 14% of all neurologic cases were associated with the wild strains.

Hence, researchers are trying to learn more about the different genetic types. Udeni Balasuriya, BVSc, MS, PhD, professor of virology at the University of Kentucky’s Maxwell H. Gluck Equine Research Center, has received funding to investigate genetic diversity among EHV-1 isolates from neurologic disease outbreaks. With enough samples (preferably from three or four major outbreaks, he says), he hopes to be able to identify and compare viruses.

He aims to clarify the relationship between the genetic type of the virus and the frequency and severity of neurologic disease in horses and to identify additional mutations that could be involved. “This information is critical, to enable us to design improved diagnostic assays, vaccines, and medications (for treating horses with the neurologic form of the disease) in the future,” says Balasuriya.

Viral DNA Over the past few years scientists have studied the neurologic form of the virus closely. A diagnostic test can now differentiate between the neurologic and the non-neurologic form, based on the virus’ genetic makeup.

“It is important to take biosecurity precautions no matter which virus is discovered,” Perkins says. “Emphasis has been put on dangers of the neurologic strain, but the non-neurologic strain is also dangerous and can sometimes cause neurologic disease as well.”

Immunity Perkins, along with Bettina Wagner, DVM, DrVetMed, at Cornell University, is studying equine immunity to herpesviruses. “This may help us develop better vaccines,” she says. “We are looking at young horses because the foal’s immune system is different from that of an older horse. We’re trying to figure out if there’s a way we can accommodate the foal’s immunological strengths and not treat the foal the same way we would an adult horse, with the same vaccine.”

In other words, they aim to devise a vaccine that will be effective in foals so they don’t get infected when young and carry the virus latently into adulthood.

Vaccination Results from several studies have shown that vaccination can reduce horses’ viral shedding from the nose as well as non-neurologic clinical signs of disease, Perkins says. But vaccinated horses can still get sick, and she reminds us that the vaccine is not labeled for protection against EHM. “There is some thought, however, that the vaccine reduces the amount of viremia (virus circulating in the bloodstream) and that this also reduces the chance for neurologic disease,” she notes.

Vaccine development for EHV-1 is difficult in part because the neurologic form is so sporadic. In two experimental studies, for instance, Perkins and colleagues tried to induce neurologic signs in 30 horses of different ages and breeds, but they were only successful in 30% of the horses.

Outbreaks occurring in well-vaccinated horses further complicates the issue. “In the Ohio-Findlay (riding school and veterinary teaching hospital) outbreak in 2003 (, about 30% of those horses developed neurologic signs; we’d never seen that many get neurologic signs in an outbreak,” Perkins says.

Most of those horses had been vaccinated multiple times with a killed-virus vaccine. “This raised the question of whether this vaccine made horses more likely to get neurologic signs, and whether vaccination pushes the immune system in the wrong direction, but none of that has been proven,” she says. “Many questions remain, but vaccination is still important for the prevention of EHV-1.”

Take-Home Message

So what have we learned? Manage your horses conscientiously, vaccinate them, and practice smart biosecurity methods both at home and when traveling to shows and events that put your horses in potential contact with other horses. Don’t panic if a horse gets sick or if you hear about an outbreak; seek advice from your veterinarian about optimal ways to protect your horses, and ask him or her any questions you might have about the situation.