EHV-1: Then and Now

Over the past few decades veterinarians and researchers have investigated major EHV-1 outbreaks in the U.S. to learn more about how to contain and prevent future occurrences.

EHV-1: Then and Now
Veterinarians now know that shared equipment and nose-to-nose contact between horses at events can spread the virus. | Photo: iStock

Research developments, biosecurity practices, and vaccination recommendations that have come out of recent EHV outbreaks

Equine herpesvirus outbreaks are nothing new, but their effects can still be dramatic—shutting down equine events, obstructing horse transport across state lines, and causing panic in corners of the industry.

Several types of equine herpesvirus infect horses, but the one that makes the news is equine herpesvirus-1 (EHV-1). This pathogen can cause respiratory disease and abortion in its milder forms, but it can also cause neurologic disease that might be fatal. Horses can become infected by inhaling airborne viral particles or by coming into contact with contaminated surfaces and equipment; human hands, clothing, and footwear; and water and feed sources.

“EHV-1 exposure happens to most horses early in life; we don’t currently have any means to avoid that early exposure and infection,” says Josie Traub-Dargatz, DVM, MS, Dipl. ACVIM, professor emeritus at Colorado State University’s (CSU) College of Veterinary Medicine and Biomedical Sciences, in Fort Collins. Yes, a vaccine exists, and veterinarians recommend it, but it’s complicated; the vaccine isn’t labeled to protect against the neurologic form of the disease.

Over the past few decades veterinarians and researchers have investigated major EHV-1 outbreaks in the U.S. to learn more about how to contain and prevent future occurrences. Here we’ll recap those notable disease events, their import, and what we’ve learned.

Learning From the Past

Our sources believe the following to be some of the most memorable and significant outbreaks:

Colorado, 1980s

Traub-Dargatz recalls CSU having an influx of EHV cases from an outbreak associated with horses from different dude ranches overwintering together. Caretakers noticed the first case in this outbreak when they found an older horse down in the pasture. They subsequently noticed multiple horses with abnormal gaits, three of which were sent to CSU and diagnosed with the neurologic form of EHV: equine herpesvirus myeloencephalopathy (EHM).

“A large number of horses were affected in this outbreak,” she says. “For me, this is memorable in that EHM is not a new disease; we’ve seen it for a long time—before some of the higher-profile outbreaks occurred. The social stress of bringing horses together, along with the potential nutritional challenges of feeding a large number of horses that were newly commingled in a group, as well as the fact that many of these horses were older animals may have prompted that outbreak. We learned that host factors beyond the virus lead to some animals developing the severe disease and some not.”

University of Findlay, 2003

The EHV-1 outbreak within this Ohio university’s equine program resulted in 12 equine deaths. Of the 135 horses at the facility, 117 showed clinical signs of EHV-1 infection, 46 of which displayed neurologic signs. Horses in the program had been vaccinated against EHV every couple of months, and new arrivals were vaccinated within 10 to 45 days of arrival. Data analysis after the outbreak (Henninger et al. 2007) revealed risk factors for developing EHM included:

  • Being older than 5;
  • Having a temperature higher than 103.5 degrees F; and
  • Having the highest rectal temperature on or after the third day of fever onset.

Ogden, 2011

This pivotal outbreak following a large cutting horse competition in Ogden, Utah, was in many ways a game changer in recognizing the importance of biosecurity and communication within the equine industry. In this case horses attending the show were exposed to EHV-1 without anyone realizing it, and veterinarians confirmed the first case of EHM after all horses had departed from the event.

The Colorado veterinarian examining the first EHM cases immediately notified state veterinary officials, who then contacted the Utah state veterinarian, says Traub-Dargatz. Together the State Animal Health Officials (SAHO), the American Association of Equine Practitioners (AAEP), and the American Horse Council recognized the potential scope of the outbreak and asked the USDA to assist.

“State animal health officials and the equine industry came together,” she says. “The USDA had not played a role in EHV outbreak mitigation prior to this request and subsequently has developed recommendations. The industry worked with state health officials in a cohesive effort to contact owners of potentially exposed horses. This outbreak was remarkable not only in scale, but in the coming together of key groups that played a role in mitigation of the outbreak, as well.”

Traub-Dargatz says an outbreak of this scale has an economic spinoff effect, causing many equine events to be canceled and equestrian professionals, event services, and their supporting communities to incur financial losses.

By the time the USDA deemed the outbreak contained a month later, more than 2,000 horses had been exposed, and 13 had died. Many sick horses had not attended the event but were infected by exposed horses returning home.

In 2013 Traub-Dargatz led a case control study of data from the horses attending the Ogden event.

“Statistically, we looked at risk factors for developing the neurologic form of the disease among horses that attended the cutting event,” she says. In this and other outbreaks, “it appears that mares are more likely to be in the neurologic cases. Mares were overrepresented, even when other factors were taken into consideration. … There have been other outbreak investigations comparing cases to controls that found that mares were overrepresented. Again, we don’t know necessarily why that would be … but it’s an interesting observation.”

Traub-Dargatz says she and her colleagues determined that horses exposed to two or more biosecurity risks (e.g., being tied outside the stall or being allowed access to a shared water source) while at the Ogden event were more likely to be among the neurologic cases. “I think that’s credence for taking precautions with your horses when you bring them into a situation in which they are commingled,” she says.

The study also showed that zinc might play an antiherpetic role. “When we compared neurologic cases to the controls (horses that attended the Ogden event and did not become sick), another factor was that horses that were on a dietary supplement containing zinc were at a reduced risk for being in the neurologic cases,” Traub-Dargatz says. “There is some evidence in the literature that zinc is a critical element in the immune response. Unfortunately, we didn’t have enough observations to say a certain level of zinc was associated with a protective effect.”

EHV-1 in Other Equids

Another outbreak Traub-Dargatz describes took place in Wyoming many years ago, with multiple horses infected with the neurologic form of EHV-1. That group, however, included mules, none of which displayed clinical signs. Since then, researchers found that mules housed together with horses in a mountain packing station in California during an EHV-1 outbreak in 2011 showed viral loads in blood samples and nasal secretions but developed no clinical signs. Only horses showed neurologic signs (

In a 2018 study European ­researchers (PA Seeber, et al.) evaluated whether travel and new living arrangements could cause dormant EHV-1 virus to reactivate. They examined stressors when transporting a captive zebra with unknown infection status to a new zoo and introducing her to the established herd there, which were known to be latently (without showing signs) infected with EHV. Researchers sampled feed tubs after each meal to monitor for viral shedding and measured fecal glucocorticoid metabolites (which indicate increased stress levels). Their results showed that the stress associated with long-distance transport and restructuring of social groups likely caused increased viral shedding in all animals.

The Silver Lining: Improved Communication

A reportable disease is one practitioners must report to federal, state, or local health officials upon diagnosis.

“Regulatory response to a contagious disease begins with the veterinary practitioner’s recognition of compatible clinical signs and laboratory identification of the disease agent,” says Katie Flynn, BVMS, MRCVS, equine veterinarian for the California Department of Food and Agriculture. “Thus, a regulatory response on the part of the SAHOs requires the disease be a reportable disease in a state. During the 2011 multistate EHV-1 outbreak, it was noted that EHV-1 was reportable in only 36 states. According to a 2016 survey of 49 state veterinarians in the United States, neurologic cases of EHV-1 were reportable in all but one state (showing regulatory updates), whereas cases of EHV-1 respiratory disease were reportable in 26 states.”

EHV-1 outbreak recognition is still evolving, says Traub-Dargatz, and might provide an incomplete picture: “I think we can have an impression that there are now more outbreaks, but we need to be cautious of that,” she says. “We need to ask ourselves, ‘Are there really more ­outbreaks, or are they more visible because there is more consistent r­eporting?’ ”

“The regulatory response to a reportable disease varies from no regulatory action (strictly monitoring) to establishing an official quarantine of infected and exposed animals,” says Flynn. “To address the identified need for consensus, the United States Animal Health Association’s Infectious Diseases of Horses Committee (now called Committee on Equine) established an EHV-1 subcommittee to develop a consensus document related to the EHV-1 regulatory mitigation. The consensus guidance document provides SAHOs with science- and field-­experience-based control guidance for an EHM incident. The guidance document, initially published in 2015, was revised in 2018 (find it at­horses) based on new findings of regulatory officials during recent EHV-1 responses.”

In 2016 the AAEP and American Horse Council launched the Equine Disease Communication Center (EDCC), a resource that assists the horse industry in staying abreast of reliable information. “There was a definite need for a national, reliable, credible source of information, so the industry came together to develop the EDCC,” says Traub-Dargatz. “People can sign up to get an email notification when there are outbreaks and check travel requirements and quarantines. The majority of SAHOs now submit their press releases on selected diseases to the EDCC.”

(You can also read about EDCC alerts, along with additional vetted equine disease information, on

Vaccination Conundrums Remain

“What we would ideally have is a vaccine that would be licensed to prevent the most severe form of the disease, the neurologic form,” Traub-Dargatz says. “None of the current vaccines carry that on their licensing documentation. On a population basis, if we use a vaccine that has been licensed and certified to reduce viral shedding, maybe we can reduce risk of exposure to a certain extent.”

She says existing EHV-1 vaccines are either licensed to:

  • Reduce risk of respiratory disease only;
  • Reduce risk of respiratory disease and abortion; or
  • Reduce risk of viral shedding.

“Each is a somewhat higher bar,” she says.

Veterinarians also face unanswered questions. “Data show horses that received an EHV vaccine in the five weeks prior to the Ogden event were actually at an increased risk of EHM,” says Traub-Dargatz. “We can say that’s an association, we can’t say it’s causal, but it does make you wonder if in certain horses recent vaccination somehow altered their response when they were exposed to the virus.

“In the Findlay outbreak there was an association with frequent EHV vaccination and EHM cases, but this was in older horses, so the vaccination history was confounded by the horse’s age,” she adds.

On the whole, she recommends working with your veterinarian to tailor a vaccine plan for your horses, as well as referring to the AAEP’s vaccine recommendations (

But it’s crucial to think about ­management beyond vaccination: “Vaccination in general is critical to the control of infectious disease, but I think we also need to be good stewards in other areas, including less stressful transportation, meeting the micronutrient needs of our horses, and keeping horses as comfortable and happy as possible while at events.”

EHV-1: Then and Now
Regulatory bodies such as the USEF now enforce quarantine protocols for affected horses. | Photo: Courtesy Dr. Katie Flynn

Enhanced Biosecurity

Keeping horses healthy on the road and at home requires implementing biosecurity measures. Organizers of many equestrian events have enacted rigorous protocols in light of recent outbreaks.

“There have been a lot of changes over the last few years,” says Heather Petersen, CEO of 2 White Feet Inc., an equine event management business that organizes various Fédération Equestre Internationale (FEI) and United States Equestrian Federation (USEF) dressage competitions. She says recent biosecurity developments include:

  • Stricter vaccination requirements. For example, horses attending USEF competitions must have proof of equine influenza and EHV vaccination within the prior six months before entering the grounds. Horses that cannot tolerate the vaccine must have their temperature recorded twice daily for seven days prior to entering the grounds to ensure they’re healthy. FEI requirements go a step further, with vaccination no more than seven days prior.
  • In addition to vaccination logs and a current negative Coggins (equine infectious anemia test), many facilities require owners to show a veterinarian-issued health certificate dated seven to 30 days prior, depending on the event. However, some facilities still have no health certificate requirements.
  • FEI competitions include microchipping and horse passport requirements; separate and secure stabling, grazing, and schooling areas; and on-site veterinary inspections.
  • The USEF now requires competition management to file an isolation protocol. This includes a plan for quarantine stabling, obtaining supplies, and treatment and handling protocols, as well as a plan to be used for horses that might have been in contact with an ill horse.

“The USEF’s isolation plan requirement for sanctioned events is a first step for a more comprehensive biosecurity plan involving entry requirements, early detection of disease, and early action plan for response if disease occurs,” says Traub-Dargatz.

However, Flynn notes, many facilities cannot fulfill isolation protocols. “Unfortunately, most equine facilities have limited isolation stabling available,” she says. “On-site and off-site stabling availability is a challenge during the busy show or fair season when temporary stabling is not available. Lack of isolation or inappropriate isolation stabling for test-positive equids poses a real challenge to regulatory officials, as an inability to isolate the index case(s) often leads to prolonged quarantines due to continued disease spread.”

Outbreak data and more advanced and rapidly available diagnostic test capabilities indicate horses might be contagious longer than previously thought.

“Historically, research has indicated that infected horses shed virus at onset of fever,” says Flynn. “However, during our outbreaks (here in California), several febrile horses tested negative on initial nasal swab samples and positive on samples submitted 72 hours later. Recommended regulatory response protocols have been revised and recommend resampling suspect exposed horses 48-72 hours later to determine EHV-1 status.”

Current diagnostic testing technology has also detected virus in nasal swabs from exposed horses beyond the previously proposed two-to-10-day shedding period. During recent incidents, says Flynn, the laboratory detected EHV-1 in horses 21-28 days post-initial infection.

“Additionally, detections in several horses were at a very low level,” she says, “which poses the question, is the polymerase chain reaction (PCR) test detecting traces of residual nucleic acid or inactive virus (basically, viral DNA)? Are these horses with a very low level of virus capable of infecting another horse?”

Special Feature: Practical Biosecurity Tips to Protect Your Horses
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​Until researchers can determine transmission risk for low-shedding horses, regulatory officials will continue to require two negative tests for quarantine release and recommend handling these horses as potentially infectious, says Flynn.

Creating a safe distance between horses needs to happen both at events and at home when horses are coming and going. “Avoid direct nose-to-nose contact with horses that aren’t part of your herd, and keep new introductions separate for two to three weeks while monitoring for signs of disease,” says Traub-Dargatz. “By keeping isolated horses in a totally separate area, you not only avoid sharing air space but also create a functional barrier, so it’s easier for people to comply with separation protocol, with separate supplies, clothing, and footwear.”

Color-coding supplies and clothing might also help with compliance.

In 2017 German researchers found that the EHV-1 virus could persist for up to three weeks in water under optimal pH and temperature conditions.

“While this is not an outbreak finding, it would potentially tell us that we should be concerned about the use of shared water sources,” says Traub-­Dargatz. This includes buckets, troughs, and hoses transferring from bucket to bucket.

Take-Home Message

While EHV-1 has been around for years, it hasn’t always been considered a disease of regulatory importance. “With the increasing number of incidents, the equine industry and state regulatory officials became aware of the impact of such a disease,” says Flynn. “The economic impact of an outbreak can be substantial. Aside from the direct cost due to horse fatalities, there are many other costs associated with treatment, quarantine, canceled events, and the inability for horses to perform and compete in equestrian events.”

With improved communication between agencies, incident documentation, and access to outbreak data, the veterinary community continues to learn more about the disease.

“This field experience and knowledge is essential for advancing equine regulatory EHV-1 responses,” says Flynn. “Equine herpesvirus myeloencephalopathy was added to California’s reportable disease list in 2011. Since that time we have investigated over 25 incidents of EHM (in the state). These incidents have provided new insights into a virus that has been around for many years.”

“We have to accept we don’t currently have the means or method to avoid the index EHM case in all situations, so control really revolves around our detection and response to that first case,” says Traub-Dargatz. “We have come a long way with our communication, information resources, and biosecurity. It’s not easy, but more and more owners, veterinary practitioners, and the industry in general are embracing those concepts. We have things to be optimistic about.


Written by:

Freelance journalist Natalie DeFee Mendik is a multiple American Horse Publications editorial and graphics awards winner specializing in equestrian media. She holds an MA in English from Colorado State University and an International Federation of Journalists’ International press card, and is a member of the International Alliance of Equestrian Journalists. With over three decades of horse experience, Natalie’s main equine interests are dressage and vaulting. Having lived and ridden in England, Switzerland, and various parts of the United States, Natalie currently resides in Colorado with her husband and two girls.

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