Cover Story | Infectious Diseases

Anatomy of an EHV-1 Outbreak: Not Their First Rodeo

By Marie Rosenthal MS

Many states west of the Mississippi River suffered an outbreak of equine herpesvirus-1 (EHV-1) in the final days of 2025. For Ben Buchanan, DVM, the outbreak began as most emergencies seem to—just as everyone was ready to call it quits for the day.

At 5:30 pm on Nov. 17, the Brazos Valley Equine Hospital (BVEH) staff in Navasota, Texas, took a call for a “colic.” The trainer noticed the horse was off, gave her a dose of an anti-inflammatory and loaded her onto the trailer, where she took a nosedive and wedged herself into the trailer.

The mare, a pro barrel racer, was subdued, in pain and did not have good spinal reflexes. And when Dr. Buchanan went to auscultate the GI tract, the mare tried to bite him, he explained at the 71st AAEP Annual Convention, in Denver, Colo.

He quickly recognized this was not a run-of-the-mill colic and included neurologic conditions in the differential. If the horse was infectious, he could not take it to BVEH. The practice’s sling stalls are in the middle of the barn, which would increase the risk for transmission to other patients. He contacted other nearby facilities to see if they could take the horse; 2 already had neurologic cases in the sling stalls, and 1 of them had been diagnosed with EHV.

Then Dr. Buchanan received a follow-up call from 1 of the facilities that changed everything: “By any chance, was your horse in Waco, Texas?”

From Nov. 5 to 9, the Women’s Professional Rodeo Association (WPRA) World Finals and Elite Barrel Race were held in Waco. Before long, “we realized that all of us had neurologic horses that had been at the WPRA,” he said.

After consultation, the owner elected to euthanize the mare. “We put the horse to sleep, sent the horse to the Texas Veterinary Medical Diagnostic Lab, and they confirmed that it was positive for equine herpesvirus,” Dr. Buchanan said.

By Dec. 11, 2025, 60 confirmed positive cases had been reported in 8 states, all of which could be traced back to the Waco event, according to Katie Flynn, BVMS, a senior veterinarian in equine health and biosecurity for the United States Equestrian Federation (USEF), who also discussed the outbreak at the AAEP convention.

Only the beginning
As the saying goes, this was not Dr. Buchanan’s first rodeo, and he knew the euthanasia of the mare was just the beginning, not the end of this case management. And as if to confirm his suspicions, he quickly placed an order for valacyclovir to help with incoming cases, but it was already on backorder. Valacyclovir can reduce viral replication of EHV-1 and its clinical signs, especially when given early in treatment.

He and his colleagues started alerting people—other veterinarians, state officials and various event facilities that recently held or planned to hold events. Many of the horses that had been in Waco had already been to or were traveling to a show in Oklahoma. They also let their clients and others in the rodeo world know about the potential exposure, since more than 700 horses had attended the Waco event.

They wanted to “give people a heads up that there are rodeo horses that are potentially infected, so expect patients and calls from owners about what to do with their horses,” Dr. Buchanan said.

One way to disseminate information was through social media, he said. “We pushed out a Facebook post Tuesday morning saying, ‘There’s a horse that tested positive for herpes. There’s potentially an outbreak developing, and here are the things to know,” he said, adding that the post also had a link to a fact sheet on EHV-1.

Although social media is a good method for quickly presenting information, it can also disseminate misinformation. Dr. Buchanan suggested that veterinarians ensure there are links where people can find high-quality, accurate information when posting on social media.

Several of the horses that were in Waco were attending or planning to attend an event in Oklahoma, so all the folks involved in that event and other events had to be notified, too. The organizers of the other events had to test and possibly isolate horses and decide whether to hold their events. Ultimately, there were more than 15 events identified by the Texas Animal Health Commission where horses tested positive for herpes related to the outbreak in Waco.

Reporting
Not every state requires reporting of EHV-1, Dr. Flynn said, and reporting requirements vary by state even among those that do require it. For some, only the most serious cases—those with neurologic disease—are reportable. Others, like Texas, for example, require reporting with laboratory confirmation unless there is a suspicion of equine herpesvirus myeloencephalopathy (EHM). EHM cases should be reported before they are confirmed.

Veterinarians must know their state’s requirements, and, regardless of the law, it is a good idea to give their state veterinarian a heads-up about potential cases in their state, Dr. Flynn suggested.

Stall-side tests frequently are used to give veterinarians an immediate idea of what is going on with the horse, but serious diseases such as EHV and all reportable diseases should be confirmed by a laboratory.

According to Dr. Flynn, the Fluxergy test is not an official test that would trigger state action because it has not yet been cleared by the Department of Agriculture. “The state animal health professionals are in agreement that it is under review by the USDA for licensure,” she explained. “Thus, they do require these tests be confirmed by a regulatory laboratory. That is a caveat that we want to make sure everyone understands.”

Quarantines, holds and other measures to contain the spread were implemented throughout the affected states, they said. However, it is important to remember which horses were affected. Because the outbreak was among rodeo and barrel racers, venues that offered Thoroughbred and Standardbred events would not have to close, reminded Nathaniel A. White II, DVM, MS, DACVS, a professor emeritus of equine surgery at the Virginia-Maryland College of Veterinary Medicine, and the director of the Equine Disease Communication Center (EDCC). The EDCC kept track of the outbreak and provided information to state and local veterinarians, owners, trainers and others in the equine industry.

As this outbreak was unfolding, reports of 2 EHM cases east of the Mississippi River caused some consternation until they could verify that the cases in Pennsylvania and Ohio were not related to the Waco outbreak.

“It was critical that the people who evaluated those horses were able to say the horse was not off the property, and there was no possible contact with barrel racing or rodeo,” Dr. White said, adding that this can be a relief to people. “The goal is to assess the risk to some level so that the horse industry can continue its activity.”

“We get individual cases,” he explained.

Vaccination was an area of controversy in which some veterinarians recommended vaccination during the outbreak, while others did not. One can see the reason for the ambiguity, Dr. Buchanan said, because some of the horses affected had been vaccinated. Still, Dr. Buchanan recommended vaccination to drive a cell-mediated response to curtail and limit the outbreak. “Vaccines will reduce viremia and reduce nasal shedding,” he said.

Because immunity following vaccination is short-lived in young animals, the AAEP recommends horses younger than 5 years be vaccinated every 6 months. However, the recommendations admit that “despite frequent vaccination, infection and clinical disease continue to occur.”

Vaccination of older horses depends on the situation and their use. (For more, see the AAEP recommendations at https://aaep.org/resource/equine-herpesvirus-rhinopneumonitis-vaccination-guidelines/)

The AAEP said there are no data on the effectiveness of using vaccination to control EHV outbreaks. Therefore, the AAEP recommends strict biosecurity that includes “isolation of infected horses, quarantine of affected premises, and monitoring of at-risk populations.”

Dr. Flynn said there are lessons to learn from this and other recent outbreaks to contain and respond to future outbreaks.

“EHV is out there every day, and we shouldn’t be freaking out every time we hear about EHV,” she said. Instead, “make sure we understand how to manage EHV because we have to live with it.”

Biocontrol
Prevention is always better than cure, so practicing biological and environmental controls is important. One of the current practices that seems to invite outbreaks is tying up a group of horses to the corral in the arena as they wait to compete. The horses are only a few feet apart for extended periods.

Another pet peeve of Dr. Flynn’s is using the same nose rag on multiple horses. “I’ve got a tissue that I’ve only used once. Do you want it?” She joked.

Don’t use water troughs and communal water sources—the virus can survive there for 21 days, making them ideal environments for spreading disease. Don’t share water buckets without washing and disinfecting them between horses.

Keep equipment clean. UV light kills EHV, so you can “throw the tack or equipment in the sunlight,” she said, but remember to wash it first. You can’t disinfect anything if you don’t wash it first.

“[Sunlight] is one of the ways we can disinfect it, but make them clean it first so that all that grime where the virus can be hiding can be eliminated, and we can penetrate it better,” she said.

The AAEP also provides advice on disinfecting equipment, such as nasogastric tubes.

Isolation
Many facilities asked about how to isolate horses. “That is a concern when we think about these sick horses,” Dr. Flynn said. “How do we isolate them and get them to an area where they are not exposing other horses?”

Sometimes people need to think outside the box. Think about where portable stalls or pipe corrals can be put up—even if it’s the parking lot or an unused arena.

And this is something the facility should consider long before any horse arrives. Address the “what-ifs” in a protocol that can be implemented immediately. “People need to understand that we have to try to prevent the perfect storm,” she said.

During the outbreak, many Interstate Certificates of Veterinary Inspection (CVIs) were required to attest to whether the horse had been exposed to EHM. The expiration dates for CVIs for transporting horses across state lines should be shorter—6 months or more is too long for a horse that is traveling to several venues, Dr. Flynn said.

Temperature checks are also important. “A horse with a temperature should not be competing,” she asserted.
Both Drs. Buchanan and Flynn discussed one of the biggest challenges—the trace-back investigation among horses that attend multiple events. Unless they are entered, they aren’t counted, but people bring noncompetitive horses for several reasons, including introducing young horses to the competition environment.

Dr. Buchanan said his index case was a perfect example. The horse went to Waco but did not compete. “She went to the event, she got infected, she developed EHM and she died. And when I was reporting that, there was no record of that horse being in Waco.”

He added, “And that is the biggest challenge of trying to track this and do the epidemiologic studies—there are going to be horses at a lot of places, and nobody knows that they were there.”

Dr. Flynn said: “Tracing these horses that went from one event to another event to another event … The biggest challenge is knowing where they have gone and who is going where.” It takes time to track these horses down.

“We don’t know everything that is on some of these showgrounds unless we require them to check in and be registered,” Dr. Flynn suggested. Implementing a method for counting all horses, not just those who compete, could help with traceback investigations. Although USEF does not regulate the events where this happened, biosecurity and traceability is the reason USEF is now mandating microchips for horses entering USEF-sanctioned competitions, Dr. Flynn explained. MeV

SUGGESTED READING
Maxwell LK, Bentz BG, Gilliam LL, et al. Efficacy of the early administration of valacyclovir hydrochloride for the treatment of neuropathogenic equine herpesvirus type-1 infection in horses. Am J Vet Res. 2017 Oct;78(10):1126–1139. doi:10.2460/ajvr.78.10.1126
https://avmajournals.avma.org/view/journals/ajvr/78/10/ajvr.78.10.1126.xml