Word of a concerning outbreak of equine herpes (EHV-1) has circulated quickly inside horse communities. It makes Wyoming State Veterinarian Hallie Hasel happy to know horse owners are paying attention. That said, some information has been wrong, she said.

“[The virus] is serious, and the word spread very quickly, which we’re glad, but there is a lot of misinformation out there,” she said in a Thursday interview. “Any horse that’s traveling or has been exposed to horses traveling, regardless of what type of horse it might be, we ask that they basically just stop traveling for 14 to 21 days.”

According to the U.S. Department of Agriculture’s Animal and Plant Health Inspection Service, Equine herpesvirus (EHV), also known as equine rhinopneumonitis, is a family of highly contagious viruses found in horses worldwide, according to the U.S. Department of Agriculture. Of the nine known herpesviruses, EHV-1, EHV-3 and EHV-4 pose the highest disease risk to U.S. horses despite not posing a risk to people.

Equine herpesvirus-1 primarily causes upper respiratory disease and abortions/stillbirths in horses. In some cases, it causes a neurological disease called equine herpes myeloencephalopathy (EHM) that can be fatal. EHV-3 causes a venereal disease called coital exanthema. EHV-4 primarily causes respiratory disease in foals and abortion in mares but can also produce cases of EHM. EHV-infected horses that don’t develop EHM generally recover with no side effects.

The primary means of EHV-1 transmission is horse-to-horse contact, so isolation of infected horses is the key in avoiding transmission. 

Virus particles are shed through secretions, which include nasal discharge and fluids and tissues associated with abortions. Any materials contaminated with secretions from infected horses can carry viral particles and should be disinfected (quaternary ammonium compounds are recommended).

The Wyoming Livestock Board currently has not confirmed a case of EHV-1 in the state, however, a Colorado horse confirmed with EHV-1 participated in events in Cheyenne, Wheatland and Casper, Nov. 15-16. This horse recently traveled to an event in Waco, Texas, developed a mild fever, and tested positive for EHV-1 on Nov. 20.

“While the EHV exposure is concerning, practicing excellent biosecurity, including movement restrictions, is our best protection against further spread,” the board reports.

The good news, Hasel said, is that unless you travel or introduce your horses in areas with multiple horses or in areas horses of unknown origin have been (think public or campground corrals), you should be OK. However, due to the ease at which EHV-1 is transmitted, even your healthy-appearing horses can be carriers.

Texas, where the infected horse had traveled for an event, and Oklahoma have been ground zero for the virus this year.

“It’s always out there, especially this time of year, but we do know that there was some significant transmission that occurred during those events,” she said. “We see it annually. This isn’t anything new for any state. However, the severity of this outbreak is significant.”

That could be a problem for equine involved in the National Rodeo Finals in Las Vegas. As it’s the “Super Bowl of Rodeo,” horses from across the country headed that way.

Hasel said the event has taken greater biosecurity measures, enhanced biosecurity and enhanced their rules “just a tad bit” due to the outbreak. Wyoming residents and horses that attend will not be required to test and isolate their horses, she said just before the roadeo began last week.

“If something happens at the event, then we’ll take further action if needed,” Hasel said. “Basically, just stop traveling unless they absolutely have to.”

The disease affects any kind of equine, whether it’s a mule, donkey or horse. That raises questions about summer events in the region, such as the 2026 Mule Days event and Cody rodeos. According to Hasel, the winter season is when the disease seems to be transmitted more often, but in the warmer months the virus can survive longer.

“When we have severely cold weather, the virus won’t last as long, maybe five to seven days,” she said. “During more moderate temperatures, it can last up to 30 to 35 days in outdoor environments. So [equine owners] have to be concerned whenever they’re an overnight or boarding and lodging in those types of facilities.”

But she warns there is no safe time with the virus; anytime you have horses that are stressed or commingled, you will see EHV.

“We encourage everybody to vaccinate for equine herpes. There isn’t a completely 100% effective vaccine, but it definitely does help.”

Testing options

Tests available include a PCR test, which can be run on nasal swabs, blood or infected tissue, according to the Wyoming State Veterinarian Lab. This test detects viral DNA in the sample, rather than host response to viral exposure. The lab also offers serum virus neutralization, which is a blood test that detects the presence of antibodies to the virus. This does not necessarily indicate an active infection, rather when this test is positive, it indicates that there has been exposure to the virus, either through natural infection or vaccination.

Results from both tests should be interpreted with the help of a veterinarian. As a reportable disease, when a test is EHV-1 positive, in addition to the submitter, the result is automatically sent to the Wyoming Livestock Board and the state veterinarian. The office of the state veterinarian follows up positive cases with recommendations related to tracing contacts, management and additional testing.

The Wyoming Livestock Board was made aware of the outbreak of both Equine Herpes Virus (EHV) and Equine Herpes Myeloencephalopathy (EHM) in horses that were at or associated with barrel racing and rodeo events in Texas and Oklahoma.

“Several cases of EHM have been confirmed in Texas and Oklahoma associated with these equine events, with numerous reports of additional cases and equine deaths pending investigation,” the board reported. “If our office receives traces from the other states, we will contact those owners directly and issue official hold orders to mitigate further spread of the disease.”   

The outbreak serves as a reminder of the potential disease risks associated with large equine events, the board reported in a recent warning to the public. It shows the importance of timely reporting of diseases and ensuring movement requirements are followed, such as, veterinary inspection, health certificates, and current Coggins test prior to travel, they said.

Recommended resources

Hasel said she personally recommends information from the American Association of Equine Practitioners at aaep.org and the Equine Disease Communication Center at equinediseasecc.org.

The Wyoming Livestock Board also maintains a list of animal diseases called the Reportable Disease List. These diseases must be reported to the state veterinarian when they are identified, per state statute. The list of diseases are important for their impact on herd health, public health and the economics and marketability of the Wyoming livestock industry.

If any person or government entity that knows or suspects that there is a contagious or infectious disease among animals under their jurisdiction, or any veterinarian who knows of or suspects a reportable disease, must report it to the state veterinarian’s office within 24 hours. All information collected in response to the reportable disease list remains confidential.

To report a disease: 307-777-7515