What to Know About the 2025 Equine Herpesvirus Outbreak
What Happened
In November 2025, an outbreak of equine herpesvirus myeloencephalopathy (EHM) began at a Women's Professional Rodeo Association (WPRA) World Finals and Elite Barrel Race event in Texas. Several horses at that event then traveled to the Barrel Futurities of America (BFA) World Championship in Oklahoma. As soon as the positive cases were identified, the BFA cancelled the remainder of their event.
A significant concern is that many horses were exposed to infected horses during these two events and then traveled home before developing symptoms and before the outbreak was identified.
EHM is caused by the virus equine herpesvirus-1 (EHV-1) and rarely EHV-4. These viruses generally cause respiratory disease (also referred to as rhinopneumonitis; this is most commonly seen in young horses) and occasionally abortion in mares, but some strains can cause neurologic disease. The neurologic form of the disease is called EHM. This virus is contagious between horses, easily transmitted through direct horse contact, airborne droplets from coughing, and contaminated items and surfaces, such as buckets, grooming tools, or even people. According to the Equine Disease Communication Center (EDCC), EHM, while serious, is not always fatal. It is estimated that 60-70% of infected horses do recover with intensive veterinary treatment.
As of December 8, 2025, there have been a total of 47 confirmed EHM-affected equines that can be traced back to these events. There were 10 EHV-1 positive equines with no neurologic signs and 4 EHV-1 positive equines with no clinical signs at all. The cases are primarily located in Texas and Oklahoma, with a few cases also in Louisiana, South Dakota, Colorado, New Mexico, Arizona, and Washington. At the time of writing, no cases have been linked to this outbreak in Pennsylvania.
There were two unrelated cases of EHV in Pennsylvania in November 2025 after the outbreak began, causing confusion. The first horse in Northumberland County was positive for EHV-4 and exhibited respiratory disease. This horse was at the Keystone Yearling and Breeder's Sale on November 7th and 8th in Bloomsburg, PA, and was not associated with the Texas outbreak.
A few days later, a farm in Lancaster County had 3 horses with EHM. Two were tested and confirmed EHV-1 positive, and the third was euthanized and not tested. These horses were not associated with the Texas outbreak or the case in Northumberland County.
It's important to remember that not all EHM cases are linked. It is thought that most mature horses have already been exposed to this virus early in life, and it may reactivate during stressful conditions (including travel and competition). It is normal to see some EHV/EHM cases every year. From the year spanning November 2024 to November 2025, there was one facility with EHM-affected horses in Pennsylvania and 26 facilities with EHM cases nationwide. All of this information is publicly available on the EDCC website.
Most horses in Pennsylvania appear to be at low risk from the EHM outbreak in Texas and Oklahoma. The highest-risk horses are the ones that attended the affected events, followed by horses exposed to these competitors. This could include horses boarded at the same home facility or horses at subsequent shows with a horse that was at an affected event.
Ideally, a horse that was at an affected event should be isolated from other resident horses when it returns from the event, or at least as soon as the exposure is communicated. If a horse at this facility becomes symptomatic and tests positive, the facility would then be placed under quarantine, so that no horses enter or leave until a set period of time after all affected horses have tested negative.
However, in the beginning of the outbreak, there is a lag between exposure, symptom development, testing, and tracing of exposed horses. Many horses had already returned home before anyone was aware of the exposure. This is why it is recommended to quarantine horses returning from shows and events, even though it is not always practical.
December Update
Effective December 8, 2025, a temporary quarantine order is in effect, carrying additional documentation requirements for Interstate Certificates of Veterinary Inspection for the transport of horses into Pennsylvania. The order includes documentation requirements to attest that a horse has not been exposed to EHM or had a suspected or confirmed diagnosis of EHM. The order reiterates documentation of temperature check requirements.Â
The full order can be found on the PA Department of Agriculture website.
Signs of EHV-1 and EHM
Horses infected with EHV-1 typically exhibit signs including fever, nasal discharge, coughing, and lethargy/depression. The respiratory form of this disease tends to be mild in mature horses. There may be an incubation period of 2-10 days after exposure, during which the horse is infected but shows no signs of illness. The horse can transmit the virus during this period.
The neurologic form of EHV-1 causes inflammation and damage to the brain and spinal cord, resulting in EHM. Horses with EHM may have fever and respiratory symptoms before neurologic signs appear (but not always). Signs of EHM include limb swelling or stocking up, weakness of the hindlimbs, incoordination, and urine dribbling. Severe cases can result in behavior changes, seizures, and inability to stand.
EHV-1 and EHM are reportable diseases, so it is essential to work quickly with your veterinarian to test your horse if you suspect you may have a case of it.
There are vaccines available for EHV-1 and EHV-4, and they are considered risk-based vaccines rather than core vaccines. They offer protection against respiratory disease and abortion. According to the American Association of Equine Practitioners, no available vaccines claim to prevent EHM, so biosecurity practices remain the best way to fight an outbreak. However, prior vaccination could help to reduce nasal shedding of EHV-1 and the spread of the disease if the horse is subsequently exposed. There is no research to determine whether vaccination during an outbreak is effective. Consult with your veterinarian to determine what is best for your horse.
If you suspect that your horse may have been exposed to EHM or any infectious disease, isolate it from other horses, call your veterinarian, and monitor its temperature twice daily. It is good practice to follow this protocol for any horse that shows signs of infectious disease as well.
Biosecurity on Horse Farms
Pennsylvania horse owners can protect their horses by implementing biosecurity plans on their facilities. These practices will reduce the risk of exposing your horses to any infectious disease, not just EHV-1 and EHV-4.
- Wash hands and change clothes/footwear between barns. Shoe or boot soles can be easily disinfected using a disinfectant spray and then allowed to dry. However, disinfectants don't work on dirt; the boots must be scrubbed clean first for this to be effective.
- Keep separate gear for each horse, such as tack, grooming supplies, blankets, buckets, halters, and leads.
- Talk to your farrier and other equine service professionals about their biosecurity practices between barns.
- Vaccinate your horses for all core vaccines and talk to your veterinarian about risk-based vaccines.
- Require a Coggins test and Certificate of Veterinary Inspection (health certificate) for any new horse entering the facility.
- Quarantine new or returning horses for 21 days. They should be able to see other horses but not contact them. Leave at least one stall empty between that horse and other horses, although a separate barn or shed is ideal. Monitor for signs of infectious disease and take temperatures twice daily.
- Isolate any horses that develop signs of infectious disease and call the veterinarian. Communicate with any parties who have recently been exposed to that horse.
- Either designate a limited number of people to care for the sick horse, or if you are the only caretaker, contact the sick horse after you have finished caring for the healthy horses. Keep a separate pitchfork, muck tub, feed and water buckets, etc. Thoroughly clean and disinfect the stall when the horse recovers.
- Keep in mind that mules and donkeys can be asymptomatic shedders of EHV-1.
When traveling off-farm:
- Follow the event or facility's rules regarding health certificates. Do not take a horse with any signs of illness to an event.
- Avoid nose-to-nose contact between your horse and other horses.
- Do not let strangers touch or feed your horses.
- Do not touch or feed other people’s horses.
- Do not borrow equipment.
- Do not use shared water troughs or feeders.
- When filling water buckets, do not dip the end of the hose into the water.
- Take your horse's temperature twice daily and monitor other vital signs.
- After you return home, monitor your horse for signs of disease. If your horse was in close proximity to unknown horses or you know that there was an infected horse at the event, isolate your horse from other resident horses. Isolate and disinfect any items that were at the event, including your trailer.
- Keep records of travel, shows, and known exposures.
Conclusion
Biosecurity should be practiced every day, not just during outbreaks. Be extra careful when you travel with your horse. Have a plan for how your facility will isolate and care for new arrivals or a horse with an infectious disease. Learn to take your horse's vital signs and identify signs of illness. If exposure is suspected, focus on disinfection and isolation, and consult your veterinarian promptly.
This website, including its text, graphics, and images ("Content"), is for educational purposes only; it is not intended to be a substitute for veterinary medical advice, diagnosis, or treatment. Always seek the advice of a licensed doctor of veterinary medicine or other licensed or certified veterinary medical professional with any questions you may have regarding a veterinary medical condition or symptom.
Resources and References
Equine Disease Communication Center
American Association of Equine Practitioners
