Late last fall, many horse owners across the southern United States found themselves navigating uncertainty as cases of Equine Herpesvirus Myeloencephalopathy (EHM) emerged across Texas, Oklahoma, and neighboring states. In a recent episode of Extension Horses Tack Box Talk: Horse Stories with a Purpose, Dr. Kris Hiney (Oklahoma State University Extension) sat down with Dr. AJ Manship, veterinarian at the OSU College of Veterinary Medicine, to reflect on what happened—and, more importantly, what we can learn moving forward.
What is EHM, and How Is It Different from “Regular” EHV?
EHM is a neurologic form of disease caused by Equine Herpesvirus type 1 (EHV‑1). EHV‑1 is extremely common in the horse population and is best known for causing respiratory illness—fever, nasal discharge, and cough—particularly in young or newly congregated horses.
In some cases, however, EHV‑1 can progress beyond the respiratory tract. After a horse develops a fever (often a biphasic fever, meaning two distinct fever spikes a few days apart), the virus may enter the bloodstream. If it reaches the blood vessels supplying the brain or spinal cord, inflammation and damage can occur, resulting in the neurologic signs we recognize as EHM.
Importantly, EHM is not caused by a separate virus. It is a less common but more severe outcome of an EHV‑1 infection.
Why Do Only Some Horses Develop Neurologic Disease?
This remains one of the biggest unanswered questions. While a “neurotropic strain” of EHV‑1 was once suspected, research has shown that both neurologic and non‑neurologic cases can occur with the same viral variants. Risk appears to be influenced by a combination of factors, including:
- Amount of virus circulating in the body (viremia)
- Stress (transport, frequent competition, illness)
- Age
- Host immune response
Because EHV‑1 is a herpesvirus, once a horse is infected it may carry the virus for life. During periods of stress, some horses can shed virus without appearing sick, creating a major challenge for outbreak prevention.
Why This Outbreak Spread So Quickly
The timing of the outbreak played a significant role. Late fall and early winter bring together several compounding risk factors:
- Heavy horse movement associated with major competitions and end‑of‑year events
- Environmental conditions (cooler temperatures and higher moisture) that favor virus survival
- Close contact as horses are stalled or housed more closely
Many horses were traveling long distances, attending multiple events in short succession—creating, as Dr. Manship described it, a “perfect storm” for disease transmission.
A Word About Blame—and Asymptomatic Shedding
One critical point emphasized in the discussion is that outbreaks like this are rarely caused by someone knowingly hauling a sick horse. Because horses can shed EHV‑1 before obvious signs appear—or without showing signs at all—it is very likely that the initial source horse looked healthy at the time of travel.
This reality underscores why biosecurity, rather than blame, must remain the focus during outbreaks.
Vaccination: What It Does—and Doesn’t—Do
No currently available vaccine is labeled to prevent EHM. However, EHV‑1 vaccination does reduce viral shedding and severity of disease, which helps lower overall risk within the population.
Key take‑home vaccination points include:
- Do not vaccinate horses with known direct exposure during an active outbreak
- Vaccination remains recommended for high‑risk horses, including those that travel frequently, show regularly, or are young and mixing with other horses
- EHV‑1 is considered a risk‑based vaccine, meaning not every horse needs it, but many do
- Immunity (from vaccination or natural infection) is relatively short‑lived—about 3–4 months
- Vaccines should be timed so immunity peaks 2–3 weeks before high‑risk periods such as major competitions
Decisions about vaccinating retired horses depend on housing, age, and exposure risk and should be made in consultation with a veterinarian.
Was the Worry Justified?
In short: yes. Horses were lost, competition schedules were disrupted, and the outbreak had real financial and emotional impacts. More importantly, it had the potential to spread across disciplines and into national events.
At the same time, the outbreak also showcased the best of the horse industry—cooperation among veterinarians, owners following quarantine recommendations, and rapid communication to contain spread.
The Biggest Lesson Moving Forward
If there is one clear message from this outbreak, it is the critical importance of everyday biosecurity. Simple practices—monitoring temperatures, isolating returning horses, using dedicated equipment, and having a quarantine plan in place before there is a problem—can significantly reduce risk.
As Dr. Manship noted, horse owners are willing to do the right thing when they understand the “why” behind the recommendations. Education, preparation, and communication remain our strongest tools against future outbreaks.
Staying informed, planning ahead, and working closely with your veterinarian can make all the difference—not just during outbreaks, but year‑round.
