EHV-1, or equine herpesvirus type 1, is a highly contagious virus that causes upper respiratory illness, late-term abortion in pregnant mares, and in some cases a potentially fatal neurological disease. Most horses are exposed to EHV-1 at some point in their lives, and the virus can hide in the body for years before flaring up again during periods of stress.
What EHV-1 Does to Horses
EHV-1 has three main clinical forms, and the same virus can cause all three. The most common is respiratory disease: fever, nasal discharge, and coughing, similar to what you’d see with a cold. In pregnant mares, the virus can trigger abortion or stillbirth, typically in the last trimester. These “abortion storms” can sweep through a breeding facility, affecting multiple mares within days or weeks of each other.
The most feared form is the neurological version, called equine herpes myeloencephalopathy, or EHM. This happens when the virus damages blood vessels in the spinal cord and brain, cutting off oxygen and nutrients to nerve tissue. The resulting inflammation and tissue death cause symptoms that range from mild coordination problems to complete paralysis of all four limbs. Other signs include difficulty urinating, loss of tail tone, head tilt, swelling in the lower legs, and in rare cases, blindness. Horses that become unable to stand often face euthanasia.
How It Spreads
The virus spreads primarily through direct contact with nasal discharge from an infected horse. It also travels through contact with aborted fetuses and placental material, making abortion cases especially high-risk for barn-wide transmission. Indirect contact matters too: shared water buckets, tack, grooming tools, and even handlers’ clothing and hands can carry the virus from one horse to the next.
Infected horses begin shedding virus from the nose within about two days of infection, with peak shedding typically occurring between days five and eight. In experimental settings, nasal shedding lasts up to 14 days, and the virus circulates in the bloodstream for up to 21 days. During real outbreaks, though, viral DNA has been detected in nasal swabs for as long as 42 days, meaning a horse can remain a source of infection far longer than many owners expect.
Latency: The Virus That Never Leaves
Like other herpesviruses, EHV-1 establishes lifelong latent infection. After the initial illness resolves, the viral genome stays tucked inside certain cells without actively replicating. The horse appears perfectly healthy and shows no symptoms during this phase. But the virus isn’t gone. It’s waiting.
Stress is the primary trigger for reactivation. Transportation is one of the best-documented stressors linked to EHV-1 flare-ups in horses. Long trailer rides, changes in social groups, heavy competition schedules, and even relocation to a new facility can spike stress hormones (glucocorticoids) enough to wake the dormant virus. Once reactivated, the horse begins shedding virus again and can spark a new outbreak, sometimes with a different clinical presentation than the original infection. This is why outbreaks so often cluster around horse shows, sales, and training facilities where horses from many different barns mix under stressful conditions.
How EHM Is Diagnosed
Real-time PCR testing is the gold standard for confirming EHV-1 infection. A veterinarian collects nasal swabs and blood samples, ideally at the same time, to check for viral DNA. Nasal swabs tend to yield the highest number of viral copies during the first week of illness, making early sample collection important for an accurate result. Urine can also be tested but generally contains lower levels of viral DNA than nasal swabs.
Because the respiratory form of EHV-1 looks similar to other equine respiratory infections, PCR testing is essential for distinguishing it from other viruses. For suspected neurological cases, bloodwork showing the virus circulating in white blood cells (viremia) alongside neurological symptoms strongly supports an EHM diagnosis.
Recovery and Prognosis
Respiratory EHV-1 is usually self-limiting. Most horses recover with supportive care within a couple of weeks, though they remain contagious longer than they appear sick.
The neurological form is more serious. Data from the large 2021 outbreak in Valencia, Spain, which affected international competition horses, put the EHM case fatality rate at 12%. All deaths in that outbreak were directly linked to worsening neurological disease. Among survivors, return to previous performance levels varies considerably. In that same outbreak, 73% of horses that recovered from EHM returned to international-level competition within 30 months, compared to 89% of infected horses that never developed neurological signs.
Severity matters a great deal for long-term outlook. One study found that half of horses with mild neurological signs recovered completely within a year. But among horses with severe signs that required intensive care, only three out of eight survivors returned to their previous performance level, and just one met the owner’s expectations. Horses that become recumbent (unable to stand) face the worst prognosis.
Biosecurity During an Outbreak
When EHV-1 is suspected on a property, stopping all horse movement in and out of the facility is the single most important step. No horses should leave or arrive until testing confirms or rules out the diagnosis. This prevents the virus from reaching other barns and competition venues.
Within the facility, limiting horse-to-horse contact is critical. Sick or exposed horses should be isolated, and handlers should change clothing and wash hands between caring for different groups. Shared equipment like water buckets, lead ropes, and grooming kits should be dedicated to individual horses or thoroughly disinfected between uses. A solution of one part chlorine bleach to ten parts water is effective for decontaminating surfaces and equipment. Accelerated hydrogen peroxide disinfectants also work well. EHV-1 doesn’t survive long in the environment compared to some other pathogens, but contaminated surfaces can bridge the gap between horses quickly in a busy barn.
Limiting human-to-horse-to-horse contact is easy to overlook but just as important as physical separation. Veterinary staff, farriers, and barn workers moving between horses are common links in transmission chains during outbreaks. Dedicated boots and outerwear for the isolation area, along with hand sanitizer stations, help break that chain.
Vaccination Limits
Vaccines against EHV-1 exist and are widely used, but they have important limitations. Current vaccines reduce the severity of respiratory disease and may lower the risk of abortion, but none are labeled as protective against the neurological form. Vaccination also does not prevent a horse from becoming latently infected or from shedding the virus during a reactivation event. This is why biosecurity measures remain the frontline defense during outbreaks, even in fully vaccinated populations. Vaccination still has value as part of a broader prevention strategy, particularly for pregnant mares and horses that travel frequently, but it’s not a guarantee against disease.