EHV-1 in Horses: Understanding Equine Herpesvirus and Neurological Risk

In early 2026, an EHV-1 outbreak at the HITS Culpeper show grounds in Virginia resulted in at least one horse being euthanized due to the neurological form of the disease. That horse went from competing to unable to stand in roughly 72 hours. If that timeline shocks you, it shouldn't. It's actually textbook EHM progression. What should concern you more is this: the horse that introduced the virus to the facility almost certainly showed no symptoms at the time of arrival. That's how EHV-1 works, and it's why this virus terrifies epidemiologists far more than it terrifies the average horse owner scrolling past another outbreak notice on social media.

EHV-1 vs. EHV-4: Different Viruses, Different Dangers

Equine herpesviruses are a family of nine distinct viruses, but two dominate clinical discussion: EHV-1 and EHV-4. They're closely related genetically but behave quite differently in the horse.

EHV-4 is the more common of the two and primarily causes upper respiratory disease in young horses. Think of it as the equine equivalent of a bad cold: nasal discharge, fever, cough, malaise, then recovery in 1-3 weeks. EHV-4 rarely causes abortion and almost never causes neurological disease. Most horses encounter it during their first few years of life and develop partial immunity.

EHV-1 is the dangerous sibling. It causes the same respiratory symptoms as EHV-4 (making clinical differentiation impossible without lab testing), but it has two additional tricks. First, it can infect the blood vessels of the pregnant uterus, causing abortion typically in the last trimester. Second, and most critically, it can infect the blood vessels of the spinal cord and brain, causing vasculitis that leads to ischemic damage of neural tissue. That second scenario is EHM, and it's the reason barns go on lockdown when EHV-1 is confirmed.

Knowing your horse's normal vital signs is your first line of defense. A horse running a fever of 102.5°F or higher warrants immediate isolation pending investigation, especially during show season or after any commingling event.

The Three Faces of EHV-1

EHV-1 manifests in three distinct clinical syndromes, and any individual horse can develop one, two, or all three during a single infection. The virus doesn't pick one lane.

Respiratory Form

The most common presentation. Fever (often biphasic, meaning it spikes, drops, then spikes again), serous to mucopurulent nasal discharge, enlarged submandibular lymph nodes, lethargy, and reduced appetite. In isolation, this looks identical to influenza, strangles, or EHV-4 infection. Most healthy adult horses recover within 2-3 weeks with supportive care. The danger isn't the respiratory disease itself. The danger is what happens during the viremic phase while the horse is coughing all over its neighbors.

Abortion Form

Pregnant mares infected with EHV-1 may abort anywhere from two weeks to several months after initial infection. The abortion typically occurs in the last four months of gestation. The virus infects the endothelial cells of uterine blood vessels, causing thrombosis and separation of the placenta. The mare usually shows no premonitory signs. She's fine at the evening check, and the foal is on the ground by morning. Foals born alive from EHV-1 infected mares are often weak, immunocompromised, and carry a poor prognosis. Abortion storms on breeding farms, where multiple mares abort within a short window, remain one of the most economically devastating events in the equine breeding industry.

Neurological Form (EHM)

EHM is the form that keeps veterinarians up at night. The virus enters white blood cells during the viremic phase and essentially hitchhikes through the circulatory system. When viral-laden cells reach the small blood vessels of the spinal cord (and less commonly the brain), the resulting vasculitis and thrombosis cause ischemic injury to neural tissue. The clinical signs depend on which segments of the spinal cord are affected, but the classic presentation progresses rapidly:

• Fever (day 1-2)
• Mild ataxia, tail weakness, reduced anal tone (day 3-5)
• Hindlimb weakness progressing to recumbency (day 5-8)
• Urinary incontinence, inability to stand, and potential euthanasia (day 6-10)

Not every horse that develops EHM reaches the recumbent stage. Some experience mild ataxia and recover completely. Others deteriorate so fast that they're unable to stand within 48 hours of the first neurological sign. The mortality rate for horses that become recumbent is approximately 30-50%, even with aggressive supportive care. Horses that remain standing generally carry a much better prognosis, with most recovering over weeks to months.

The D752 Mutation: Why Some Strains Are More Dangerous

Not all EHV-1 strains carry equal neurological risk. Research identified a single nucleotide polymorphism in the viral DNA polymerase gene at position 2254, resulting in an amino acid change from asparagine (N) to aspartate (D) at position 752. This is the D752 mutation, sometimes called the "neuropathogenic" variant.

Strains carrying D752 replicate more efficiently in white blood cells, producing higher and more prolonged viremia. Higher viremia means more virus reaching the spinal cord vasculature, which translates to greater risk of EHM. Studies have shown that D752 strains are significantly overrepresented in neurological outbreaks compared to abortion or respiratory-only outbreaks.

However, and this is critical, D752 is neither necessary nor sufficient for neurological disease. Horses have developed EHM from non-neuropathogenic (N752) strains, and many horses infected with D752 strains develop only respiratory disease or no clinical signs at all. Host factors like age, immune status, stress level, and possibly genetics play an enormous role in determining outcome. Older horses (over 20) appear disproportionately susceptible to EHM for reasons not fully understood.

Latent Carriers: The Hidden Reservoir

Here is the fact that makes EHV-1 control so maddeningly difficult: the vast majority of adult horses carry latent EHV-1. Estimates range from 60-80% of the general horse population. The virus establishes latency in the trigeminal ganglia and lymphoid tissue after primary infection, sitting quietly in the host's cells without causing disease or triggering an immune response.

Latent virus can reactivate under stress. Transport, competition, surgery, foaling, corticosteroid administration, commingling with unfamiliar horses, extreme weather changes: any significant stressor can trigger reactivation. When the virus reactivates, the horse sheds it through nasal secretions, potentially for 7-14 days. The horse may or may not show clinical signs during this shedding period. This means any horse at any show, any clinic, any breeding farm could be actively shedding EHV-1 without anyone knowing.

This latent carrier state is why EHV-1 will never be eradicated from the equine population. The virus has evolved a near-perfect survival strategy: widespread latency with stress-induced reactivation ensures perpetual transmission opportunities.

Biosecurity at Shows: What Actually Works

Given the reality of latent carriage, what can you actually do to reduce risk? Quite a bit, actually, though none of it is foolproof.

Temperature monitoring: Take your horse's rectal temperature twice daily starting 3 days before travel and continuing throughout any event. A temperature above 101.5°F (or more than 1°F above that individual's established baseline) should trigger isolation. Fever precedes viral shedding by 24-48 hours in most cases, making it the earliest detectable warning sign.

Minimize nose-to-nose contact: EHV-1 spreads primarily through direct contact with nasal secretions and short-distance aerosol (up to about 35 feet in research settings, though 10-15 feet is more typical in real-world conditions). Don't let your horse touch noses with unfamiliar animals. This sounds simple. At a busy show ground, it's remarkably hard to enforce.

Don't share equipment: Water buckets, feed tubs, twitches, lip chains, bits: anything that contacts the nose or mouth can transfer virus. Bring your own everything. Clean and disinfect stalls before your horse enters them. EHV-1 is susceptible to most common disinfectants (bleach at 1:10 dilution, accelerated hydrogen peroxide, quaternary ammonium compounds), but organic matter inactivates these products. Clean first, then disinfect.

Isolation on return: After any event where your horse commingled with unfamiliar animals, a 14-21 day monitoring period at home is ideal. Full quarantine (separate airspace, separate handlers, separate equipment) is the gold standard. Temperature monitoring throughout. Many barns compromise on a shortened isolation of 7-10 days with temperature checks. Better than nothing, but not without risk.

Vaccination: Necessary but Not Sufficient

Current EHV-1 vaccines are frustrating. They exist, they help, but they don't do what most people assume vaccines do.

Available vaccines (both modified-live and inactivated) can reduce the severity and duration of respiratory disease. Some are labeled for abortion prevention in pregnant mares (administered at months 3, 5, 7, and 9 of gestation). None are labeled for prevention of EHM because none have demonstrated reliable protection against the neurological form in controlled studies.

More importantly, no current vaccine prevents infection or viral shedding. A vaccinated horse can still become infected, still shed virus, and still transmit the disease to other horses. The vaccine may reduce the amount and duration of shedding, but it doesn't eliminate it. This means vaccination creates a false sense of security if barn managers believe vaccinated horses can't spread the virus.

Should you still vaccinate? Yes. The AAEP includes EHV-1 as a core vaccine for horses at risk of exposure (show horses, breeding horses, any horse that travels). A solid vaccination schedule reduces the overall viral burden in the population and lessens disease severity in individual animals. Just don't mistake vaccination for protection.

Quarantine Protocols: The 21-Day Standard

When EHV-1 is confirmed on a property, the standard quarantine period is 21 days from the last confirmed case or last fever. Here's what that looks like in practice:

• No horses enter or leave the property
• Affected horses are isolated with dedicated handlers who do not contact other horses
• All horses on the property have rectal temperatures taken twice daily
• Any horse with a temperature above 101.5°F is immediately isolated and tested (nasal swab for PCR)
• Dedicated equipment, boots, and clothing for the isolation area
• Foot baths at transition points between clean and contaminated areas
• The 21-day clock resets with every new case

That last point is the killer. On large facilities with hundreds of horses, an EHV-1 outbreak can extend quarantine for months as new cases continue to appear. The economic impact is severe: lost competition entries, breeding schedule disruptions, facility overhead without revenue, and the veterinary costs associated with managing affected animals.

Treatment: Supportive Care and Antivirals

There is no cure for EHV-1. Treatment is supportive and directed at managing symptoms while the horse's immune system clears the virus.

For respiratory cases: anti-inflammatories (flunixin meglumine or firocoxib), rest, and monitoring for secondary bacterial infection. Most recover uneventfully.

For EHM cases: the antiviral drug valacyclovir has shown some benefit in reducing viremia when administered early, though evidence is mixed and the drug is used off-label at significant cost (roughly $100-200/day for an average-sized horse). Anti-inflammatory therapy, IV fluids, urinary catheterization for incontinent horses, and sling support for those struggling to stand represent the bulk of EHM management. Physical therapy becomes important during recovery.

The decision point that every veterinarian and owner dreads: when a horse becomes recumbent and cannot rise. Horses are not built to lie down for extended periods. Recumbency causes myopathy, pressure sores, and organ compromise. The humane threshold varies, but most clinicians consider euthanasia when a horse has been recumbent for more than 48-72 hours without improvement despite aggressive support. That was the scenario at HITS Culpeper. It happens at every major EHM outbreak.

What You Can Do Right Now

You cannot eliminate EHV-1 risk. You can manage it intelligently. Start by establishing your horse's normal temperature baseline over a week of twice-daily readings. Know your horse's normal so you recognize abnormal. Keep your vaccination protocol current. Practice basic biosecurity at every event. And when an outbreak is reported in your region, take it seriously. Skipping one show is infinitely preferable to managing an EHM case in your barn.

Understanding the anatomy of the equine nervous system helps you appreciate why EHM causes the specific clinical signs it does. Explore equine anatomy to see the spinal cord and associated structures that become damaged during neurological herpesvirus disease.

Frequently Asked Questions

Can EHV-1 spread to humans or other animals?

No. EHV-1 is strictly an equine pathogen. It does not infect humans, dogs, cats, or livestock species. However, humans can mechanically carry the virus on their hands, clothing, and equipment from an infected horse to a susceptible one. This is called fomite transmission, and it's a major route of spread in barn settings.

How long is a horse contagious with EHV-1?

Horses typically shed the virus in nasal secretions for 7-14 days after infection, though some individuals may shed for up to 21 days. Shedding can begin 24-48 hours before clinical signs appear, which is why fever monitoring is so important: it's the earliest detectable indicator that a horse may be entering the shedding phase.

Can a horse get EHV-1 more than once?

Yes. Natural infection provides partial immunity that wanes over months. Reinfection is common, and latent virus can reactivate independently of new exposure. Horses can experience clinical EHV-1 disease multiple times throughout their lives, though subsequent episodes are often milder than the first.

Is there a test to know if my horse carries latent EHV-1?

No reliable test exists for detecting latent EHV-1. PCR testing of nasal swabs and blood detects active infection and shedding, but latent virus resides in nervous tissue and lymphocytes in a dormant state that standard diagnostics cannot identify. For practical purposes, assume any adult horse could be a latent carrier.

Should I avoid shows during an EHV-1 outbreak?

If an active outbreak is occurring at a venue you planned to attend, yes, avoid it. If outbreaks are reported in your general region but not at your specific venue, the decision is more nuanced. Implement strict biosecurity measures (own water, no shared equipment, temperature monitoring, minimize contact) and monitor your horse closely for 21 days after returning home.

Sources

• American Association of Equine Practitioners (AAEP) - EHV-1 Fact Sheet and Vaccination Guidelines
• USDA Animal and Plant Health Inspection Service (APHIS) - Equine Herpesvirus Myeloencephalopathy surveillance data
• Equine Disease Communication Center (EDCC) - 2026 outbreak reports
• University of Kentucky, Gluck Equine Research Center - Research on EHV-1 latency and neuropathogenic mutations
• Journal of Veterinary Internal Medicine - Published studies on valacyclovir efficacy and EHM prognosis
• Cornell University College of Veterinary Medicine - EHV-1 biosecurity protocols