The first time you see a horse that can’t get up, you don’t forget it.
EHV-1 can do that. Not in every case. Most horses exposed to equine herpesvirus-1 develop a fever, feel rough for a few days, and recover without incident. But the neurological form is something else. It can move from a mild temperature to a horse down in a stall faster than most people expect the first time they watch it happen.
That’s why experienced horse people take EHV-1 seriously, and why barn managers start pulling out thermometers the moment they hear the word “outbreak” anywhere near an event their horses attended.
What EHV-1 Is
Equine herpesvirus-1 is a highly contagious virus that can affect horses in several ways: respiratory illness, abortion in pregnant mares, death in newborn foals, and neurological disease. Horse owners sometimes hear it called “Rhino,” a reference to rhinopneumonitis, which is one of the respiratory conditions associated with equine herpesviruses.
The neurological form has its own name: Equine Herpesvirus Myeloencephalopathy, or EHM. This is the form that triggers quarantines, canceled events, and the kind of phone calls nobody wants to make. The USDA-APHIS designated EHV-1 as an emerging disease, and it’s reportable in many states.
Not every horse exposed to EHV-1 develops EHM. But because neurological cases can escalate fast, veterinarians consistently approach potential outbreaks with caution from the start.
How It Spreads
EHV-1 moves easily anywhere horses congregate. The virus travels through nose-to-nose contact, coughing and sneezing, shared water buckets or feed tubs, contaminated tack and grooming tools, stalls, trailers, and human hands and clothing after contact with infected horses.
The piece that makes outbreaks hard to control: a horse can shed the virus before showing any obvious symptoms. By the time you know there’s a problem, horses may have already been loading into trailers, competing, or sharing barn space with others.
Stress matters too. Long trailer rides, competitions, crowded show grounds, changes in routine, intense training periods. All of these can increase the chances of viral shedding or reactivation of a dormant infection. It’s one reason many barns build in a monitoring period after horses return from shows, clinics, sales, or large events. Not paranoia. Just pattern recognition.
What the 2006 Pimlico Outbreak Looked Like
In early 2006, an outbreak at Maryland’s Pimlico Racecourse demonstrated how quickly EHV-1 can move through a facility. Two horses were euthanized after testing positive for the virus, one developing neurological signs so severe he could no longer rise on his own. Within days, hold orders were placed on affected barns, more than 80 horses were under observation, and five of six horses in isolation had tested positive on PCR. It was a contained outbreak by many standards. It was also the kind of situation that sharpens everyone’s awareness of how fast the window closes between “we have a fever” and “we have a problem.”
Symptoms to Watch For
Symptoms vary considerably from horse to horse. Some horses spike a fever and seem quiet for a couple of days. Others progress to neurological involvement. The two categories look different and need to be treated differently.
Early and respiratory signs:
- fever
- nasal discharge
- coughing
- reduced appetite
- lethargy
- swollen lymph nodes
Neurological signs (EHM):
- hind-end weakness
- stumbling or loss of coordination
- urine dribbling
- difficulty standing
- inability to rise in severe cases
If you’re seeing anything in that second list, call your vet and isolate the horse before you do anything else.
During known outbreaks, many barn managers take temperatures twice daily as a matter of routine. Fever is often the earliest detectable sign, and according to AAEP guidelines, it can appear before nasal shedding even begins. That window matters. A horse isolated at the fever stage is a horse that hasn’t been shedding on its neighbors yet.
The Neurological Form: What It Can Mean
The mortality rate for EHM is significant. Research puts it at 30 to 50 percent in neurological cases. Horses that survive can face a long road. Recovery time ranges from several days to more than a year, depending on how severely they were affected and how quickly intervention began.
Treatment is primarily supportive. There is no antiviral medication approved for EHV-1 in horses, though antivirals have been used off-label in some cases. Anti-inflammatory medications, IV fluids, and in severe cases, full-body support slings for horses that can’t stand. These are the tools available. Antibiotics do nothing against the virus itself, though they may be used if a secondary bacterial infection develops.
A published case study from 2024 followed a 17-year-old mare who became recumbent with EHM. She needed a support sling for 21 days, parenteral fluids, bladder catheterization, and weeks of intensive nursing care. At eight months post-illness, she had achieved marked improvement in mobility. That’s a best-case outcome. It’s also an enormous undertaking. The honest picture of EHM recovery includes the time, the cost, and the uncertainty. Experienced horse people know that going in.
Risk factors for developing EHM include being older than five years, running a high fever (above 103.5°F), and having a fever that peaks later in the illness rather than early. These aren’t guarantees in either direction, but they give veterinarians and owners a clearer picture of which exposed horses need the closest watching.
Vaccination: What It Does and What It Doesn’t
Vaccination is an important management tool, with clear limits.
No vaccine currently on the market carries a label claim for preventing EHM. The neurological form remains outside what vaccines can reliably block. What vaccines can do: reduce severity of respiratory illness, decrease nasal shedding, and lower the overall viral load an infected horse contributes to its environment. That’s meaningful. It’s just not a complete answer.
Vaccination schedules are worth discussing directly with your veterinarian based on your horse’s age, travel schedule, exposure level, and breeding status. AAEP guidelines serve as the baseline. Many competition venues now require proof of recent EHV vaccination, often within six months of entry, and some have tightened that window further after recent outbreaks.
One important note from the AAEP: vaccinating horses that have already been exposed to an active outbreak is not recommended. That’s a conversation to have with your vet, not a decision to make on your own based on a social media post about a nearby event.
Biosecurity in Practice
Biosecurity during an EHV-1 concern isn’t complicated. It’s just consistent.
Isolate any horse showing fever or neurological signs immediately, before you know whether EHV-1 is confirmed. Separate water buckets and feed tubs. Disinfect stalls, tack, and grooming tools. Wash hands between horses, and change clothes or footwear before moving between isolated and healthy animals. Limit unnecessary traffic in and out of the barn.
For horses that have been potentially exposed, isolation for a minimum of 21 days is the standard recommendation, with temperature monitoring twice daily throughout that period. Any new fever during quarantine restarts the clock.
If EHM is confirmed on your premises, the situation becomes reportable in many states. The state animal health official may place a formal hold order and set testing and biosecurity requirements. Working closely with your veterinarian from the beginning, rather than waiting on lab confirmation before taking precautions, is consistently the right call.
The things that spread the virus most predictably at events: shared hose nozzles at communal water stations, nose-to-nose contact between horses from different barns during warm-up, and handlers moving between horses without changing gloves or washing hands. None of these require elaborate solutions. They require attention.
Latent Infection and Reactivation
Like other herpesviruses, EHV-1 can remain dormant in a horse’s system after initial infection and reactivate later during periods of stress, illness, transport, or immune suppression. A horse can appear completely healthy while carrying a latent infection.
This is one reason outbreaks sometimes seem to appear without warning. A horse that last showed symptoms years ago may shed virus again after a hard competition season, a change in management, or transport stress. It doesn’t mean every recovered horse is a ticking clock. It means that baseline biosecurity around horse movement makes sense year-round, not only during named outbreaks.
EHV-1 Now
The 2006 Pimlico outbreak is a reference point. EHV-1 has continued circulating since then, with significant outbreaks at racetracks, show facilities, veterinary hospitals, and boarding barns. A major outbreak moved through barrel racing events in Texas in November 2025, prompting responses from USEF and widespread biosecurity advisories across disciplines.
The pattern is consistent: horses congregate, travel home before symptoms appear, and the virus moves with them. The response that works is also consistent: early fever detection, fast isolation, honest communication between barn managers, and veterinary involvement before things escalate.
Most horses exposed to EHV-1 don’t develop EHM. Awareness and sensible precautions matter more than fear. Call your vet if you have concerns. Take temperatures when there’s a reason to. Don’t wait for certainty before you start isolating.
When to Call Your Veterinarian
Call your vet promptly if your horse develops a fever with no obvious explanation, shows any neurological signs, has recently traveled or been exposed to unfamiliar horses, or seems off in a way you can’t account for. Early testing and isolation can change the outcome, for your horse and for the other horses around it.
Sources
- UC Davis Center for Equine Health: Equine Herpesvirus 1 (EHV-1) / Equine Herpesvirus Myeloencephalopathy (EHM)
- Merck Veterinary Manual: Equine Herpesvirus Infection
- American Association of Equine Practitioners (AAEP): EHV-1 & EHV-4 Infectious Disease Guidelines
- AAEP: FAQ for Horse Owners Regarding EHV-1
- University of Minnesota Extension: Equine Herpesvirus (EHV-1) and EHM
- EquiManagement: What Did We Learn From Last Year’s EHV-1 Outbreak? (March 2026)
- ScienceDirect / Equine Veterinary Education: Medical management and positive outcome after prolonged recumbency in EHM (2024)
- PubMed: Outbreak of neurologic disease caused by EHV-1 at a university equestrian center
- USEF: EHV-1 Update and Biosecurity Reminders (November 2025)
This article is written from the perspective of an experienced horse owner and is not a substitute for veterinary advice. If you have concerns about EHV-1 exposure or symptoms in your horse, contact your veterinarian.
Last updated: May 23, 2026
Originally published: June 4, 2010
