Equine Herpesvirus Myeloencephalopathy (EHM) in Horses: Emergency Neurologic Signs
- See your vet immediately if your horse develops sudden incoordination, hind-end weakness, trouble standing, urine dribbling, or tail weakness.
- EHM is the neurologic form of equine herpesvirus type 1 (EHV-1). It can spread quickly in barns, showgrounds, and training facilities through respiratory secretions, contaminated hands or equipment, and reactivated latent infection.
- Diagnosis usually involves a neurologic exam plus PCR testing on a nasal swab and EDTA whole blood. Your vet may also recommend isolation, twice-daily temperature checks for exposed horses, and state reporting when required.
- There is no vaccine labeled specifically to prevent EHM, so prevention relies heavily on biosecurity, movement control, isolation, and risk-based vaccination to reduce shedding of EHV-1.
- Prognosis depends a lot on severity. Mildly affected horses may recover with supportive care, while recumbent horses or those with severe bladder dysfunction have a more guarded outlook.
What Is Equine Herpesvirus Myeloencephalopathy (EHM) in Horses?
Equine herpesvirus myeloencephalopathy, or EHM, is the neurologic form of equine herpesvirus type 1 (EHV-1). This virus is common in horse populations, and many adult horses carry it in a latent state for life. In some cases, EHV-1 causes respiratory disease or abortion. In others, it damages blood vessels supplying the spinal cord and brain, leading to sudden neurologic signs such as ataxia, weakness, and difficulty urinating.
EHM is an emergency because signs can worsen quickly over hours to days. A horse may start with a fever and mild hind-end wobbliness, then progress to stumbling, inability to rise, loss of tail tone, or urine dribbling. Horses with neurologic disease should be treated as potentially infectious until your vet says otherwise, because EHV-1 can spread before the cause is confirmed.
This condition also matters beyond the individual horse. Suspected EHM can trigger isolation, movement restrictions, and facility-wide monitoring because exposed horses may be shedding virus even if they look normal. Early recognition and fast biosecurity steps can help protect the rest of the barn.
Symptoms of Equine Herpesvirus Myeloencephalopathy (EHM) in Horses
- Fever, sometimes before neurologic signs appear
- Sudden ataxia or wobbliness, especially in the hind limbs
- Weakness or paresis that may start in the rear end
- Toe dragging, stumbling, or crossing limbs
- Difficulty backing, turning, or standing square
- Urine dribbling, bladder distension, or trouble passing urine
- Reduced tail tone or tail weakness
- Decreased sensation around the perineum or hindquarters
- Dog-sitting posture or inability to rise
- Recumbency or paralysis in severe cases
Mild cases may look like vague hind-end weakness or an unusual gait after a recent fever. More serious cases can progress to collapse, inability to stand, or loss of bladder function. EHM often affects the spinal cord, so signs are commonly worse in the hind limbs than the front.
When to worry: immediately. Any horse with fever plus neurologic signs, sudden incoordination, or urinary problems needs urgent veterinary evaluation and prompt isolation from other horses. While not every neurologic horse has EHM, your vet will usually approach these cases as contagious until testing and the clinical picture say otherwise.
What Causes Equine Herpesvirus Myeloencephalopathy (EHM) in Horses?
EHM is caused by infection with equine herpesvirus type 1 (EHV-1). The virus spreads mainly through respiratory droplets, direct horse-to-horse contact, and contaminated people, tack, buckets, twitches, thermometers, trailers, and other shared equipment. It can also reappear when a previously infected horse experiences stress and begins shedding virus again.
The neurologic form develops when EHV-1 causes a cell-associated viremia and inflammation in blood vessels supplying the central nervous system. That vascular injury can reduce blood flow to the spinal cord and brain, which is why affected horses may show weakness, incoordination, bladder dysfunction, and sometimes recumbency.
Risk tends to rise anywhere horses mix closely or move on and off the property often, such as boarding barns, showgrounds, racetracks, breeding farms, and training facilities. Fever clusters, recent travel, new arrivals, and horses under physiologic stress can all increase concern. Importantly, vaccination may help reduce respiratory disease and viral shedding in some settings, but it does not guarantee protection from EHM.
How Is Equine Herpesvirus Myeloencephalopathy (EHM) in Horses Diagnosed?
Diagnosis starts with an urgent physical and neurologic exam. Your vet will look at gait, strength, tail tone, bladder function, and whether the horse can safely stand and move. Because several serious diseases can look similar, your vet may also consider differentials such as equine protozoal myeloencephalitis, West Nile virus, rabies, trauma, cervical spinal cord disease, and toxic causes.
For suspected EHM, the most useful first-line tests are usually PCR on a nasal swab and EDTA whole blood, collected together. Cornell's Animal Health Diagnostic Center specifically recommends paired samples because viral shedding and viremia can vary over time. A negative result does not always rule EHV-1 out, especially if sampling timing is not ideal.
Your vet may also run CBC and chemistry testing, assess hydration and kidney values before certain medications, and monitor rectal temperatures in exposed horses twice daily. In many states and facilities, suspected neurologic EHV-1 cases also trigger reporting and biosecurity steps while results are pending. If a horse is severely affected, diagnosis and stabilization often happen at the same time.
Treatment Options for Equine Herpesvirus Myeloencephalopathy (EHM) in Horses
Spectrum of Care means you have options. Here are treatment tiers at different price points.
Budget-Conscious Care
- Urgent farm call or clinic exam with neurologic assessment
- Immediate isolation and basic biosecurity setup
- PCR testing on nasal swab and blood when feasible
- Anti-inflammatory medication and supportive nursing directed by your vet
- Bladder monitoring, sling-free assisted standing if safe, and strict stall rest
- Twice-daily temperature monitoring for exposed horses
Recommended Standard Treatment
- Hospitalization or intensive on-farm management under close veterinary supervision
- PCR testing on nasal swab and EDTA whole blood, plus baseline lab work
- Anti-inflammatory treatment and supportive fluids as indicated
- Consideration of antiviral therapy such as valacyclovir when your vet feels timing and case details support it
- Urinary bladder management, pressure sore prevention, assisted mobility, and nutritional support
- Facility outbreak management plan with movement restriction, temperature surveillance, and disinfection protocols
Advanced / Critical Care
- Referral hospital or ICU-level equine care
- 24-hour nursing for recumbent or non-ambulatory horses
- Advanced supportive care including IV fluids, repeated bloodwork, urinary catheterization when needed, and intensive skin and limb protection
- Mechanical lift or sling support when appropriate and safe
- Aggressive management of complications such as pressure sores, muscle damage, aspiration risk, and inability to urinate
- Comprehensive outbreak consultation for large facilities or competition barns
Cost estimates as of 2026-03. Actual costs vary by location, clinic, and individual case.
Questions to Ask Your Vet About Equine Herpesvirus Myeloencephalopathy (EHM) in Horses
Bring these questions to your vet appointment to get the most out of your visit.
- Based on my horse's neurologic exam, how concerned are you about EHM versus other causes of ataxia?
- Should we collect both a nasal swab and EDTA blood sample for EHV-1 PCR today?
- Does this case need referral, or can we safely manage it on the farm right now?
- What signs would mean my horse is getting worse, especially with bladder function or ability to stand?
- Would antiviral treatment be reasonable in this case, and what benefits or limits should I expect?
- What biosecurity steps should everyone in the barn follow starting today?
- How long should exposed horses have temperatures checked, and when can movement restrictions be lifted?
- What is the likely cost range for conservative, standard, and advanced care in this situation?
How to Prevent Equine Herpesvirus Myeloencephalopathy (EHM) in Horses
Prevention starts with biosecurity, not vaccination alone. New arrivals should be separated from resident horses for a monitoring period set by your vet, and horses returning from shows, sales, racetracks, or clinics should be watched closely for fever or respiratory signs. Avoid sharing water buckets, bits, lead ropes, thermometers, and grooming tools between horses unless they are cleaned and disinfected first.
If a horse develops fever or neurologic signs, isolate that horse immediately and stop horse movement until your vet advises otherwise. AAEP guidance for suspected infectious neurologic disease recommends acting as though EHM is possible until proven otherwise, with a clear biosecurity perimeter, limited personnel access, and twice-daily temperature checks for exposed horses.
Vaccination still has a role, but expectations should be realistic. Current AAEP guidance states there are no licensed vaccines labeled for prevention of neurologic EHV-1 disease (EHM). Some EHV vaccines may help reduce nasal shedding and viremia in certain settings, which can support outbreak control, but they are not a substitute for isolation, hygiene, and movement control. Your vet can help build a risk-based vaccine plan for your horse's age, travel schedule, breeding status, and facility exposure level.