Equine Herpesvirus Myeloencephalopathy in Horses: EHV-1 Neurologic Disease
- See your vet immediately. Equine herpesvirus myeloencephalopathy, or EHM, is the neurologic form of EHV-1 and can worsen quickly over hours to days.
- Common early clues include fever, hind-end weakness, stumbling, urine dribbling, reduced tail tone, and trouble standing. Some horses have respiratory signs first, while others do not.
- Diagnosis usually involves a neurologic exam plus EHV-1 PCR testing on both a nasal swab and EDTA whole blood. Your vet may also recommend bloodwork and spinal fluid testing to rule out other neurologic diseases.
- Treatment is supportive and may include anti-inflammatory medication, fluids, bladder care, sling support, nursing care, and sometimes antiviral medication such as valacyclovir, depending on timing and severity.
- Because EHV-1 spreads easily between horses, immediate isolation, twice-daily temperature checks for exposed horses, and strict biosecurity are essential.
What Is Equine Herpesvirus Myeloencephalopathy in Horses?
Equine herpesvirus myeloencephalopathy, often shortened to EHM, is the neurologic form of equine herpesvirus type 1 (EHV-1) infection. EHV-1 is a common, contagious virus in horses. Many horses are exposed during their lives, and the virus can remain latent in the body and reactivate later during stress or illness.
In some horses, EHV-1 does more than cause fever or respiratory disease. The virus can damage blood vessels in the spinal cord and brain, leading to inflammation, poor blood flow, and neurologic signs. That is why affected horses may suddenly look weak, wobbly, or unable to urinate normally.
EHM is especially concerning because it can spread through a barn before the first neurologic horse is recognized. Horses may shed virus from the nose, and infected horses can also have virus circulating in the blood. Not every horse with EHV-1 develops EHM, but when neurologic disease happens, it is considered an emergency and often triggers isolation and reporting steps through your vet and state animal health officials.
Symptoms of Equine Herpesvirus Myeloencephalopathy in Horses
- Fever, sometimes in two phases
- Hind-end weakness or incoordination
- Stumbling, swaying, or a wobbly gait
- Difficulty rising or inability to stand
- Urine dribbling or trouble emptying the bladder
- Reduced tail tone or weak anal tone
- Lethargy and decreased appetite
- Nasal discharge or mild respiratory signs
Some horses start with fever and mild respiratory signs, then develop neurologic problems a few days later. Others are first noticed because they are weak behind, dragging their toes, leaning, or having trouble backing or turning. Bladder dysfunction is an important clue in EHM and may show up as urine dribbling, straining, or a distended bladder.
See your vet immediately if your horse has fever plus weakness, wobbliness, or urinary changes. A horse that cannot rise, is falling, or seems rapidly worse needs urgent veterinary care and strict isolation from other horses.
What Causes Equine Herpesvirus Myeloencephalopathy in Horses?
EHM is caused by infection with EHV-1, a herpesvirus that spreads mainly through respiratory secretions, close horse-to-horse contact, and contaminated hands, tack, buckets, trailers, or equipment. The incubation period is usually about 2 to 10 days. Horses may also carry the virus silently after earlier exposure because herpesviruses can become latent and reactivate later.
The neurologic form develops when EHV-1 causes damage to blood vessels supplying the spinal cord and sometimes the brain. This vascular injury can lead to swelling, tiny hemorrhages, and nerve dysfunction. That is why signs often center on the hind limbs, tail, and bladder.
Stress, transport, competition, hospitalization, crowding, and mixing horses from different groups may increase the chance of viral shedding and spread. Vaccination can help reduce respiratory disease and viral shedding in some situations, but currently available vaccines are not proven to reliably prevent EHM itself. Your vet can help you weigh risk factors for your individual horse and facility.
How Is Equine Herpesvirus Myeloencephalopathy in Horses Diagnosed?
Your vet starts with a full history and neurologic exam. They will look at gait, strength, tail tone, anal tone, bladder function, and whether the signs fit a spinal cord problem. Because several serious diseases can look similar, your vet may also consider equine protozoal myeloencephalitis, West Nile virus, cervical vertebral stenotic myelopathy, trauma, rabies, and toxic causes.
Testing for suspected EHM usually includes EHV-1 PCR on both a nasal swab and EDTA whole blood. Using both samples matters because viral shedding and viremia can vary over time. Bloodwork may show changes linked to infection, and some horses also have cerebrospinal fluid testing if it is safe and useful.
Diagnosis can be challenging, especially early in the course of disease or after shedding has decreased. A negative test does not always rule EHV-1 out if timing was poor. Because EHM is a reportable concern in many situations, your vet may contact state animal health officials and recommend immediate isolation while results are pending.
Treatment Options for Equine Herpesvirus Myeloencephalopathy in Horses
Spectrum of Care means you have options. Here are treatment tiers at different price points.
Budget-Conscious Care
- Urgent farm exam and neurologic assessment
- Isolation and barn biosecurity plan
- Temperature monitoring for exposed horses
- PCR testing on nasal swab and blood when feasible
- Anti-inflammatory medication as directed by your vet
- Basic nursing care, hydration support, and careful footing management
Recommended Standard Treatment
- Hospitalization or intensive on-farm management
- PCR testing on nasal swab and EDTA whole blood
- CBC/chemistry and additional neurologic rule-out testing
- Anti-inflammatory treatment and IV or enteral fluid support as needed
- Bladder management, including catheterization when needed
- Frequent nursing care, pressure sore prevention, assisted standing, and reassessment
- Discussion of antiviral therapy such as valacyclovir when timing and case details support it
Advanced / Critical Care
- Referral hospital or ICU-level equine care
- Continuous or near-continuous monitoring
- Sling support or assisted recovery systems for non-ambulatory horses
- Repeated bloodwork and follow-up testing
- Aggressive bladder care, fluid therapy, nutritional support, and skin care
- Advanced imaging or spinal fluid testing when indicated
- Extended hospitalization and outbreak-level biosecurity management
Cost estimates as of 2026-03. Actual costs vary by location, clinic, and individual case.
Questions to Ask Your Vet About Equine Herpesvirus Myeloencephalopathy in Horses
Bring these questions to your vet appointment to get the most out of your visit.
- Do my horse's signs fit EHM, or are there other neurologic diseases we need to rule out right away?
- Should we test both a nasal swab and EDTA whole blood for EHV-1 PCR today?
- Does my horse need referral or hospitalization, or is careful on-farm treatment reasonable?
- What supportive care does my horse need for bladder function, hydration, footing, and pressure sore prevention?
- Would antiviral treatment such as valacyclovir be appropriate in this case, and what are the likely benefits and limits?
- What is my horse's short-term prognosis based on whether they are standing, urinating normally, and improving or worsening?
- What isolation steps should we use for this horse, and how long should exposed horses have twice-daily temperature checks?
- Which horses on the property are highest risk, and should any vaccination or movement plans change during this outbreak?
How to Prevent Equine Herpesvirus Myeloencephalopathy in Horses
Prevention focuses on biosecurity, early detection, and risk reduction. New arrivals should be separated from resident horses when possible, and horses returning from shows, sales, or hospitals should be monitored closely. Avoid sharing water buckets, bits, lead ropes, thermometers, and grooming tools between groups. Hand hygiene, dedicated clothing, and cleaning high-touch surfaces matter during any suspected outbreak.
Twice-daily temperature checks are one of the most practical early-warning tools for exposed horses. Fever may appear before neurologic signs, so catching a temperature spike early can help your vet isolate that horse sooner. If a horse on the property develops fever, nasal discharge, or neurologic signs, stop horse movement and call your vet promptly.
Vaccination remains part of many herd health plans because it may help reduce respiratory disease and viral shedding, but it should not be viewed as complete protection against EHM. The American Association of Equine Practitioners notes that currently available vaccines are not labeled to prevent the neurologic form. During a confirmed incident, strict isolation, direct observation, and at least 21 days of monitoring after the last relevant case milestone are commonly recommended before quarantine release, based on your vet's guidance and local animal health requirements.