EHM, or equine herpes myeloencephalopathy, is a neurological disease in horses caused by equine herpesvirus type 1 (EHV-1). It occurs when the virus, which commonly causes respiratory illness, travels through the bloodstream and damages blood vessels in the spinal cord and brain. The result is sudden, often severe neurological problems that can leave a horse unable to walk or stand. EHM is one of the most feared complications of a virus that most horses encounter at some point in their lives.
How EHV-1 Becomes EHM
EHV-1 is extremely common in the horse population. Many horses carry it without obvious illness, and most infections cause nothing more than a fever and nasal discharge. The virus spreads through respiratory droplets, direct nose-to-nose contact, and contaminated surfaces like shared water buckets, tack, or handlers’ clothing and hands.
In a small percentage of cases, the virus doesn’t stay confined to the respiratory tract. It enters white blood cells and hitches a ride through the bloodstream, a phase called viremia. When the virus reaches the blood vessels supplying the spinal cord, it triggers inflammation and clotting that cuts off blood flow to nervous tissue. That loss of blood supply is what causes the neurological damage. Not every horse infected with EHV-1 develops EHM, and researchers still don’t fully understand why some horses progress to the neurological form while others don’t. Certain genetic variants of the virus appear more likely to cause viremia, but the “neuropathogenic” strain isn’t the only one capable of triggering EHM.
Signs to Watch For
EHM typically appears 4 to 6 days after exposure to the virus, though the incubation period can be as short as 24 hours. The first sign is often a fever, sometimes accompanied by the mild respiratory symptoms common to any EHV-1 infection. Neurological signs then develop rapidly, often within a day or two of the fever spike.
The hallmark of EHM is a lack of coordination, most severe in the hind limbs. A horse may appear wobbly, drag its hind feet, or sway when turning. Other common signs include urinary incontinence or dribbling, constipation, and a weak or limp tail. In severe cases, horses become recumbent and unable to stand. A horse that was perfectly normal yesterday may be staggering or down today. That speed of onset is what makes EHM so alarming.
If your horse develops sudden coordination problems, urine dribbling, or an inability to stand, with or without a concurrent fever, contact a veterinarian immediately. Early recognition makes a meaningful difference in management.
How EHM Is Diagnosed
Diagnosing EHM involves a combination of clinical signs and lab testing. When a vet sees sudden hind-limb weakness in a horse, particularly during an outbreak or after recent contact with unfamiliar horses, EHM is high on the list of possibilities.
Confirmation requires detecting the virus through PCR testing, which identifies viral genetic material. Vets typically submit both a nasal swab and a blood sample, because the timing of viral shedding and the bloodstream phase can vary. A horse may test positive on one sample but not the other depending on where it is in the course of infection, so paired samples are critical. In fatal cases, the virus can also be identified in spinal cord tissue during post-mortem examination.
Treatment and Supportive Care
There is no cure for EHM. Treatment focuses on supporting the horse through the acute phase and limiting further damage. The antiviral drug valacyclovir is commonly used during outbreaks. It doesn’t kill the virus outright, but it can reduce viral replication and may shorten the period of shedding. Anti-inflammatory medications help manage the vascular inflammation in the spinal cord that drives the neurological damage.
For horses that can still stand, the goal is keeping them upright, hydrated, and eating. Horses that become recumbent need intensive nursing care, including sling support when possible, bladder management, and prevention of pressure sores. This level of care is labor-intensive, emotionally draining, and expensive. The decision to continue treatment for a down horse is one of the hardest calls an owner and vet face together.
Survival and Recovery
Survival rates in EHM outbreaks have been reported as high as 75%, but outcomes depend heavily on the severity of neurological damage. Horses with mild ataxia that remain standing have a much better outlook than those that go down. Recovery is not quick, easy, or inexpensive, even in horses that survive.
Some horses recover fully and return to their previous level of work. Others are left with residual neurological deficits that may never completely resolve, ranging from subtle hind-limb weakness to persistent coordination problems. The uncertainty around long-term recovery is one of the most frustrating aspects of the disease. There’s no reliable way to predict early on which horses will bounce back and which will have lasting limitations.
Why Vaccines Don’t Prevent EHM
EHV-1 vaccines exist and are widely used, but none of them carry a label claim for preventing the neurological form of the disease. Current vaccines can reduce respiratory symptoms and may decrease the amount of virus a horse sheds, which helps slow spread during an outbreak. However, they offer limited protection against EHM specifically. The European Food Safety Authority has recommended promoting EHV-1 vaccination broadly while acknowledging that it provides only limited defense against the neurological form.
This gap was highlighted during a major EHV-1 outbreak in Europe in 2021, which affected competition horses across several countries. Many of the affected horses had been vaccinated. The international equestrian community acknowledged that improved vaccines are needed, but for now, biosecurity remains the primary line of defense against EHM.
Biosecurity During an Outbreak
Because the virus spreads through respiratory secretions and can survive on surfaces, outbreaks at shows, training barns, and veterinary hospitals can escalate quickly. Infected horses shed the virus through nasal secretions for 7 to 10 days on average, though shedding can last longer in some cases.
When EHM is confirmed or suspected at a facility, the standard response includes immediate isolation of affected horses, a halt on all horse movement in and out of the property, and twice-daily temperature monitoring of every horse on site. Fever is typically the earliest detectable sign of infection, so catching a temperature spike before a horse starts shedding virus is one of the best tools for containing spread. Dedicated equipment, clothing changes between horses, and hand hygiene become essential.
For individual horse owners, the practical takeaway is straightforward: any time your horse has been at a show, sale, or shared facility where an EHM case is later confirmed, monitor its temperature twice daily for at least two weeks. A rectal temperature above 101.5°F (38.6°C) warrants a call to your vet. Isolate the horse from others on your property until the exposure window has passed. These simple steps can prevent a single case from becoming a barn-wide crisis.