Outbreak Awareness: Equine Herpesvirus Myeloencephalopathy (EHM)

A recent disease outbreak in horses in Minnesota and Wisconsin has left many in the equine community concerned. To date, since early March 2014, 10 confirmed cases of a neurologic and sometimes fatal disease in horses has occurred as a result of infection with a mutated form of Equine Herpes Virus-1 (EHV-1), also known as Equine Herpesvirus Myeloencephalopathy (EHM). All 10 confirmed cases have been found in eastern Minnesota and northwestern Wisconsin. At this point, there have been NO cases reported in Southern Wisconsin.

Here is basic information regarding the disease, signs to look for in your own horse, prevention strategies, and what to do and whom to contact if you are concerned or have suspicions of EHV-1 or EHM. 

What Is EHV-1?

By 2 years of age, the majority of horses have been exposed and infected with the EHV-1 virus. In most infected horses, the virus can become inactive in the horse’s body yet sets up a life-long latent carrier state. In the latent state, no signs of disease are seen. Sometimes, however, the virus can be activated by stressful events including strenuous exercise, long distance transport, or weaning. In the active disease, the EHV-1 virus is able to cause four different forms of disease including respiratory disease, abortion, neonatal death, and finally neurologic disease.

Equine Herpesvirus Myeloencephalopathy (EHM) refers to the neurologic disease associated with certain highly contagious mutated strains of EHV-1. Patients infected with this specific strain of virus can exhibit neurologic signs as a result of damage to blood vessels in the brain and spinal cord. This interference with blood supply leads to tissue damage and abnormal brain and spinal cord function leading to signs such as incoordination, urine dribbling, weakness, and inability of the horse to rise.  

How the Virus Is Spread

The most common way for the EHV-1 virus to spread is by direct horse to horse contact, through the respiratory or nasal secretions of horses infected with the neurologic form. Less commonly, horses can be infected by objects (people, tack, feeding buckets, gates, etc.) that have been contaminated by the infectious virus.

Signs to Watch For

Most affected horses will exhibit a fever, lethargy, and possible inappetance approximately 7-10 days prior to showing any neurological signs. After that period of time, a horse affected by EHM will show decreased coordination; urine dribbling; loss of tail tone; hind limb weakness; leaning against a wall, tree or fence to maintain balance; lethargy; and inability to rise (dog-sitting appearance). If you or anyone you know see any of these signs in your horses, contact your veterinarian immediately to report the disease and begin diagnostic and treatment plans. In order to diagnose EHM, your veterinarian will submit a nasal swab and blood work to a lab. 


The best practice to prevent the spread of EHM, other than complete isolation, is to practice good biosecurity which includes washing down all objects and surfaces that your horse may contact. People should wash hands or use hand sanitizer before and after contact with their horse and other horses. Additionally, avoid sharing any equipment including hoses, water buckets, and tack.

If your horse has been exposed or is suspected of being exposed to horses infected with EHM, call your veterinarian for further instructions and keep your horse home. Do not allow unexposed horses near those that have been exposed. Isolate the sick horse and do not share equipment between healthy and sick horses. If disease is suspected, it may be important to take a rectal temperature twice daily and report any abnormal temperatures. Any temperature over 102.0 F warrants notifying your veterinarian.


Although there are several types of vaccines available that protect against EHV-1, the products available are only proven to help prevent disease against the respiratory and abortive forms of the disease. At this time, there is no known vaccine that will help prevent against the neurologic form of equine herpes virus. 

Treatment Options

For horses infected with EHM, supportive treatment is the primary treatment option. This includes keeping the horse comfortable and standing, maintaining nutrition and hydration, and providing anti-inflammatory medication. Anti-viral medication has been used for treatment and considered somewhat effective in curbing the disease, however it can be somewhat cost prohibitive. The specific dosing and effectiveness of anti-virals are still being evaluated. Antibiotics have no effect on Equine Herpesvirus, but can help if a secondary infection is involved.

Concerns for Southern Wisconsin

Horse owners in Southern Wisconsin should consider their veterinarian as a valuable resource to help them determine if travel is safe for their horse.

“For those with performance horses that travel near regions with current outbreaks, our recommendation is to minimize or eliminate travel to reduce exposure between horses. For those with horses that don’t travel frequently, risk of exposure is significantly reduced. While we encourage equine activities (trail riding, local shows, and events), we strongly recommend preventative biosecurity measures be taken.” - Scott Spaulding, DVM, Badger Veterinary Hospital

“Never hesitate to contact your veterinarian for guidance regarding equine herpes virus. Since the virus is endemic in the majority of horse population, outbreaks with the neurologic form of the virus are almost impossible to predict. In circumstances where there is concern of EHM, working with your veterinarian promptly can mean the difference between one and many infected horses.”  - Toria Waldron, DVM, Badger Veterinary Hospital