Washington County horse tests positive for virus leading to neurological disease

Publish Date
Author
Dan Callahan

St. Paul, MN: On Friday, April 25, 2025, the Minnesota Board of Animal Health (Board) received notification of a Washington County horse exhibiting neurologic signs and testing positive for equine herpesvirus type 1 (EHV-1). EHV-1 is highly infectious between horses and can develop into a debilitating neurological disease known as Equine Herpesvirus Myeloencephalopathy (EHM). Because the horse, a 16-year-old mare, exhibited symptoms consistent with EHM, the Board has issued a quarantine for the remaining 60 horses on the property to control the spread of the virus.

The horse’s owner first noticed clinical symptoms on Tuesday, April 22, 2025, and voluntarily instituted biosecurity measures to reduce the risk of potential spread, such as limiting visitors to the farm and isolating the affected horse and those considered to be higher risk exposures. A veterinarian examined the animal April 23, noting significant balance and muscle control issues and dribbling urine. The horse has shown marked improvement in the last 48 hours. Treatment for EHM consists of supportive care to allow the nervous system time to heal. Some horses with EHM recover completely and go back to their previous level of function, while others never fully recover. 

“EHM is highly contagious, and horse owners should take their biosecurity seriously to avoid it,” said Dr. Heather Damico, senior veterinarian in charge of equine programs at the Board. “Thankfully, we have a plan in place to control the spread of the disease when it does pop up.”

The Board developed the EHM Control Plan in collaboration with Minnesota’s equine industry. All quarantined horses must be monitored closely for symptoms consistent with EHV-1 and have rectal temperatures taken and recorded twice daily. Horses displaying clinical signs with temperatures of 102°F or higher must be tested for EHV-1. All exposed horses will remain quarantined until 21 days from when the last elevated temperature was noted, or clinical signs were observed.

The last case of EHM diagnosed in Minnesota was in June 2024. Multiple cases continue to be identified throughout the United States.

Horse owners are reminded of the importance of adhering to stringent biosecurity practices to prevent the spread of contagious diseases. Viruses like EHV-1 can be carried from one place to another by other horses, people, insects, trailers, and equipment (including tack, blankets, feed and water buckets, brushes, sponges, hoses, or veterinary supplies). The virus can remain inactive inside the horse for long periods of time. Horses can be exposed and not suffer serious illness, yet some develop EHM. The virus is shed from infected horses via the respiratory tract and is capable of surviving in organic material like manure and feed.

The Board encourages horse owners to follow these biosecurity tips when traveling with their horse: 

  • Use your own trailer and equipment. If you must borrow; clean and disinfect items thoroughly before exposing your horse and again before returning.

  • Don’t let your horse touch other horses, especially nose to nose.

  • Never put the end of a shared hose in your horse’s water bucket without disinfecting first. Don’t hand-graze your horse where other horses have recently grazed.

  • Don’t let strangers touch your horse. If you touch other horses, wash your hands with soap and water, and dry them well. Use disinfectant wipes or hand sanitizer if soap and water are not available.

  • Clean and disinfect tack, boots, equipment, and grooming supplies before returning home. Make sure to remove dirt and manure before disinfecting.

  • Shower, blow your nose (germs can survive a long time in nasal secretions), and put on clean clothes and shoes upon your return.

  • Keep horses returning from shows/exhibits separate from your other horses for up to a month. When doing feeding and chores, work with the returning horses last. Wear boots and coveralls and remove them before working with your other horses.

  • Don’t forget to wash your hands.

Vaccines are available for EHV-1. However, these vaccines are not labeled or considered effective for prevention of the neurological form of the virus (EHM). 

Owners and exhibitors entering Minnesota from other states are required to have a Certificate of Veterinary Inspection verifying the horse has been examined by a veterinarian and is free from signs of communicable and contagious diseases.

For more information EHM, visit the Board’s Reportable Diseases webpage.