The neurological disease Equine Herpesvirus Myeloencephalophy (EHM) caused by Equine Herpesvirus type 1 (EHV-1) has been in the news recently, with horses and farms affected across the United States and Canada. The outbreak appears related to initial cases at a cutting horse show in Ogden, Utah, which was held April 29 – May 81. While the true extent of the outbreak is unclear at this time, there is an elevated risk of EHM cases in horses that were at the show or in contact with horses that were at the show. Pfizer Animal Health is encouraging anyone with questions or concerns about the disease to contact their local veterinarian.

EHV-1 (Rhinopneumonitis) also causes upper respiratory infections in horses and abortions in pregnant mares2. However, the neurological disease EHM, affects the horse’s brain and spinal cord and may result in paralysis and death. Neurological symptoms include incoordination that can progress to the inability to stand, lower leg swelling, the inability to urinate or pass manure, urine dribble and reduced tail tone2. Some of the symptoms of EHM can be confused with other neurological diseases such as rabies, Equine Protozoal Myeloencephalitis (EPM) and West Nile virus infections, so it is important for animals showing any clinical symptoms to be examined by a veterinarian as soon as possible.

EHM is the most infrequent but potentially devastating form of EHV-1 infections and is difficult to treat once neurological symptoms are observed.

“It’s important for horse owners to understand the facts about the disease,” said Tom Lenz, DVM, MS, Diplomate ACT and Senior Director, Equine Veterinary Services for Pfizer Animal Health. “EHV is extremely contagious and many horses may become latently infected – infecting other horses but may never display symptoms themselves. Working with a local veterinarian to develop a comprehensive disease prevention program including good barn hygiene is the best defense horse owners can provide their animals.”

The EHV-1 virus is transmitted through both direct and indirect contact with infected horses, and can spread via water buckets, feed tubs, tack, grooming equipment and even the hands and feet of people caring for affected animals. Proper biosecurity measures should be followed to ensure the best protection against an outbreak. For example, isolate all infected horses, and limit movement of horses on and off the premises. The incubation period of EHV-1 infections is typically 1-2 days, with clinical signs and fever then occurring over the following 10 days3. The neurological form of the disease typically occurs 8-12 days after the primary infection. Horses can shed the virus up to 21 days after they stop showing clinical signs.  Disinfect all areas where the disease may have spread including halters, lip chains and feed buckets.

Please report any cases or suspect cases to your state/provincial animal health department as soon as possible.

Visit for frequently asked questions, resource information from the American Association of Equine Practitioners (AAEP), USDA, state and provincial animal health departments, and other related information regarding this outbreak, and the disease. Also, remember your local veterinarian is the best resource on the risk of an outbreak in your region.


Email comments or questions to [email protected]

Write A Comment