Equine herpesvirus-1 (EHV-1, rhinopneumonitis) is a highly contagious infection that can cause respiratory disease in weanlings and young horses, abortion in pregnant mares, and neurologic disease in adult horses. Herpesvirus is widespread in the equine population.

A neurologic form of EHV-1 called equine herpesvirus myeloencephalopathy has become more prevalent—likely as a result of more horses traveling from different regions and commingling. The virus damages the blood vessels in the brain and spinal cord, causing tissue damage, necrosis and loss of neurologic function. This neurologic form is most often seen in adult horses. Outbreaks are relatively rare; however, there is evidence that the number of EHM cases reported in the United States has been increasing.