Equine Herpesvirus Myeloencephalopathy (EHM)

Disease Overview

Equine Herpesvirus Myeloencephalopathy (EHM) is a neurologic disease associated with equine herpesvirus type 1 (EHV-1) infection. EHM is highly contagious and life-threatening. Neurologic signs result from inflammation of the blood vessels, blood clots, and death of neurologic tissue. While there is no vaccine labeled specifically for prevention of EHM, EHV-1 vaccination protocols decrease the severity of respiratory disease and reduce nasal shedding in infected horses.


EHV spreads directly in aerosolized droplets dispersed by coughing or direct contact (nose-to-nose). It spreads indirectly through fomite transmission—hands, clothing, vehicles and trailers, stalls, and common use articles such as brushes, buckets and bits can all spread EHV-1. It can also be transmitted through contact with placental and fetal fluids and tissues from fetuses aborted due to EHV-1.

Clinical Signs

  • Fever (>101.5°F)
  • Lethargy
  • Nasal discharge
  • Loss of tail tone
  • Urine dribbling
  • Inability to stand
  • Incoordination and weakness, especially in the hind limbs

Risk Factors

  • Advanced age (although EHM can affect horses of any age)
  • Compromised or immature immune system
  • EHV-1 associated fever >103.5°F
  • Females are more commonly affected
  • Travel
  • Frequent contact with large numbers of horses
  • Exposure to horses that have traveled

Note: Ponies and smaller breeds are less commonly affected


“Equine Herpesvirus (Rhinopneumonitis),” American Association of Equine Practitioners, copyright 2021, aaep.org/risk-“Equine Herpesvirus Myeloencephalopathy (EHM),” Equine Disease Communication Center, accessed July 15, 2021, aaep.org/sites/default/files/Documents/DiseaseFactsheetEHM.pdf.

“Equine Herpesvirus Myeloencephalopathy (EHM) Quick Facts,” Merck Animal Health, copyright 2021,