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.

Transmission

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
Incoordination and weakness, especially in the hind limbs
Loss of tail tone
Urine dribbling
Inability to stand

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

References

“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.