11/21/2025
This information is from The Equine Disease Communication Center.
Equine Herpesvirus Myeloencephalopathy (EHM) Outbreak
The Equine Disease Communication Center is monitoring an outbreak of EHM that originated at the Women’s Professional Rodeo Association (WPRA) World Finals and Elite Barrel Race event Nov 5-9. To date, officially confirmed cases have been reported in the following states:
State
EHV-1 cases - PCR positive, no neurologic signs
EHM cases - PCR positive, with neurologic signs
Texas
7
Oklahoma
1
3
Louisiana
3
Colorado
1
The EHM case reported in MD on 11/18/25 was not associated with the Waco, TX outbreak.
Equine herpesvirus-1 (EHV-1) is the virus causing this outbreak. While EHV-1 most commonly causes respiratory disease, some strains cause outbreaks of neurologic disease. EHV-1 causes neurologic signs as a result of inflammation of the blood vessels and activation of blood coagulation within the brain and spinal cord. Neurologic signs occur as a result of a lack of blood flow and resulting damage to the brain or spinal cord; this disease manifestation is known as Equine Herpesvirus Myeloencephalopathy (EHM). Classic clinical signs of EHM include fever, swelling/stocking up of the limbs, hindlimb weakness, and urine dribbling. Some horses are more severely affected and display behavior changes, signs of cranial nerve dysfunction, seizures, and inability to stand. While EHM is a serious disease, the majority of cases are not fatal. In most outbreaks of EHM, 60-70% of affected horses recover with treatment and supportive care. Veterinary evaluation and isolation of exposed or affected horses is crucial.
