Equine Herpesvirus Myeloencephalopathy (EHM) is a neurologic disease caused by the Equine Herpesvirus-1 virus, however in rare cases, the Equine Herpesvirus-4 virus can also cause EHM. Myeloencephalopathy refers to disease of the brain and spinal cord.

Incubation: The incubation period is the time from exposure to the virus to when a horse shows clinical signs of infection. The incubation period for EHM varies from two to six days.

Transmission: EHV-1 is spread via horse-to-horse contact with nasal discharge. It can also be spread via nasal aerosol droplets in the air. Contact with contaminated surfaces and fomites may also transmit the disease.

Shedding Period: Horses can shed the virus during the incubation period. EHV-1 can be shed when no obvious clinical signs are present. Approximately 40-60% of infected horses can be lifelong carriers of EHV-1 and intermittently shed the virus.

Clinical Signs: For EHV-1 cases that progress to EHM, clinical signs include fever, incoordination, hind limb weakness, loss of tail tone, loss of bladder control, leaning against surfaces to maintain balance and recumbency.

Diagnostic Testing: Nasal swabs can be collected along with blood to be tested by PCR. A positive PCR in nasal swab indicates an active shedding case of EHV-1.

Treatment: Supportive care is the best form of treatment for EHM. Medications such as phenylbutazone (Bute) and flunixin meglumine (Banamine) can be administered to help reduce fever, inflammation, and pain.

Prevention: There is no vaccine for EHV-1 that also protects against EHM. However, EHV-1 vaccines can reduce the viral load in a horse and reduce shedding of virus, so keeping horses up to date on EHV vaccines is the primary method of prevention. New horses should always be quarantined and kept separate from the general population before they can be introduced to horses that reside on the property.

Prognosis: Prognosis for those horses that test positive for EHV-1 and have neurologic symptoms can be poor, especially if they become recumbent and can’t rise. Fatality can be as high as 30%.