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Equine Health Resource

Equine Herpes Myeloencephalitis, EHM

Synonyms: Neurologic Rhino

Equine Herpes Myeloencephalitis (EHM) is a neurologic condition caused by EHV-1 (Equine Herpes Virus-1). The virus causes inflammation of the blood vessels in the spinal cord resulting in loss of neurologic function.

Horses affected by EHM usually have severe neurologic disease that affects the hind limbs, bladder and rectum. Horses are often wobbly, incontinent, and weak, but are usually alert and hungry. Horses in the early stages have a fever. Severely affected horses often are down and cannot rise.

Not all horses that acquire the EHV-1 infection develop these neurologic signs.

EHM is a reportable disease, meaning that if a horse has or is suspected of having this disease, vets are required to report it to agricultural authorities (usually the State Veterinarian). These authorities may investigate the case as part of a larger effort to monitor equine health and coordinate with other states and the USDA APHIS in preventing the spread of illness or disease on a national and international level.

Diagnosis is made using nasal swabs, PCR and blood tests.

Treatment is mostly nursing care and anti-inflammatories. Anti-viral drugs have been shown to be helpful in treatment.


  • How should I manage my other horses in order to minimize risk for this condition?
  • Are anti-viral drugs of any use in treating this condition?
  • How should my horses be vaccinated to minimize the risk of this disease?

    Vaccines for Equine Herpesvirus (rhino) are not considered protective against EHM but vaccination is still recommended to reduce the incidence and spread of EHV-1 in a population of horses.

    There were severe outbreaks of this syndrome in the United States in 2011, and it is thought to be on the rise.

    Helpful Outside ResourcesCredible Equine Health Information on the Internet

    Author: Doug Thal DVM Dipl. ABVP


    Anatomy of an Epidemic: The 2011 EHV-1 Outbreak. UC Davis Center for Equine Health, The Horse Report 2011 July: 6-7.


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