Horse herpes: The virus behind the outbreak

As an outbreak of highly contagious horse herpes infects horses across Western states, leaving some horses dead and prompting event organizers to cancel competitions, a closer look at the virus causing all the trouble would seem in order.

But first, as Reuters reports: “Horses cannot infect humans but for the animals the symptoms of the virus include respiratory problems and hind-leg weakness, decreased coordination, nasal discharge and fever.”

Nine strains of equine herpes virus have been identified; the strain in the news, EHV-1, is very common. By age 2, almost all horses have contracted the virus, usually from their mothers (dams), but the virus generally lies inactive, according to an information sheet from the U.S. Department of Agriculture.

As can happen with human herpes, the virus kicks back into gear when the animals are stressed — such as during long-distance transportation (often required for competitions) or vigorous exercise. After the virus is reactivated, the viral infection can spread to other horses, through a sneeze, for example, or through contact with rags, buckets or people.

The disease can cause respiratory illness or abortion; EHV-1 can sometimes become a neurological infection, called equine herpes myeloencephalopathy, which destroys blood vessels and leads to damage of the brain and spinal cord. Horses with the neurological form may have to lean against a wall for balance or be unable to stand up.


Vaccines exist for the respiratory form of EHV-1, but there isn’t enough evidence of how well they work, nor is there a vaccine labeled for the neurological form, according to USDA assessment of what’s known about the virus. Dr. Barry Meade, a researcher in equine infectious diseases at the University of Kentucky, wrote this recommendation in the report:

“Development of a vaccine that could prevent latency and the neurological form of EHV should be a top priority for research related to this disease.”

For now, veterinarians recommend owners quarantine their horses—see this veterinarian’s down-to-earth advice on protecting horses from EHV-1.

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