Veterinarians confirmed a 15-year-old pony mare displaying neurologic signs of equine herpesvirus myeloencephalopathy (EHM) on June 27, 2018. The mare originated from a San Mateo County facility, and an additional horse originating from the same property—a 24-year-old Tennessee Walking Horse mare—displayed a fever and tested positive for equine herpesvirus-1 (EHV-1).

Authorities have isolated and placed both mares under quarantine, and enhanced biosecurity is in effect. Exposed horses will be monitored for clinical signs, and temperatures will be taken twice daily. Any horse displaying a fever or compatible clinical signs will be tested. The California Department of Food and Agriculture (CDFA) will continue to monitor the situation and release new information as it becomes available.

Equine herpesvirus infection in horses can cause respiratory disease, abortion in mares, neonatal foal death, and neurologic disease. EHM might be caused by damage to blood vessels of the brain and spinal cord associated with EHV-1 infection. It’s often due to the neuropathogenic strain of EHV-1 but can occasionally be caused by the non-neuropathogenic strain of the virus.

EHV-1 spreads easily and typically has an incubation period between two and 10 days. Respiratory shedding of the virus generally occurs for seven to 10 days but can persist longer in infected horses. For this reason, veterinarians recommend a 21-day isolation period of confirmed-positive EHM cases, along with stringent biosecurity protocols. Like herpesviruses in other species, the latent form of EHV-1 can reactivate later but generally with a low viral load, posing a low risk of infecting other horses. Humans are not at risk of contracting the virus; however, humans can act as indirect modes of transmission.