A horse with neurologic signs was diagnosed with equine herpesvirus myeloencephalopathy (EHM) in Marion County, Oregon, according to a Equine Disease Communication Center (EDCC) alert.


The horse began showing acute neurologic signs on Saturday March 18 and was immediately referred to Oregon State University-College of Veterinary Medicine (OSU-CVM). The diagnosis was made on March 20 by a positive test for both the EVH-1 wild type and neurotropic genetic markers. When horse’s condition deteriorated, it was euthanized.

All horses at the Marion County property have been quarantined and strict biosecurity measures initiated. A second location in Marion County with a high-risk contact horse has also been quarantined. Quarantined horses are being closely monitored and there is no indication at this time that the virus has spread to other horses beyond those being quarantined, and there are no traces to other states.

The Oregon Department of Agriculture will continue to work closely with all involved parties and is monitoring the situation very closely.

Equine Herpes Virus (EHV-1) infection in horses can cause respiratory disease, abortion in mares, neonatal foal death and neurological disease. The neurological form of the disease is known as Equine Herpes Myeloencephalopathy (EHM) and may be caused by damage to blood vessels of the brain and spinal cord associated with EHV-1 infection. EHM is most often due to the neuropathogenic strain of EHV-1, but may occasionally be caused by the non-neuropathogenic strain of the virus.

EHV-1 is easily spread and typically has an incubation period between 2-10 days. Respiratory shedding of the virus generally occurs for 7-10 days, but may persist longer in infected horses. For this reason, a twenty-one day isolation period of confirmed positive EHM cases is recommended along with stringent biosecurity protocols. Similar to herpes viruses in other species, the latent form of EHV-1 can reactivate at a later date, 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 an indirect mode of transmission.