Horses in Wyoming Test Positive for Equine Herpesvirus (EHV-1) Myeloencephalopathy

January 26, 2022

The Wyoming Livestock Board (WLSB) received laboratory confirmation of a Park County horse infected with Equine Herpesvirus (EHV-1) Myeloencephalopathy Wednesday, January 26. The affected horse and approximately 40 herd mates are quarantined to the operation near Cody where they have resided without addition of new horses for the past 6 weeks. The epidemiological investigation revealed contact between the affected herd and two outside herds. Horses in the contact herds are being monitored for respiratory and neurologic disease and have remained free of disease since the exposure date. Both

EHV-1 is the very common Rhinopneumonitis virus which typically causes mild respiratory symptoms in infected horses, but in rare cases may cause severe neurologic signs.

To date, while infection has been confirmed in only one horse, multiple horses have shown clinical signs consistent with Equine Herpesvirus Myeloencephalopathy. The first affected horse began showing neurologic signs early the second week of January. The most recent horse to develop clinical signs became apparent Saturday, January 22. This affected herd will remain under quarantine until 21 days after the resolution of clinical signs.

The source of infection in the quarantined herd is unknown. EHV-1 often takes up residence in host nerve cells after the initial infection. From that nerve cell residence, EHV-1 can then recrudesce (basically, establish a new clinical infection with shedding of infectious virus) when a horse’s immune system falters. The epidemiologic evidence indicates that was the source of the outbreak in this herd.

More information on Equine Herpes Virus can be obtained by contacting the Wyoming Livestock Board Office at 307-777-7515. Detailed background information is available from the American Association of Equine Practitioners at

Source: Wyoming Livestock Board