Disease outbreak hits John Day River bighorn sheep population

THE DALLES, Ore. (KTVZ) -- Several California bighorn sheep along the John Day River have tested positive for Mycoplasma ovipneumoniae, marking the first known detection of the respiratory pathogen in the herd.

The Day River bighorn sheep herd is Oregon's largest, with an estimated 1,000 individuals.

The pathogen, commonly known as M. ovi, is associated with severe pneumonia outbreaks in wild sheep populations and there is no known vaccine or medical treatment available. Oregon Department of Fish and Wildlife staff are conducting necropsies on dead sheep, submitting samples to the Washington Animal Disease Diagnostic Laboratory and surveying the canyon to assess the extent and specific strain of the outbreak.

Trent Gianella, an assistant wildlife biologist with ODFW in The Dalles, noted that the herd had previously avoided the disease. “Many bighorn populations in the state have felt the impacts of pneumonia over the years,” Gianella said. “But the Day bighorn herd has stood out as a stronghold for its size and health both in Oregon and across the West as until now we had not detected M. ovi in this herd. So, this is disappointing news.”

In the past, M. ovi outbreaks have heavily affected bighorn populations in areas such as Hells Canyon and the Owyhees. Disease management generally focuses on reducing or eliminating contact between wild bighorns and potential disease vectors, including domestic goats, domestic sheep and exotic aoudad.

Aoudad, also known as Barbary sheep, were introduced to central Oregon through a private game ranch in the late 1970s. Gianella explained that agency efforts rely heavily on regional cooperation. “As an agency we have had an exceptional collaborative relationship with landowners adjacent to the Day canyon and have worked to ensure we collectively managed bighorns to reduce the risk of contact between potential disease vectors and wild sheep,” Gianella said. “A newer challenge to that management goal is a growing herd of aoudad in the region.”

Infection outcomes vary depending on the specific bacterial strain, with some herds experiencing short-term impacts while others face chronic health issues lasting for decades. Wildlife managers across the West have used tactics such as culling infected individuals or lowering herd density to curb transmission rates and local biologists will evaluate potential management options as more data on disease prevalence becomes available.

ODFW will work alongside key partners, the Confederated Tribes of the Warm Springs and the Confederated Tribes of the Umatilla Indian Reservation in the coming months to review findings and develop plans to manage the disease and prevent its spread to other herds.

State officials ask anyone recreating in the Day River canyon this fall to contact their local ODFW office if they spot any dead or potentially sick bighorn sheep.