Potentially fatal bacteria found in Oregon bighorn sheep herd

11 Sep 2026, 19:43 by Jayson Jaco · The Seattle Times

Bacteria that can cause fatal pneumonia in bighorn sheep and has been affecting Baker County’s two bighorn herds since 2020 was recently detected for the first time in the state’s largest bighorn herd, along the John Day River in north-central Oregon.

Several bighorn sheep from the John Day River herd, which numbers an estimated 1,000 animals, recently tested positive for Mycoplasma ovipneumoniae — M. ovi — a respiratory pathogen, according to the Oregon Department of Fish and Wildlife.

“Many bighorn populations in the state have felt the impacts of pneumonia over the years,” Trent Gianella, assistant wildlife biologist at the ODFW office in The Dalles, said in a news release Tuesday. “But the John 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.”

ODFW is trying to gauge how widespread the bacteria is, and to identify the strain of bacteria by having samples from dead sheep tested at a Washington lab.

In early 2020, tests from samples from dead bighorns found in eastern Baker County, in the Lookout Mountain unit, contained a strain of M. ovi that had not been detected before in Oregon.

Other strains of the bacteria have infected other herds and caused widespread deaths among bighorns, including in the Owyhee Mountains and in Hells Canyon, said Brian Ratliff, district wildlife biologist at ODFW’s Baker City office.

Although the bacteria has not been killing adult bighorns in the Lookout Mountain for the past few years, Ratliff said few if any lambs have survived the infection, which threatens the future of what has been Oregon’s biggest herd of Rocky Mountain bighorns, one of two subspecies in the state.

The John Day River herd sheep are California bighorns, which are somewhat smaller than the Rocky Mountain subspecies.

The Lookout Mountain herd, which was started when ODFW transplanted sheep in the early 1990s, once numbered about 400 animals, but the population has dropped to around 250 due to the bacterial infection.

Ratliff said that because of his ongoing experience with a bighorn herd infected with M. ovi, he will be part of a group of ODFW officials who will come up with a strategy to try to protect the John Day River herd.

The Fox fire, which burned 79,000 acres in late July and early August, also poses a threat to the surviving bighorns, as it burned most of the herd’s habitat, Ratliff said.

The strain of M. ovi detected in the Lookout Mountain herd also has infected the Burnt River Canyon herd, a group of California bighorns in southern Baker County that biologists believe mingle at times with the Lookout Mountain sheep.

Ratliff has said in previous interviews that ODFW officials don’t know the original source of the bacteria, and likely never will.

There is no vaccine or treatment for the bacteria, according to ODFW.

One of the first goals when disease infects a bighorn herd is to reduce or eliminate contact between the sheep and domestic animals, such as sheep, goats and the exotic species aoudad, a Barbary sheep, that can spread bacteria and viruses to bighorns.

“As an agency we have had an exceptional collaborative relationship with landowners adjacent to the John 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 in the news release. “A newer challenge to that management goal is a growing herd of aoudad in the region.”

Although there is no cure for M. ovi, wildlife managers across the West have used multiple tactics to reduce the spread and longevity of disease outbreaks, such as removing infected individuals or reducing herd densities to slow the rate of spread, according to the news release.

In the case of the John Day herd, ODFW biologists will explore options as they gather data about the severity of the disease outbreak.

ODFW will be working with the Confederated Tribes of the Warm Springs and Confederated Tribes of the Umatilla Indian Reservation to evaluate findings and develop plans to manage the disease and keep it from spreading to other herds.

In Baker County, Ratliff said ODFW officials have been trying to find bighorns that are “chronic shedders” — sheep that are not sick but are infected with the bacteria and can spread it to other bighorns.

Ratliff has said it’s possible there is only one chronic shedder in the herd — a sort of bighorn Typhoid Mary. ODFW will kill that sheep if the agency can find it and confirm through testing that it is shedding the bacteria.

ODFW is asking hunters and others recreating in the John Day River Canyon this fall to notify an ODFW office if they see any dead bighorns, or sheep that appear to be ill. Bighorns with pneumonia often cough and wheeze and struggle to climb steep slopes, something at which healthy bighorns excel.

ODFW allocated 27 hunting tags this fall for bighorn rams in the John Day River area, along with 28 tags for a ewe hunt. The hunts are in September, October and November.

Adam Baylor, an ODFW spokesperson, said on Thursday that ODFW doesn’t plan to cancel any hunts. He said the agency is hoping to collect sheep tissue samples from successful hunters.