BAKER CITY — Two herds of bighorn sheep in Baker County continue to be threatened by a bacterial infection first detected in early 2020 that can lead to fatal cases of pneumonia.

The larger of the two herds is faring better, said Brian Ratliff, district wildlife biologist at the Oregon Department of Fish and Wildlife’s Baker City office.

That’s the herd of Rocky Mountain bighorns in the Lookout Mountain unit of eastern Baker County. The herd, numbering around 280, mainly ranges along the breaks of the Snake River between Richland and Huntington.

Prior to the illness outbreak, the Lookout Mountain herd, with around 400 sheep, was the biggest group of Rocky Mountain bighorns in Oregon.

Ratliff isn’t so optimistic, though, about the bighorns in the Burnt River Canyon in the southern part of the county, west of Durkee.

Based on extensive surveys, including from the air, Ratliff said he doesn’t believe any lambs born in that herd the past two years survived for long.

The herd, he said, is “definitely in jeopardy.”

At the end of 2022, biologists counted 37 sheep in the Burnt River Canyon herd. Ratliff said that’s considered a “minimum” population since no survey can tally every animal.

Nonetheless, the herd is shrinking. Prior to the disease outbreak the estimated population in the Burnt River Canyon was about 85 bighorns.

Sickness detected in 2020

Lab tests in early 2020, from samples of dead bighorns found near Brownlee Reservoir between Richland and Huntington, confirmed the sheep were infected with a strain of Mycoplasma ovipneumoniae bacteria not previously found in Oregon bighorns.

Later in 2020 biologists also confirmed the bacteria in bighorns in the Burnt River Canyon.

The two herds, separated by Interstate 84, sometimes mingle.

ODFW has not identified the original source of the bacteria.

Ratliff said that although bighorn sheep are hardy animals, known for negotiating with aplomb steep, rocky terrain where deer and elk don’t normally venture, the sheep are quite vulnerable to a variety of viruses and bacteria that the sheep can quickly spread through nose-to-nose contact.

“Bighorn sheep getting sick is not new,” he said. “They’re a very susceptible species.”

The animals can be infected by contact with other bighorns and with domestic sheep and other livestock.

The initial wave of illness in 2020 devastated that year’s crop of lambs in both the Lookout Mountain and Burnt River Canyon herds, Ratliff said. The lamb toll in Lookout Mountain totaled around 65 to 70, he said.

Dozens of adult sheep also perished that year, including an estimated 75 in the Lookout Mountain unit.

In the past two years, by contrast, mortality among adult bighorns has dropped dramatically, Ratliff said.

The problem — and it’s one that could potentially imperil the long-term survival of both herds — remains with the lambs.

Three-state experiment

The appearance of the bacteria strain for the first time in Oregon helped lead to a three-state, multiagency experiment.

The project, with financial and technical assistance from the Confederated Tribes of the Umatilla Indian Reservation, Foundation for North American Wild Sheep and the federal government, is intended to measure the effects of what’s known as the “test and remove” strategy.

The idea, Ratliff said, is to identify adult sheep that are “chronic shedders” by capturing the animals and testing their nasal swabs to see if they are carrying the bacteria. These sheep generally don’t get sick, but they can easily spread the bacteria to other sheep. The biggest concern is lambs, since they, unlike the adults, haven’t been previously exposed to the bacteria, Ratliff said.

“Remove,” in this case, means to kill the chronic shedders.

Removing chronic shedders is vital because even a handful of these sheep can keep the bacteria circulating among a herd, dooming each year’s crop of lambs, Ratliff said.

Biologists also test the blood of captured sheep to check for antibodies that show the animals have been exposed to the bacteria even if their nasal swabs are negative, meaning they aren’t shedding the bacteria.

The experiment involves three bighorn herds in Oregon — Lookout Mountain, Burnt River Canyon and Lostine — as well as two in Idaho and two in Washington.

Biologists have captured, tested and fitted with tracking collars close to 100 bighorns between the two Oregon herds, the majority in Lookout Mountain.

That makes it easier to recapture and retest sheep, and it also has given biologists vastly more data than they had before about how the bighorns move throughout the year, Ratliff said.

“It is a unique experience, and an incredible research opportunity,” he said.

The strain of bacteria was found in the Idaho herds around the 1980s, in one Washington herd in 2020 and in the other around 2009-10, according to a report from the team overseeing the experiment.

Ratliff, along with Jenny Dalton, represents ODFW on that team.

So far, ODFW has killed one chronic shedder from Lookout Mountain and six from Burnt River Canyon. Most were killed in 2022 or 2023.

Mixed effects in Baker County herds

Ratliff said there’s clearly at least one chronic shedder in the Burnt River Canyon herd — hence the failure of any lambs to survive there in the past two years.

Lambs were born to ewes in that herd each spring — biologists saw them — but none seemed to have survived even to the end of the year, Ratliff said.

If that trend continues, the herd is doomed, he said.

Although bighorn ewes can in theory give birth up to around age 16, the likelihood that they’ll have a lamb decreases once they reach age 10 or so.

So unless biologists can remove all the chronic shedders from the Burnt River Canyon herd, which would in theory result in lambs surviving, then the herd’s fate is all but certain, Ratliff said.

The chronic shedders will die eventually, too, but he said that might not happen soon enough to save the herd.

In the Lookout Mountain herd, by contrast, Ratliff said 12 lambs born this spring are still alive, up from five surviving lambs from the 2022 crop.

Although having 12 lambs survive is a disappointing number by historical standards, when the Lookout Mountain herd typically produced 70 or so lambs per year, it’s far better than in 2020 and 2021, when it appeared no lambs survived, he said.

The Lookout Mountain herd, in addition to having a much larger population than the Burnt River Canyon herd, has the advantage of distributing sheep among multiple subgroups that typically don’t mingle with others, at least during the spring and summer, Ratliff said.

He suspects that those subgroups which have surviving lambs do not include a chronic shedder.

But because those subgroups can congregate during the winter, even lambs from subgroups without a chronic shedder can be exposed to a chronic shedder, putting them at risk during the winter.

Ratliff said the research team wants to capture and kill one ewe in the Lookout Mountain herd that is a chronic shedder. That sheep has proved elusive, however.

The future

The research team’s plan for 2024 is similar to the previous two years, Ratliff said.

They will continue to trap and test sheep to try to identify chronic shedders, and to trap and kill known chronic shedders.

In the meantime, Ratliff said ODFW has no plans to resume sport hunting of bighorns in either Baker County herd.

Before the bacteria arrived, the state sold a few tags per year for rams in the Lookout Mountain herd, and one annually for Burnt River Canyon.