Brian Ratliff is embroiled in a mystery nearly three years old, and the fate of Baker County’s biggest herd of bighorn sheep might well depend on the solution.
The list of clues, fortunately, is growing longer.
Ratliff is not a detective.
He’s a wildlife biologist.
He oversees the Oregon Department of Fish and Wildlife’s (ODFW) Baker City office.
For almost three years, Ratliff and his colleagues, both at ODFW and from the Confederated Tribes of the Umatilla Indian Reservoir, have been investigating a bacterial disease that threatens the future of bighorn sheep in the Lookout Mountain unit east of Baker City.
Lab tests in early 2020, from samples of dead bighorns found near Brownlee Reservoir, confirmed a strain of Mycoplasma ovipneumoniae bacteria not previously found in Oregon bighorns. The bacteria causes pneumonia that can kill bighorns.
Other strains of the bacteria, which have previously infected bighorn herds in Oregon, can also lead to fatal pneumonia.
Ratliff said bighorns, despite being stout and otherwise hardy animals, 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.”
Ratliff estimated that about 75 adult sheep, and the entire 2020 crop of lambs, around 65 to 70 animals, died that year.
ODFW has not identified the source of the bacteria.
The Lookout Mountain herd, which consisted of about 400 sheep, was the biggest group of Rocky Mountain bighorns in Oregon.
Biologists later in 2020 confirmed that the bacteria had also infected California bighorns, a somewhat smaller subspecies, in the Burnt River Canyon herd in southern Baker County. The Burnt River Canyon herd is smaller, with around 85 animals typically.
The two herds, separated by Interstate 84, sometimes mingle.
Although biologists relatively quickly identified the infectious culprit in both the Lookout Mountain and Burnt River Canyon herds, protecting the sheep has proved to be a much more daunting challenge.
Since 2020, ODFW, with financial assistance from the Umatilla tribe, Oregon and national wild sheep foundations and the federal government, has trapped about 200 bighorns, testing them to see if they had been exposed to the bacteria or were still infected, and fitting GPS tracking collars to several dozen animals so biologists can monitor their movements.
The ultimate goal, Ratliff said, is to identify “chronic shedders” — adult sheep that survive the infection but can easily infect other sheep — and then kill those shedders.
So far, ODFW has killed two adult sheep, one from Lookout Mountain and one from Burnt River Canyon, that were tested three times and each time were identified as chronic shedders.
The Lookout Mountain sheep was killed in February 2022, after its third positive test, and the Burnt River Canyon sheep in March 2022, Ratliff said.
Removing chronic shedders is vital, he said, because even a handful of these sheep can keep the bacteria circulating among a herd, dooming each year’s crop of lambs.
Based on an aerial count done in the fall of 2022, Ratliff estimated the Lookout Mountain bighorn population at 250 to 270 (biologists actually counted about 230 animals). As few as four or five chronic shedders, given the social structure of bighorns, which tends to bring together groups of sheep during the winter, could thwart efforts to eradicate the bacteria, he said.
The trapping and testing task continues. Workers from Baker Aircraft shoot net guns from helicopters to immobilize the bighorns, and then biologists do the testing and collar fitting on site, so the animals don’t have to be moved, Ratliff said.
In late December ODFW trapped and tested 12 sheep in the Burnt River Canyon. Ratliff is still waiting for lab results to determine which, if any, of those sheep is a chronic shedder and thus will be killed.
And this week the agency was scheduled to catch around 35 sheep from the Lookout Mountain herd.
Ratliff said the sheep will be tested for the bacteria, and biologists will fit 30 of them with a GPS collar. That will bring the total of collared sheep to 82, most from the Lookout Mountain herd.
An education in infectious diseases
In the era of COVID-19, some of the terms Ratliff uses have become familiar — nasal swabs and PCR tests and antibody levels.
And indeed the surveillance of bighorns is comparable to efforts to reign in the pandemic, although in the case of the sheep the infectious agent is bacterial rather than viral, as with COVID-19.
Biologists conduct two tests on each trapped bighorn.
The nasal swab can reveal the presence of the bacteria in the sheep’s nasal cavity, an indication that it is actively shedding the bacteria, Ratliff said.
The animal’s blood is scanned for antibodies to the bacteria.
The presence of the specific antibody means the sheep has been exposed to the bacteria, and the concentration gives biologists an estimate of how long ago the most recent exposure was, he said.
Testing over the past two years indicates that the initial 2020 outbreak, when the bacteria was widespread and both adult sheep and lambs were dying, has waned, Ratliff said.
“We’re not picking up much mortality, if any, among adults,” he said.
That’s not the case, however, with lambs.
It’s likely that all lambs born in 2020 and 2021 died.
Biologists saw lambs in the spring and summer of both years, but none was counted during the late fall aerial census.
As with many infections, the youngest animals tend to be especially vulnerable to the bacteria-induced pneumonia, Ratliff said.
The situation is slightly more promising now.
One group of Lookout Mountain sheep had five lambs, born in the spring of 2022, that were still alive during the fall survey, Ratliff said.
The voluminous data from the more than 50 GPS collars shows that the group with surviving lambs didn’t mingle during the summer and fall with other groups which, due to the lack of lambs, likely include at least one chronic shedder, he said.
That those lambs survived also implies that the group of which it is a member does not have a chronic shedder.
Ratliff said he’s concerned that the group with lambs will end up congregating with other groups this winter, possibly putting the lambs at risk.
One reason for optimism, though, is the absence of lambs in those other groups.
Ratliff said lambs, like children, tend to play together. So even if the group with surviving lambs mingles this winter with groups that include an adult that’s a chronic shedder, it’s possible that the lambs won’t have nose-to-nose contact with that shedding sheep, and thus survive.
In any case, those lambs, as they near their first birthday, are likely to be hardier and more likely to survive even if they are exposed to the bacteria, he said.
Pinpointing the problem
Ratliff said his confidence is growing in ODFW’s ability to identify and remove the chronic shedders from both Lookout Mountain and Burnt River Canyon.
“Things are starting to line up, allowing us to use a more targeted approach,” he said on Tuesday, Jan. 10. “We’re starting to understand how to move forward.”
That’s due largely to tracking collar data and previous test results, both of which give biologists stronger evidence of which bighorn social groups include chronic shedders.
As ODFW eliminates more of those sheep, Ratliff said the survival rate among lambs should increase.
The long-term survival of the herds depends on that.
He said the current lamb ratio in the Lookout Mountain is about 3 per 100 ewes. In the Burnt River Canyon herd it appears that at most one lamb from the 2022 crop has survived, yielding a ratio of about 1.8 lambs per 100 ewes.
To even maintain a herd population requires a ratio of about 30 lambs per 100 ewes, Ratliff said.
The current ratios, he said, “might as well be zero.”
Ratliff is eager to get test results from both the Lookout Mountain and Burnt River Canyon herds, and to use that data to hone the strategy to find chronic shedders.
“It’s like a murder mystery, and we’re trying to figure it all out,” he said. “Sometimes it can be a little overwhelming. You’re never going to have every piece of data when you’re dealing with a wild population. But it’s starting to look better.”
Ratliff said the campaign to protect Baker County’s bighorns is part of a wider project, including Idaho, Washington and Nevada, that includes the testing of multiple strategies to deal with disease outbreaks in bighorn herds.
The local strategy is to identify and removing chronic shedders. In Washington officials are experimenting with reducing the population of herds, and another strategy is to let the outbreak run its course for a couple years and then reassess the situation, he said.
Ratliff is preparing a presentation he’ll give at the annual Sheep Show, a gathering of bighorn sheep hunters and biologists, set for Jan. 12-14 in Reno, Nevada.