BAKER CITY — As the helicopter flew over the ridges and canyons of eastern Baker County recently, Brian Ratliff was seeing more bighorn sheep than he expected.

But the news wasn’t exclusively of the good variety.

It was the sheep Ratliff didn’t see during the flight over the Lookout Mountain unit — lambs — that define the continuing threat to the future of Oregon’s biggest herd of Rocky Mountain bighorns.

Ratliff, the district wildlife biologist at the Oregon Department of Fish and Wildlife’s Baker City office, counted just four lambs during the aerial census.

All of those lambs, born in 2021, were in small groups of sheep in the northern part of the unit, which is bordered on the north by state Highway 86 and on the south by Interstate 84.

The scarcity of lambs shows that a bacterial infection remains widespread in the Lookout Mountain unit and puts the long-term health of the herd in peril.

ODFW biologists believe all of the 65 to 70 lambs born in the unit in spring 2020 died due to the same strain of Mycoplasma ovipneumoniae bacteria.

It was first detected in the Lookout Mountain herd, which included about 400 bighorns, in February 2020 when dead sheep were found near the Snake River Road above Brownlee Reservoir.

Lab tests of tissue samples from dead sheep confirmed the strain of bacteria, the first time it had been found in bighorn sheep in Oregon.

Half a herd

During an aerial count in late 2018, biologists counted 403 bighorns in the Lookout Mountain unit. A survey in late 2020 showed about 250 sheep.

Ratliff said he expected about 225 in this year’s count, so he was somewhat pleased by the total of 274 animals. About 62% of the bighorns, however, were distributed among several large groups in one canyon in the south part of the unit. Ratliff didn’t see any lambs in that area.

He said that’s strong evidence the bacteria, which sheep can easily spread among themselves, is still present in those larger groups and likely killed all of this year’s lambs.

“In the smaller subgroups there’s less chance of lambs being infected,” Ratliff said.

Biologists were initially optimistic in 2020, with no dead lambs found as of mid-June. They knew, from earlier testing, that lambs aren’t infected by their mothers prior to birth.

But later in summer 2020, as ewes and lambs started to congregate in larger “nursery” groups, lambs started to sicken and die.

Finding the ‘shedders’

The key to saving the Lookout Mountain herd — and the smaller herd of California bighorns, a smaller subspecies, in the Burnt River Canyon south of I-84 — is finding the sheep that are chronic “shedders” of the bacteria that causes potentially fatal pneumonia, Ratliff said.

That’s the focus of a multi-agency effort that started in 2020 and likely will continue for several years.

This campaign relies heavily on temporarily capturing bighorns, testing them for the bacteria, and fitting them with GPS tracking collars.

Chronic shedders often don’t get sick, but they can quickly spread the bacteria to other sheep that lack high levels of immunity, Ratliff said.

Ewes that are chronic shedders are especially problematic because they mingle with other ewes, and lambs, more often than rams, which are either solitary or with other rams much of the year.

Lambs are especially vulnerable, as the nearly wholesale loss of the youngest animals the past two years attests.

Last fall, ODFW, with financial aid from the Confederated Tribes of the Umatilla Indian Reservation, as well as the Oregon and national chapters of the Foundation for North American Wild Sheep, captured 25 bighorns from the Lookout Mountain herd. Although all 25 of those sheep had antibodies in their blood showing they had been infected with the bacteria, just four of the 25 were shedding bacteria at that time, Ratliff said.

Biologists fitted all those sheep with tracking collars so they can be captured again and retested.

‘We’ve just got to figure out who it is’

So far this fall, ODFW has captured, tested and collared 14 more sheep from Lookout Mountain.

None was a chronic shedder, Ratliff said, although test results from two of the bighorns were inconclusive, so it’s not clear whether they are shedders or not. One of the 14, a ewe, was a chronic shedder identified in 2020, but was not shedding this fall.

Sheep that are trapped twice and are chronically shedding both times will be euthanized, Ratliff said.

So far, ODFW hasn’t euthanized any bighorns from Lookout Mountain since none has twice been identified as a chronic shedder.

Ratliff said it’s possible the herd could recover from the outbreak by virtue of the chronic shedders dying naturally. Over time, more sheep are likely to gain immunity from a previous exposure to the bacteria, as well.

On the other hand, just a few chronic shedders could potentially keep the bacteria circulating within the herd, and decimating each year’s crop of lambs.

“We’ve just got to figure out who it is,” Ratliff said.

To maintain the herd population requires a minimum of 20 lambs per 100 ewes, he said.

The average ratio for the Lookout Mountain herd is 38 lambs per 100 ewes, and the number has ranged from a high of 67 per 100 to a low of 24.

Ratliff said he expects to see a typical crop of lambs born in the Lookout Mountain herd late in the coming spring.

During the aerial survey this month, he said he didn’t see any evidence of sick sheep — bighorns that develop pneumonia from the bacteria typically cough and stumble. And based on 2020 and 2021, it doesn’t appear the bacteria is affecting the reproductive capacity of the herd.

Ratliff said biologists don’t know how the Lookout Mountain herd was initially infected with the bacteria.

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