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Pneumonia threatens success of bighorn sheep introduction near Deadwood

Nearly four years after bighorn sheep made their return to the northern Black Hills, a pneumonia outbreak is hampering the herd’s population growth. But by removing a few troublesome individuals, wildlife officials say they can return the herd to health.

Mortality in the Deadwood herd has been high since 2016. Trenton Hafley, Regional Terrestrial Resources Supervisor for South Dakota Game, Fish and Parks, says that pneumonia has been the biggest killer. Still, it’s not the only culprit.

“We had some cliff falls, some vehicle collisions, and one actually got her head stuck under a log and she starved to death,” Hafley said.

Those accident- and disease-related deaths bring the population of the Deadwood herd to 17 sheep, according to GF&P’s most recent count in the summer of 2018.

Bighorn sheep disappeared from the northern Black Hills around the early 1900s. That changed in 2015, when GF&P captured 26 bighorn sheep from Alberta Canada and released them in the Deadwood area to re-establish a wild population.

But in 2016, one of the bighorn sheep contracted pneumonia — likely from a domestic sheep. That individual then spread the bacteria to other members of the Deadwood herd, said Thomas Besser, a bighorn-sheep disease researcher at the Washington State University College of Veterinary Medicine.

“In most outbreaks, only a single animal contacts the source of the bacteria [domestic sheep] and gets infected,” said Besser, whose lab has monitored the herd’s health. “From then on, it’s all wild sheep to wild sheep.”

The epidemic has wreaked havoc. Although many of the relocated Canadian sheep reproduced successfully in the Black Hills, 25 of those original 26 animals have since died.

“They’re all gone except for one,” Hafley said. “The sheep we have left now are 1- and 2-year-old sheep that were born in South Dakota.”

According to Hafley, pneumonia is particularly deadly for a seemingly counterintuitive reason: the disease doesn’t always kill the sheep it infects.

“Some of the adults will live through it [pneumonia],” Hafley said. “They’re outwardly healthy… but they'll still be carrying the bacteria that causes it.”

This becomes a problem when lambs are born.

“Lambs are most susceptible,” Hafley said. “So when they [infected adults] come in contact with the lambs, the lambs die.”

For Hafley, this cycle is reminiscent of “Typhoid Mary,” an Irish-American cook in the early 20th century who carried the bacteria that causes typhoid fever. “Typhoid Mary” didn’t get sick herself, but reportedly infected dozens of people who came in contact with her.

Besser said that up to 10-20 percent of sheep who survive pneumonia may continue to infect new lambs in this way.

Other bighorn sheep herds in the Black Hills are also suffering from pneumonia, according to Hafley. The Rapid City herd has been particularly hard-hit, having dropped from about 300 sheep in the early 2000s to only 40-50 individuals currently. Custer State Park’s herd has also seen a dramatic population decrease due to pneumonia.

But herds in Jewel Cave National Monument, Badlands National Park, and Elk Mountain remain healthy.

Hafley said that GF&P can treat pneumonia outbreaks by removing the infectious adults — the so-called “Typhoid Marys” — from the population. The agency recently removed two infectious sheep from the Custer State Park herd. Currently, they are removing infectious adults from the Rapid City herd to “clean it up,” Hafley said.

Once the disease is purged from the Rapid City herd, Hafley said, they may begin working on the Deadwood herd.

But until that happens, Hafley expects population growth to be slow. And GF&P won’t issue hunting permits for the herd until the population gets bigger.

Despite the low numbers, Hafley said it’s relatively easy for wildlife viewers to spot members of the Deadwood herd.

“They hang right around Deadwood,” Hafley said. “You can often see them walking down Main Street.”

Even though population growth isn’t as strong as he would like, Hafley remains optimistic about the herd’s future.

“I'm 100 percent confident that in the next couple years, we'll be able to clean up this pneumonia outbreak,” he said. “We’re working in Rapid City right now to get this thing cleaned up, and I expect that Deadwood will go just as well.”

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