BISMARCK, N.D. (AP) — At least 20 bighorn sheep died from pneumonia last month in western North Dakota's badlands after mingling with domestic sheep, the state Game and Fish Department said.

Biologist Brett Wiedmann said 14 of the dead bighorns were among a group of two dozen animals that had been transplanted from Alberta, Canada in February.

He said the bighorns from Canada may be especially vulnerable because they were pristine previously from outside contact with domestic sheep.

“It went from being a great project to a nightmare. I hate to see it,” Wiedmann told the Bismarck Tribune. “It's terrible, the worst thing in the world. They are such a magnificent animal.”

Wiedmann knows that 20 are dead for sure because each was wearing a radio collar and he could pinpoint its location. He hiked out to each corpse and took field samples for the veterinarian. One was a state-record ram, with the biggest horn set yet found, he said.

Bighorn sheep are susceptible to a respiratory infection from domestic sheep. The form of pneumonia is fatal to wild sheep, and has killed bighorns in the badlands before, the agency has said.

“They have no resistance to the bacteria carried by domestic sheep. It's deadly,” Wiedmann said.

North Dakota has about 350 bighorn sheep located mostly in the badlands.

Bighorn sheep are native to the state, but they were wiped out by 1905 because of over-hunting and disease. Biologists say the animals were reintroduced in North Dakota in 1956.

Dan Grove, a wildlife veterinarian with the agency, said like in humans, a death from pneumonia means the animal drowns in fluid buildup in its lungs.

“It's pretty gruesome,” he said.

Grove said it's the worst bighorn die-off he's seen.

“We're trying to be optimistic. The mortality has slowed down, but to think this is completely over would be foolish on our part,” Grove said

Wiedmann said this year's bighorn sheep hunting season, which opens Oct. 31, likely won't be affected.

Only five licenses were issued, including one at an auction that raised $70,000.

“At this point we feel we can proceed,” he said.

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Information from: Bismarck Tribune, www.bismarcktribune.com