The Bighorn Mountains are a sister range to the Rocky Mountains and were named for their herds of wild, big-horned sheep. Today, the herd is mostly found around Devil’s Canyon, which is located in the Bighorn Canyon National Recreation Area, just north of the border between Wyoming and Montana.

The sheep were driven out of the area by hunters in the 1800s but were reintroduced in the 1970s. Their population has steadily grown to around 250 to 300 individuals, but the recent outbreak has wiped out over 10 percent of the herd.

GPS collars were previously added to 24 members of the herd to monitor their distribution, habitat use, seasonal migrations, and survival rate. It was one of these GPS collars that alerted the Game and Fish Department to the mysterious deaths, combined with a report from a local landowner.

Corey Class, the wildlife management coordinator for the Cody Region, said that the majority of deaths occurred in ewes and lambs. Nasal and tonsil swabs were gathered from the sheep carcasses and analyzed in a lab in Laramie to identify any potential infections, and the results returned a clear culprit: Mannheimia haemolytica.

Mannheimia haemolytica is a species of bacteria that is known to cause lethal pneumonia in sheep, killing its victims relatively quickly. Class said: “The source of the infection is unknown.”

“Mannheimia haemolytica was present in the herd already, but this is a different strain; [a] more virulent strain I guess, [a] more pathogenic strain,” Class said in a video produced by the Game and Fish Department.

Wildlife officials are working to remove any sheep carcasses in an effort to stop the spread of the disease while the source of the infection is still unknown. Class said that the GPS collars will now also play a useful role in tracking the disease as well as the herd’s movements.

“The collars will now help us monitor bighorn sheep populations from a disease perspective and document additional mortalities,” he said.

Do you have an animal or nature story to share with Newsweek? Do you have a question about bighorn sheep? Let us know via nature@newsweek.com.

References:

Mason, C., Errington, J., Foster, G. et al. Mannheimia haemolytica serovars associated with respiratory disease in cattle in Great Britain. BMC Vet Res, January 3 2022, https://doi.org/10.1186/s12917-021-03121-3