The Wyoming Game and Fish Department (WGFD) is investigating a large die-off of pronghorn antelope in the western part of the state. According to a press release, approximately 200 animals have died since mid-February in an area just south of Pinedale. Preliminary lab results determined that the antelope died from pneumonia brought on by a bacterial pathogen called Mycoplasma bovis.

“This is a nasty, nasty pathogen,” WGFD Wildlife Disease Specialist Hank Edwards told Field & Stream. “Whether it’s in domestic stock or otherwise, it’s just incredibly fatal—particularly in pronghorn. The pneumonia that they’re getting is substantial. It’s killing them pretty damn quickly.”

According to the National Institute of Health (NIH), M. bovis was first detected in domestic cattle in the United States in 1961 and has since spread around the globe, largely via the international cattle trade. It now exists on all continents where cattle are kept. The pathogen was first discovered in mule deer in 2019. It’s also turned up in pen-raised whitetails and domesticated bison.

Though the WGFD says about 200 antelope have died from the disease so far this winter, Edwards thinks the death toll could tick higher once spring comes and Wyoming’s deep snowpack begins to melt away. “We have had a nasty winter, one of the worse we’ve had in a long time,” Edwards said. “I’m sure that didn’t help in terms of these animals getting the disease.”

Severe winter weather has exacerbated the M. bovis outbreak. Wyoming Game and Fish Department.

This is the second time that biologists in Wyoming have seen significant pronghorn antelope deaths due to an outbreak of M. Bovis. “This is similar to an outbreak that occurred in the Gillette area, which is on the northeastern side of the state,” Edwards said. “That outbreak went on for two years in 2020 and 2021, and we lost four or five hundred animals. We were hoping that that outbreak was the last of it—and we haven’t seen it reoccur in the Gillette area—but we were really surprised to see it pop up again on the other side of the state.”  

Related: How to Plan a Pronghorn Antelope Hunt in Wyoming

The WGFD will continue to monitor the disease. So far, the agency hasn’t been detected in any other free-ranging wildlife herds in the Cowboy State.

“We’ve checked our wild bison in the state by both swabs and blood analysis, and we have yet to find any evidence of M.bovis in our bison, but this bug can be really hard to find,” Edwards said. “We’re really hoping that once spring rolls around this outbreak resolves. Unfortunately, we’re a long way from spring here, so it’s hard to know how much longer this outbreak is going to persist.”