Testing from a northwest South Dakota prairie dog confirms plague as the cause of a late May prairie dog die off, according to the National Park Service and the U.S. Forest Service. At stake is the largest free-ranging black-footed ferret population in the world.

“We are concerned about the impacts to the black-footed ferret population and are taking necessary action to protect this important keystone species,” said Eric Veach, superintendent of Badlands National Park.

The disease has been detected in Badlands National Park, Buffalo Gap National Grassland, and the greater Conata-Badlands ecosystem, where the largest free-ranging black-footed ferret population in the world now resides. The disease has not been detected in Conata Basin since 2009, when it was responsible for killing more than 80% of the remaining population black-footed ferrets, one of the most endangered mammals in the world.