Chet Brokaw, Associated Press
A prairie dog stands at a burrow in South Dakota. Prairie dogs have spread the sylvatic plague to ferrets.

INTERIOR, S.D. — On the grasslands a few miles from the pinnacles and spires of Badlands National Park, federal wildlife officials have been waging a war since spring to save one of the nation's largest colonies of endangered black-footed ferrets.

The deadly disease sylvatic plague was discovered in May in a huge prairie dog town in the Conata Basin. The black-tailed prairie dog is the main prey of ferrets, and the disease quickly killed up to a third of the area's 290 ferrets along with prairie dogs.

The disease stopped spreading with the arrival of summer's hot, dry weather, but it poses a serious threat to efforts to establish stable populations of one of the nation's rarest mammals, said Scott Larson of the U.S. Fish and Wildlife Service in Pierre.

The plague, which is carried by fleas, is the biggest danger to ferrets' survival in the Conata Basin and other sites that still have ferrets, said Larson, who is coordinating ferret conservation efforts among five federal agencies.

"It has the capacity to take out more ferret habitat than anything we've run up against, and do it in such a short order," Larson said. "For ferrets, it's the most challenging issue we face."

The ferrets were once considered extinct. But one colony was discovered in Wyoming in 1981, and a captive breeding program succeeded in increasing their numbers. Since then, ferrets have been reintroduced at 17 sites in South Dakota, Wyoming, Montana, Colorado, Utah, Kansas and Mexico, said Nancy Warren, endangered species program leader in the Rocky Mountain Region of the U.S. Forest Service.

Reintroduction efforts failed in some locations, and plague has hit most of the ferret colonies to some degree, Larson said.

Of the 25,000 acres of prairie dog habitat managed for ferrets in the basin, the plague had spread to about 9,700 acres before its growth halted in August. Officials expect the plague might start spreading again this fall or next spring. The disease has not been found inside Badlands National Park itself.

About 5 to 15 people are infected by plague in the United States each year, but it can be cured with antibiotics if treatment is prompt.