"We've been working on bison conservation for over 100 years," Plumb said. "This population exemplifies that success."

Plumb said the brucellosis carried within the park herd most likely was contracted from infected livestock brought in by early settlers.

Other large animals, including elk, also carry brucellosis.

Elk were blamed for transmitting the disease to a Montana cattle herd this spring. The outbreak led to the slaughter of almost 600 cows from the infected ranch and put Montana at risk of losing its brucellosis-free designation, which could still happen if a second outbreak occurs.

If that happened, ranchers across the state likely would be forced to implement a costly testing and vaccination program.

After a 2003 outbreak in Wyoming, it took the state three years to regain its disease-free certification, costing the state and its livestock producers an estimated $9 million.