Some fear that bison could transmit disease to livestock. Credit: I. ARNDT/MINDEN PICTURES/FLPA

A record number of bison foraging outside Yellowstone National Park have been captured and killed this winter as part of a US plan to try to keep the animals from spreading the disease brucellosis to livestock. The bacterium Brucella abortus can cause spontaneous abortion in cattle.

Although the effort has successfully prevented bison–cattle contact, last week the Government Accountability Office reported that the plan has fallen behind in its other goal of allowing more bison to range outside the park. Since 2000, federal and state agencies have struggled to secure additional land for grazing and find a way to deliver the RB51 brucellosis vaccine to bison calves and yearlings.

An aerial survey last month revealed that the Yellowstone bison population fell from 4,700 last summer to roughly 3,000.