BILLINGS, Mont. – Federal officials say the shipment of bison to slaughter from Yellowstone National Park is on hold after the plan was challenged in court by wildlife advocates.

Almost 400 of the animals were being held Friday in corrals inside the park for testing to see if they have been exposed to the disease brucellosis.

Officials had planned this week to begin sending to slaughter bison that test positive.

Environmental and American Indian groups on Thursday asked a federal judge in Helena to block the shipments. Yellowstone spokesman Al Nash says no shipments are expected while the legal challenge is reviewed by the Park Service.

Brucellosis can cause wildlife and livestock to prematurely abort their young. About half the park's bison carry the disease, although no bison-to-cattle transmissions have been recorded.