BILLINGS, Mont. (AP) — US officials were transferring 33 disease-free bison Monday from Yellowstone National Park to an American Indian reservation in Montana as part of efforts to reduce the slaughter of bison that migrate from the park.

Robert Magnan with the Fort Peck Tribes said the shipment was expected to arrive on the northeastern Montana reservation later in the day.

The shipment included bison and descendants trapped by park administrators under a legal agreement with the state of Montana that’s intended to prevent the spread of the disease brucellosis.

The bison were tested repeatedly during a Yellowstone-area quarantine run by the U.S. Department of Agriculture to make sure they were disease-free.

“We’ve run enough animals through this process that I’ve got confidence it’s effective,” Montana State Veterinarian Martin Zaluski said.

Brucellosis can cause pregnant animals to abort their young. It’s been eradicated in the U.S. except for the Yellowstone area, where it persists in herds of elk and bison.

Yellowstone has captured thousands of bison and sent them to slaughter for disease control over the past few decades.

The quarantine program has been in place for more than a decade but struggled in its early years amid strong opposition from the livestock industry. Some elected officials worried about cows becoming infected.

Fort Peck has previously received Yellowstone bison, including 55 bulls over the summer. But those moved Monday included the first group of females to be transferred under current brucellosis testing protocols. The disease typically is transmitted through females, meaning they present a higher risk to livestock.

“The many years of trying to come together between the state, the tribes, the feds and (outside groups) have finally come to fruition,” said Chamois Andersen with Defenders of Wildlife, which helped pay for the shipping costs.

“This is a really good sign when you have these females that are the most scrutinized” being transferred, she added.