HELENA (AP) — The National Park Service has decided against shooting wild bison with vaccine-laced “biobullets” to prevent the spread of an animal disease.

Park Service Regional Director Sue Masica signed a record of decision earlier this month that leaves in place the occasional syringe vaccination of bison calves and yearlings that are captured outside the northern boundary of the park.

For more than a decade, wildlife officials have weighed shooting Yellowstone bison with absorbable, vaccine-laced bullets to prevent the spread of the disease brucellosis to livestock. The concept was supported by cattle ranchers.

About half of Yellowstone’s 4,600 bison test positive for the disease, which causes pregnant animals to prematurely abort their young.

Yellowstone’s chief scientist, David Hallac, has said vaccinations would cost $300,000 annually but do little to drive down infection rates.