Young bison that stroll out of Yellowstone National Park could be vaccinated for brucellosis as early as this winter, according to a new federal proposal.

The Animal and Plant Health and Inspection Service is seeking approval for a plan that would allow state and federal workers to inject bison calves and nonpregnant yearlings with a vaccine in an effort to reduce the spread of the disease in the area.

Increasing immunity Although by no means a "silver bullet," the vaccination program could help increase immunity levels in individual bison and the herd overall, said Tom Linfield, Montana state veterinarian and chair of the Greater Yellowstone Interagency Brucellosis Committee. "Vaccination is just another tool," he said. "We would certainly hope it lessens the infection rates."

Brucellosis is a contagious bacterial disease that can affect bison, elk and cattle. In infected animals, the disease can cause abortions, infertility and other problems.

Federal officials estimate that 35 to 50 percent of bison in Yellowstone may be infected with the disease. That notion led to a federal and state plan in 2000 intended to limit contact between Yellowstone bison and cattle that graze just beyond the park's boundaries.

The plan gives agents the authority to push bison back into Yellowstone when they wander out in search of food. In some cases, the bison are captured and sent to slaughter. The plan also encourages the development of vaccines. The APHIS proposal, which was published Friday in the Federal Register, would allow young bison that are captured outside the park to be vaccinated for brucellosis after it's shown they do not have the disease.

In a shot to the neck or shoulder, each eligible bison would get a dose of a vaccine called RB51, which was developed in the early 1980s. The effectiveness of the vaccine "has not been definitively determined," according to APHIS, but there are indications that it could be as effective as Strain 19, an older vaccine that has traditionally been used in cattle.

2 forms of ID Each bison that's vaccinated will receive an ear tag or another kind of permanent identification before being released. Although vaccinated bison may have slightly more freedom in wandering outside Yellowstone during the winter, the program probably won't have a significant effect in the near future on hazing, capturing and sometimes killing bison that leave the park. "We would continue with the agreed-upon ongoing efforts," Linfield said.

All of the bison would have to be cleared from those areas in the spring before cattle begin arriving, Linfield said. "This wouldn't be intended as a year-round, outside-the-park grazing opportunity," he said. Although APHIS officials can't predict exactly how effective the vaccination program would be, their plan states that it should have a positive effect overall.

"This would reduce the potential threat of infection with brucellosis for (Greater Yellowstone Area) cattle and improve the health of wild, free-roaming bison in the GYA," the proposal says. APHIS officials could not be reached for comment Tuesday. The public can comment on the proposal until Jan 5.

Members of the Buffalo Field Campaign, a conservation group that monitors bison management activities and has strongly opposed hazing, capture and slaughter, said they were caught off-guard by the APHIS proposal.

John Osher, a coordinator for the group, said he was at a brucellosis meeting this fall with state and federal officials, and no one mentioned an impending vaccination proposal. "All of a sudden, here comes this program and it's already been decided," Osher said.

Aside from objecting to a lack of the proposal, Osher said his group also questions how effective the vaccination program would be. The effectiveness of RB51 has not been sufficiently proven, he said. "The science is pretty weak," he said. "I don't think they've made a very good case for themselves."

The better option is to vaccinate cattle, he said. But if APHIS wants to pursue the bison vaccine, it should scrap the current proposal and begin again, this time including the public at the outset, he said. While the APHIS plan specifically targets bison, it mentions that a vaccine program for elk "will likely be considered" when a comprehensive brucellosis eradication program is developed for the Yellowstone area.