BOZEMAN, Mont. (AP) - Some calves and yearling bison that test negative for exposure to brucellosis will be vaccinated against the contagious disease for the first time this winter, a wildlife biologist in Yellowstone National Park said Monday.

"It's a step in trying to get to a disease-free population eventually," said Rick Wallen.

But no more than 125 buffalo will get the vaccine in the pilot project, he said.

There is disagreement about the effectiveness of the vaccine, known as RB 51.

It will be given to certain animals that enter the National Park Service's bison trap northwest of Gardiner, if they go there this winter.

Any animals that test positive for exposure to the disease will be shipped to slaughter, Wallen said.

The nonpregnant yearlings and calves that are vaccinated will then be held in a pasture until enough green grass sprouts in the park interior to keep them there.

Last winter, all captured animals were shipped to slaughter without being tested for the disease. There are about 4,200 bison in the park, which is at or near a record number.

A joint state and federal bison management plan completed in 2000 called for vaccinating bison as soon as the vaccine could be proven to be safe, Wallen said, and the federal government believes the vaccine is safe. However, it may take years to find out how well it works.

A small ear tag will be attached to the animals that are vaccinated. If they are captured again in future years, they will be tested again for the disease to get some indication of the vaccine's effectiveness, he said.

The Montana Department of Livestock, which operates two bison traps on the west side of the park, won't begin vaccinating animals before state and federal environmental analyses are complete, state veterinarian Tom Linfield said Monday.

Once those documents are complete, he said, DOL will consider starting a similar program, depending on the results of the studies.

He said vaccinating a relatively small number of young animals won't eliminate the disease, but could reduce its frequency.

"It's obviously not the silver bullet, but it is one of the tools in the toolbox," he said.

RB 51 hasn't yet been proven effective in bison, and capturing and vaccinating the young bison subjects them to needless handling with no proof the disease situation will improve, said Michael Scott, executive director of the Greater Yellowstone Coalition.

"This is demonstrating the political nature of this, rather than the scientific nature," Scott said.

Grazing animals usually contract brucellosis by exposure to infected birthing material or aborted fetuses.

The bacterial disease can cause animals - including domestic cattle - to abort during the first pregnancy after exposure. It also can cause flu-like undulant fever in humans.

In experiments, about 60 percent of vaccinated bison gave birth to viable calves after being exposed to the disease. Less than 20 percent of unvaccinated bison gave birth to viable calves after exposure, Wallen said.

Information from: Bozeman Daily Chronicle, http://www.gomontana.com