BILLINGS, Mont. -- Government agencies killed more than 6,000 wild bison leaving Yellowstone National Park over the last two decades -- the grisly result of efforts to contain a serious livestock disease carried by the animals.

But the crosshairs are shifting to a new target, elk, as the disease infects cattle in parts of Wyoming and Montana where bison haven't roamed for decades.

The disease brucellosis causes pregnant cattle to abort their young. To halt recurrent transmissions, state officials say elk from infected herds around Yellowstone must be culled.

That's an explosive proposition for a prized big game species that has thrived under the protection of a dedicated constituency of sporting groups. Nevertheless, pressure is mounting to kill or capture more elk in more places.

"We've got way too many elk," said John Scully, a rancher living in Montana's Madison Valley. "Clearly with so many elk, the risk rises. We need to reduce their numbers."

Outfitters and hunters are digging in against the prospect, concerned that too much hunting could shrink elk herds.They contend wildlife managers should focus on vaccinating cattle or eradicating the disease in bison -- not capturing and killing elk.

"I will fight that tooth and nail. As a sportsman, those wildlife are a public resource," said Bill O'Connell with the Gallatin Wildlife Association.

An estimated 95,000 elk populate the greater Yellowstone area in Idaho, Montana and Wyoming. Experts estimate only a small percentage carry brucellosis.

There is no effective brucellosis vaccine for wildlife, and cattle vaccines are only 60 to 70 percent effective.

Humans are susceptible to the disease, but cases are rare and usually limited to those who work with infected cattle.

Eradicated everywhere else in the nation, brucellosis surfaced seven times in the Yellowstone area this decade, including twice since mid-June. With the recent cases, Montana ranchers near Yellowstone face severe restrictions on out-of-state cattle sales and Wyoming ranchers could face a similar fate if another cow in the state tests positive for brucellosis in the next two years.

For bison, the strategy to prevent transmissions has been brutally straightforward. When deep snows prompt large numbers of the animals to migrate outside Yellowstone, they are rounded up and sent to slaughter or hazed back into the park.

An estimated $19 million has been spent on those efforts since 2002. Montana Gov. Brian Schweitzer said the recent brucellosis infections exposed the program as a failure.

"Managing a disease means more than chasing buffalo back into the park," Schweitzer said.

As investigations into the cases continue, state officials say elk appear a likely culprit. However, addressing brucellosis in elk presents daunting challenges.

In terms of sheer numbers, the Yellowstone region's 25 elk herds dwarf the three herds of bison. And whereas bison stick together and travel in groups, elk move freely over the region's numerous mountain ranges, often alone or in small numbers.

Since late 2006, federal officials and the governors of Idaho, Wyoming and Montana have been negotiating for a sweeping brucellosis plan that would deal with the disease across borders and among different species.

A tentative proposal, drafted by federal officials, sets a goal of eliminating the disease -- not just controlling it in bison. It also puts new emphasis on elk.

Still, prospects for an agreement remain uncertain given the states' divergent approaches to wildlife.

Wyoming's controversial use of artificial feedgrounds, for example, remains a sticking point among the states. Researchers say the feedgrounds concentrate elk herds and foster the spread of disease.

Kurt Alt, a state wildlife biologist from Montana, said the feedgrounds perpetuate the problem by constantly "pumping brucellosis into the Yellowstone system."

But Wyoming officials say the elimination of the feedgrounds could make the brucellosis problem worse, if hungry elk scattered into areas where cattle range. Near Pinedale, the state has begun capturing elk and slaughtering any that show signs of the disease.

In Montana, state officials hope to increase elk hunting and hazing near Yellowstone and expand a testing program to gauge which herds are badly infected.

Once identified, the state could argue they have the right to access private land to manage elk -- a tricky proposition given Montana's tradition of individual property rights. Montana's governor cited a recent case in which the state removed a group of bison from private land over the owners' objections.

"There's a precedent," Schweitzer said, adding the state was considering all its options to deal with the disease in wildlife.

Associated Press writer Matt Joyce in Cheyenne contributed to this report.