BILLINGS, Mont. - Federal animal health agents on Monday tested cattle near Yellowstone National Park for a notorious livestock disease to determine if a recent outbreak extends beyond a single herd, federal officials and industry representatives said.

The fate of Montana's livestock industry hangs in the balance: If blood tests in Paradise Valley, north of the park, reveal a second herd is infected with brucellosis, federal authorities would revoke Montana's disease-free status, and ranchers could be forced to adopt a costly testing and vaccination program for the state's 2.5 million cattle.

Brucellosis causes pregnant cows to abort their calves. Widely eradicated from livestock last century, it has persisted in wildlife such as elk and bison. Recent outbreaks in Wyoming and Idaho - both linked to Yellowstone-area elk - have cost livestock producers in those states millions of dollars. Wyoming has regained its brucellosis-free status.

The first confirmed case out of Montana came in a cow shipped to Iowa in early May. That animal was destroyed May 8 at Iowa State University. On Friday, six more cows from a ranch near Bridger, Mont., were diagnosed with the disease.

The probe into how far the infection has spread now centers on a ranch near Emigrant, 20 to 30 miles north of Yellowstone in the Paradise Valley, according to industry sources.

The seven cows confirmed with the disease were owned by an Emigrant rancher who moved a portion of his herd to Bridger after last year's grazing season, said Dennis McDonald, past president of the Montana Cattlemen's Association. Some of those animals, including the original infected animal, were later shipped to Iowa.

"My concern is that there is a neighboring herd (in Emigrant) that's infected," said McDonald. "That would be the worst of all scenarios, because that would mean we have a second herd and would lose our brucellosis-free status."

George Harris with the Montana Department of Livestock said additional test results are not expected until later this week.

Whether the disease came from elk, bison or other livestock is not yet known, said Teresa Howes with the U.S. Department of Agriculture's Animal and Plant Health Inspection Service.

"We're looking at how the cows came in and out of the original herd, and that will lead us to other places," Howes said. "We've got to do all the livestock first, and then we look at all the other pieces and parts. It takes some time."

McDonald and others from his group said the outbreak underscores the need for changes in how livestock and wildlife are managed in Yellowstone and on surrounding forest lands. They want a larger buffer zone around the park to better separate livestock from wildlife that carry the disease.

Under current rules, bison are separated from livestock - at times through the controversial practice of slaughtering bison - but elk are not.

Elk and bison first contracted the disease from livestock brought to the Northern Rockies by early European settlers. Those wildlife are now considered one of the last remaining reservoirs of the bacteria that causes brucellosis.

Montana was certified as brucellosis-free by the federal government in 1985. Gov. Brian Schweitzer, citing the continued proximity of Yellowstone wildlife and cattle, has said "it may be simply a matter of time" before that status is revoked.

Approximately 350 cows from the Bridger herd remain under quarantine, including 50 in Iowa that were shipped with the original infected case. All the cows face slaughter under U.S. Department of Agriculture rules.

The disease is rarely passed along to people, said Mark Quinn with Montana State University's Department of Veterinary Molecular Biology.

Known as undulant fever in its human form, the illness is most often diagnosed in ranchers, veterinarians or others who come in frequent contact with livestock. It causes recurring fevers that can be treated with antibiotics, Quinn said.