BILLINGS, Mont. - Federal animal health agents are testing cattle from a ranch near Yellowstone National Park for a notorious livestock disease to determine if a recent outbreak extends beyond a single herd, federal officials and the ranch operator said.

The fate of the state's livestock industry hangs in the balance: If blood tests 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.

The herd being tested belongs to Bruce Malcolm, a Republican state legislator and rancher in Emigrant. Malcolm said Tuesday that seven cows diagnosed with the disease last week, and later traced to a ranch owned by other members of his family in Bridger, originated with his herd.

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.

The first confirmed case in Montana came in a cow that originated in Bridger and was later shipped to Iowa in early May. That animal was destroyed May 8 at Iowa State University. On Friday, six more cows from the Bridger ranch - owned by Malcom's son and daughter-in-law - were diagnosed with the disease.

The probe into how far the infection has spread now centers on Malcolm's ranch near Emigrant, 23 miles north of Yellowstone in the Paradise Valley.

The seven cows confirmed with the disease had been moved by Malcolm to Bridger over the last several years, Malcolm said. Some of those animals, including the original infected animal, were later shipped to Iowa.

Malcolm said authorities performed blood tests on some of his cows Monday and were testing the remainder Tuesday.

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

Malcolm speculated the disease was passed to his herd from an infected elk, which in turn got it from Yellowstone bison.

"The bison don't intermingle with our herd. They're not allowed to get that far out," he said. "But when you have a brucellosis free state and you have a source of infection, it's only a matter of time before you transport that infection out."

Teresa Howes with the U.S. Department of Agriculture's Animal and Plant Health Inspection Service said authorities are working to determine the extent of the outbreak in livestock before attempting to find the source of the disease.

"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."

George Harris with the Montana Department of Livestock said additional test results are not expected until later this week, but Malcolm said he was told preliminary results could come as early as Tuesday.

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. 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.