BILLINGS, Mont. (AP) -- Results of disease tests received Friday for a cattle herd in Montana's Park County have confirmed that some of the animals were infected with brucellosis.
The tests were done on milk samples collected from six cows in the 150-head herd, Montana livestock officials said. Those animals were slaughtered Thursday after previous blood tests indicated the animals might be infected.
Brucellosis can cause pregnant animals to prematurely abort their young. It has been largely eradicated nationwide but persists in wildlife in parts of Montana, Idaho and Wyoming around Yellowstone National Park.
The remainder of the unidentified Park County herd has been quarantined. The infection marks the region's 15th found in cattle or domestic bison since 2004.
Until recently, infections brought harsh sanctions on states where the outbreaks occurred.
But stricter testing and disease requirements over the past several years prompted federal agriculture officials to adopt a more lenient response to occasional infections. The government also no longer requires that entire herds be slaughtered when infections are found in just a few animals.
Because of the changes, Montana can retain its brucellosis-free designation from the U.S. Department of Agriculture despite the latest infections, said agency spokeswoman Lyndsay Cole.
"As long as the state quarantines the affected herd and a herd plan is developed for the affected herd, it won't affect their state status," she said.
A herd plan would include testing remaining animals from the ranch where brucellosis was found and removing those that test positive.
The infected cattle from Park County had all been vaccinated. Because five were still pregnant, veterinarians believe the vaccine was working, said Steve Merritt with the Montana Department of Livestock.
"Normally with unvaccinated cows that had been exposed, they would have lost those calves," he said. But he added that they still could have potentially spread the disease to other cows, necessitating their removal from the herd.
Merritt said cattle on surrounding ranches also would be tested for brucellosis to ensure they were not exposed.
Although the source of the infection has not yet been determined, state officials suspect it may have come from elk.
Despite efforts to contain the disease, so far it has proven impossible to eliminate it from wildlife.
Thousands of Yellowstone bison have been captured and sent to slaughter over the past decade to prevent transmissions to cattle when the bison attempt to migrate to lower elevations in Montana. That has not stopped periodic transmissions to cattle from the tens of thousands of elk that roam freely across the Yellowstone region.
Copyright 2011 The Associated Press.