Montana livestock officials announced Thursday that a Paradise Valley cow that tested positive for brucellosis this spring most likely got the disease from elk, not bison.

Marty Zaluski, the Montana state veterinarian, said a recently completed genetic analysis of the bacteria that sickened the cow, along with other studies of the case, suggest the cow did not get the disease from bison in and around Yellowstone National Park, nor did the disease come from domestic stock.

"We've now had two cases of the disease in two years, and no contact with bison in either case," Zaluski said. "It supports our conclusion that elk were the source in both cases."

Brucellosis is a serious disease in livestock and wildlife that causes cows to abort their first calves. The disease has been eradicated in the United States, except for bison and elk in and around Yellowstone Park, where the animals contracted the disease from domestic stock almost 100 years ago.

Montana had been considered free of brucellosis since 1985. However, in the past 18 months, cattle have twice tested positive for the disease. Both outbreaks were linked to elk in and around Yellowstone.

The second incident, detected this spring, will cost the state its official "brucellosis-free" status. The change means Montana ranchers will have to conduct more tests of their stock before shipping them to the out-of-state markets where almost all Montana's cattle crop ends up.

Federal scientists in Ames, Iowa, analyzed the genes of the specific bacteria that infected the Paradise Valley cow. Those tests came back to the state Monday, said Steve Merritt, a spokesman for the Montana Department of Livestock.

Those tests concluded that the disease most likely did not come from fellow cattle. However, Zaluski said, the strain of brucellosis found in that part of the Greater Yellowstone area is not very specific between elk and bison. Consequently, the DNA testing could not conclude whether the disease came from elk or bison.

To make that determination, Zaluski relied on the department's own investigation of the case. Most notably, he said, the infected cow lived on a ranch near Pray, several miles away from where any bison have been spotted for several years. The closest bison to the ranch were stray bulls, which cannot spread the disease, Zaluski said, and even those animals have never been near this ranch.

Bison cows do not calve alone, as elk do, but give birth in groups, Zaluski said. It defies logic that a group of bison would be in the area and no one would notice them.

"That's just unfathomable," he said. "That would be the least likely scenario."

He said it is impossible to know exactly how the cow got the disease, but the most reasonable conclusion is elk.

Zaluski said the latest results mean managers must begin dealing with brucellosis in elk to avoid transmission to cattle. Thus far, state and federal officials have only looked at bison as a potential source of the disease and are managing only bison to prevent the spread of brucellosis.

In an effort to control the spread of the disease, government agencies have killed some 6,000 wild bison leaving Yellowstone over the last 20 years, including 1,600 last winter alone.

The Montana Board of Livestock on Tuesday instructed Zaluski and others to begin looking at ways to control the spread of the disease from elk to domestic stock.

Zaluski said simple steps could be taken to reduce the risk of transmission, like giving hunters more access to large herds of elk in the area which are currently staying on private lands closed off to public hunting. Ranchers could also fence off their hay and feed lines to make their property less attractive to elk. Or they could move their herds to different pastures if they notice large numbers of elk co-mingling with their animals during calving season.

Calving season is an important factor in preventing the spread of the disease. It is typically spread when an animal sniffs or eats the tissue of an aborted calf.

Ben Lamb, conservation director for state and federal issues at the Montana Wildlife Federation, a group that supports public hunting and fishing, said he is a little apprehensive about the idea of elk coming into the crosshairs of the brucellosis controversy.

Lamb said he didn't think Montana's many hunters would support "radical" plans that treated wild elk like domestic livestock. He also said he was skeptical of hopes to completely eradicate brucellosis from the wildlife of Yellowstone.

"Given the tools that we have now, we would have to depopulate the elk and bison coming out of the park," he said, adding that such a plan would only polarize an already tense situation.

Lamb said the rules surrounding brucellosis management in the U.S. were written before milk was pasteurized, when it was more common for people to contract the human form of the disease from eating milk products.

"This disease does not pose the same level of human health risk that it did before," he said.

Reporter Jennifer McKee: 443-4920 or jennifer.mckee@lee.net