CHEYENNE - State and federal livestock officials have launched an investigation into the possibility of a second Wyoming cow testing positive for brucellosis, jeopardizing the state's disease-free status even as the rancher at the center of the first outbreak decides to slaughter his herd.

Officials have preliminarily traced the recent detection of an infected cow at a Nebraska slaughterhouse to a herd in Wyoming's Sublette County, a rural area about 100 miles southeast of Yellowstone National Park, state veterinarian Walter Cook said Monday.

"The blood came up positive and then they started working on the paper trail, and basically at this time it's indicating that it came out of a herd in Sublette County," Cook said.

Cook said all sexually intact animals at least 6 months old in the herd will be tested to determine if other cattle are infected with brucellosis, a disease that can cause pregnant cows to abort their calves.

If an infection is confirmed, the U.S. Department of Agriculture would likely revoke Wyoming's brucellosis-free status because of the discovery of two infected herds within two years. Loss of the status would mean costly testing requirements for all Wyoming cattle before they're sold or moved across the state line.

Wyoming has been classified as brucellosis-free since September 2006, after losing the status in 2004. The disease reappeared in June when two cows from the Daniel area of Sublette County tested positive for the infection at a sale barn.

Cook said he learned late last week that the Daniel-area rancher had decided to slaughter his herd. Earlier this summer, the rancher had told state officials that he would forgo slaughtering and instead put the animals under quarantine until the entire herd tested negative three times over the course of a year.

If the rancher had opted against slaughtering the herd, Wyoming would have lost its brucellosis-free status.

Cook said the rancher was made aware of the possibility of a second infected herd in the state, but intended to follow through with slaughtering his cattle anyway.

"It really didn't impact his decision, because it would be so difficult for him to operate under quarantine for a year," Cook said.

Brucellosis has been the target of a federal eradication program for more than 70 years, but it persists in Yellowstone's bison, elk and other wildlife, and is occasionally transmitted to cattle.

Montana lost its brucellosis-free status this summer after the disease was found twice in that state in the last two years.

Fellow stockmen have said the rancher shouldn't slaughter his herd over concerns about the state's disease-free status, because there was the possibility that another infected herd would be discovered within two years - thereby revoking the state's status regardless of the rancher's actions.

Cook said there was no known contact, such as shared fence lines, between the Daniel-area herd and the newly suspected herd.

Wyoming law prohibits state officials from releasing the names of ranchers whose cattle test positive for infectious diseases. Attempts by The Associated Press to reach the Daniel-area rancher have been unsuccessful.

Cook said about 40 cattle of the rancher's 650-head Angus herd have already tested positive and been killed.

"I knew he was disappointed that we seemed to have positives every time we tested his herd," Cook said.

Cook said he was surprised to hear from the rancher about his decision to reverse course and send the remainder of the herd, about 600 animals, to slaughter.

"The herd owner made this decision because he believed it was the best choice for his herd," Cook said. "I think that was pretty untenable for him, to go another 12 months under quarantine. I'm sure it was real tough decision for him."

Cook said the herd's brucellosis status shouldn't affect its sale price for slaughter. It is not dangerous to eat the meat of animals infected with brucellosis, as long as it is cooked.

"The meat is perfectly safe to eat so there's no issue there," he said.