HELENA, Montana — For the first time in a decades, a case of bovine tuberculosis has been confirmed in a Montana beef cattle herd, the state Department of Livestock says.

The Montana Department of Livestock confirmed TB in a Blaine County herd following detection of the disease in a cow at slaughter. The infected cow was identified by meat inspectors during routine inspection at a Minnesota plant. Identification collected from the infected cow at the time of slaughter linked the animal to a Blaine County beef herd and subsequent herd testing revealed additional TB infected animals. The herd has been placed under quarantine, the MDOL says.

While bovine TB was been found in the 1990s in other species in Montana, it was not clear how long it has been since a case of bovine TB was found in a beef herd in the state.

"It's been long enough that we're having a hard time figuring out how long it's been," said Dr. Marty Zaluski, Montana state veterinarian.

The best estimate is that there were some cases in the 1960s, he said, noting that a 1959 report spoke of a really low state rate.

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"Our guess is we probably had a few stragglers in the '60s," he said.

Bovine TB is a rare diagnosis, with "maybe a dozen cases or so a year in the country," Zaluski said.

Bovine TB is caused by the bacteria Mycobacterium bovis. The disease causes granulomatous lesions inside the lymph nodes, lungs, liver, spleen, and skin of affected animals. The primary route of spread is aerosol transmission to other animals in close contact. The bacterium is also capable of infecting wildlife, such as deer, and people. The disease has an incubation period that can range from months to years and infected animals rarely show clinical signs, meaning cattle appearing healthy may be infected with the bacteria.

Although TB is a zoonotic disease capable of infecting people, Zaluski explained that it is not a threat to meat safety because of robust inspection programs. Meat inspectors identify the lesions associated with bovine TB, and the infected meat is taken out of the meat supply. Zaluski said that is how the Blaine County case was identified, then herd testing identified three additional infected animals.

Consumption of infected raw (unpasteurized) milk is another documented route of transmission to people.

According to Montana government documents pertaining to bovine TB, cases within the past five years have been reported near the Montana border, including a 2016 case in a beef cow northwest of Medicine Hat, Alberta. In 2017, South Dakota reported cases in beef cattle in Harding County, in the northwest corner of South Dakota, and and Tripp County, in south central South Dakota. All three cases appeared to be from bovine TB strains found in Mexico. In the South Dakota cases, officials confirmed that the cattle had not had contact with foreign born cattle or cattle originating in Mexico. Wildlife surveillance in the cases in Canada and South Dakota did not reveal bovine TB.

Finding bovine TB in the Blaine County herd triggers a full epidemiological investigation to evaluate the extent of disease and to mitigate further spread, MDOL says. This effort includes the testing of adjacent herds and herds that have shipped animals into or received animals from the source herd. Zaluski said the MDOL is working with the herd owner, U.S. Department of Agriculture, tribal, and wildlife officials to determine if other herds or wildlife are involved, and to try to determine the source of disease introduction.

"We don't have all the information yet. We don't have all our questions answered. A lot of work remains. We will be testing adjacent herds and following up on additional information that might be related to how this infection got into this herd or potentially left this herd," he said. "There's a lot of work to be done."

Options for TB affected herds are limited and include whole herd depopulation or extended quarantine while the herd is tested repeatedly (test and removal). Herds subject to whole herd depopulation are reimbursed, or indemnified, by the federal government based upon assessment of the animals’ value. Test negative animals from depopulated herds are able to go through slaughter channels in order to salvage the meat of the animal.

While bovine TB is rare, Zaluski said taking precautions remains important.

"I would be remiss if I didn't hammer biosecurity when I have the opportunity," he said.

Biosecurity steps to remember include knowing the source of animals and recognizing and isolating animals with possible health concerns when they are brought into a herd. Zaluski said supporting national eradication programs also is important.

"Really, as far as tuberculosis, because the disease is so slow moving and our tests are really imperfect, there's not a lot that individual producers can do," he said.