The herd has been quarantined and officials say the Nebraska Department of Agriculture is working with the U.S. Department of Agriculture also want to find out how the disease was introduced to the herd.
The outbreak is unwelcome to the state's roughly $10 billion cattle industry, which started the year with 6.35 million head.
A spokesman for an industry group, the Nebraska Cattlemen, said it's too early to worry too much.
"We don't know the extent of the infection," said Michael Kelsey, executive vice president of the Nebraska Cattlemen. "Is it one herd, is it three herds? It's far too early to say what the effect might be."
At stake is the state's tuberculosis-free designation from the USDA. Were that to change, Nebraska producers shipping cattle to other states might have to prove their cattle are disease-free - an expensive proposition when margins are already tight, said Kelsey.
The cattle would need two visits from a veterinarian, who would test for the TB.
"If we lost that tuberculosis-free designation it would put us at a competitive disadvantage" to other states, he said.
The decision on the state's status rests with federal authorities.
"We just don't know when USDA will make that determination. If the past is any indication, it would take as long as four-five weeks," Kelsey said.
State spokeswoman Christin Kamm said the state's last outbreak occurred in the early 1990s and that the state been designated tuberculosis-free for the past 17 years.
She would not say Monday how large the quarantined Rock County herd is, who owned it or how many of the cattle have tested positive.
Kamm said the outbreak was discovered three weeks ago during routine tests of cattle by a federal inspector at a slaughterhouse. She said she didn't know where the slaughterhouse was located.
The cow-calf herd was placed in quarantine as soon as officials could trace back the cattle to the producer, but she didn't know how many days passed before the connection was made - hence the concern about possibly infected cattle being shipped elsewhere.
There is no mandatory animal ID program, Kamm said, so DNA tests of the infected animal were compared against sale barn records, which traced the animal back to the seller.
Federal experts say tuberculosis is a contagious disease of animals and humans, caused by three types of bacteria.
Bovine TB can be transmitted from livestock to humans, but that's exceedingly rare. It can be transmitted to other animals.
The disease is easily transmitted through a herd of cattle, which often share breathing space. It causes severe coughing, fatigue, emaciation and debilitation.
Bovine TB is considered untreatable, so both infected and noninfected cattle in a herd usually are killed. Kamm said the USDA makes the final determination on whether to kill an entire herd.
The other option is continued quarantine and testing.
An entire herd would have to have eight consecutive clean tests before the quarantine could be lifted, a process that could take years.