FRESNO, California // More than 4,800 dairy cows at risk of carrying tuberculosis are being slaughtered this week in central California, where nearly 16,000 cattle in the country's largest milk-producing region have been quarantined, federal officials said. The Undersecretary of Agriculture Bruce Knight met privately with local dairy operators on Tuesday, along with the state veterinarian and other industry officials monitoring three new cases of TB recently discovered in Fresno County dairies. Federal and state agriculture officials were tight-lipped about the identities and locations of the three dairies where cows tested positive for TB. The disease can be transmitted to humans and other mammals through the air or through drinking unpasteurised milk from an infected cow. The discovery of the highly contagious respiratory disease has prompted changes in interstate shipping regulations. Pasteurisation kills the TB bacteria, but trade organisations still worry about the possible financial impact on California's US$7.3 billion (Dh26.3bn) dairy industry. "One of the concerns is with trade agreements," says Ray Souza, a Turlock dairy operator and board member of the Western United Dairymen, a trade group that represents 1,100 dairies across the state. "Things like this can be used to renegotiate." One dairy owner has accepted a buyout of up to $3,000 a head from the United States Department of Agriculture (USDA ), and his cows are headed for slaughterhouses. The two other dairies are weighing up whether to operate under quarantines that could last for years. One of the affected dairies milks more than 10,000 cows and sells semen and embryos from high-production cows and bulls internationally. The operation faces losing 50 years of genetic development if the dairy operator chooses to slaughter his cattle, said Michael March, chief executive of Western United Dairymen. "It's tragic when you have that kind of investment and build up that legacy and genetic bloodline," Marsh said. "He's facing a very difficult choice." A routine inspection of a slaughterhouse cow in January found TB lesions on its lymph nodes, prompting the California Department of Food and Agriculture's inspection of 150,000 cattle so far. About 90 per cent of infected cattle do not show symptoms, which include weight loss, cough and rough coats. California had been free of the disease since 2005, two years after cases detected in Kings and Tulare counties prompted the testing of 876,000 cattle and the slaughter of more than 8,000. State officials are still trying to understand how the cows got exposed. DNA testing shows that two of the cows that tested positive this spring share a strain of the bacteria that originated in Mexico. In June, the state Department of Food and Agriculture issued a warning against contact with cattle of Mexican origins, and the USDA is drafting an order that restricts transport of California cattle across state lines without testing by a veterinarian. "They're going to have to go through additional testing so we can keep this from spreading," said Jay Van Rein of the California Department of Food and Agriculture. The restrictions on transport make it more difficult for ranchers to sell cattle out of state, transport them for out-of-state grazing during California's dry season and for breeders to haul livestock to shows.