SAN JOSE, Calif. — In a move that could raise new questions about food safety and result in economic setbacks to California’s multi-billion-dollar agricultural industry, the U.S. Department of Agriculture on Tuesday confirmed that a case of mad cow disease has been found in a dairy cow in California’s Central Valley.

The incident is the first case of the disease ever found in California — and the first detected in the United States since 2006.

John Clifford, the USDA’s chief veterinary officer, said the cow was detected at a rendering plant, where the animal is now being held as part of an investigation. The USDA did not release the name of the farm the cow was from or the name of the rendering plant. The public is not at risk, he said.

The disease can be transmitted to humans through eating meat tainted with infected brain or nerve tissue of an infected animal. It is not transmitted through consumption of milk, the USDA reiterated on Tuesday. The human form is rare but can be fatal.

After a dairy cow in Washington state was diagnosed with mad cow disease in 2003, it devastated the U.S. beef industry. Dozens of countries refused to import U.S. beef, and U.S. beef shipments plunged 82 percent.

Numerous questions remained unanswered Tuesday, including where the cow came from, how it got the disease, whether other animals in the herd might be infected and whether any meat from them has been sold for public consumption.

California agriculture officials issued a statement Tuesday saying the discovery is an indication of the success of the state’s food safety system.

“The detection of BSE shows that the surveillance program in place in California and around the country is working,” said Karen Ross, secretary of the California Department of Food and Agriculture. “Because of the strength of the food protection system, the cow did not enter the food or feed supply.”

But critics said the incident shows shortcomings in food safety regulations.

“For years, Consumers Union has been trying to get the USDA to test more cows each year. Since the Bush administration, the number of cows tested each year has diminished,” said Elisa Odabashian, director of the West Coast office of Consumers Union, the advocacy arm of Consumer Reports.

“Only 40,000 cows a year — of millions of millions slaughtered — are tested,” she said. “We don’t know if this is an isolated, unusual event — or if they are not finding it because they are not looking.”