The U.S. Agriculture Department said Tuesday that a California dairy cow tested positive for mad-cow disease, the first appearance of the brain-wasting illness in the U.S. since 2006.

Agency officials said the finding didn't pose any immediate threat to the safety of the food supply. They said no meat from the animal had entered the food chain, and that people aren't at risk of contracting the disease through consumption of milk from an infected cow.

But...