From Businessweek:

A California Holstein discovered to have mad cow disease in April was an isolated case and didn't pose a threat to the food supply, a report issued Friday by the U.S. Department of Agriculture said.

A three-month investigation looked into the movements of the infected dairy cow, her offspring and the food eaten by the herd. The investigation turned up no other cases of bovine spongiform encephalopathy.

"The results of this thorough investigation confirmed that at no time was the U.S. food supply or human health at risk, and that the United States' longstanding system of interlocking safeguards against BSE continues to be effective," said John Clifford DVM, USDA's chief veterinary officer.

The 10-year-old dairy cow, only the fourth with the sickness ever discovered in the United States, was found as part of an Agriculture Department program that tests for the fatal brain disease in about 40,000 of the 35 million cows slaughtered each year.

For more, see: Report: Mad cow in California was isolated case