The U.S. Agriculture Department has confirmed a case of mad cow disease, the nation's fourth, in a dairy cow from central California.

Chief Veterinary Officer John Clifford said Tuesday that at no time was human health at risk. He said the affected cow was never meant to be slaughtered for meat and that milk does not transmit the disease.

Clifford said agriculture officials are sharing the laboratory results with the World Organization for Animal Health.

Clifford said longstanding safeguards by the United States and other countries to protect humans from mad cow disease are working. He noted there were just 29 cases of the disease worldwide in 2011 — dramatically down from the peak of more than 37,000 cases in 1992.

Mad cow disease attacks the brains of affected cattle. It is always fatal. Doctors believe people can come down with a human form of the disease by eating tainted meat.