"It was never presented for slaughter for human consumption, so at no time presented a risk to the food supply or human health," Clifford said.
The Associated Press reported Tuesday that the cow tested positive at a rendering facility in Hanford operated by Baker Commodities. The company has 21 plants across the United States that convert animal by-products into pet food, poultry feed, and tallow, used in soaps, paints, and cosmetics.
Mad cow disease, bovine spongiform encephalopathy, or BSE, is a progressive brain disease always fatal in cattle.
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 Tuesday. The human form, known as variant Creutzfeldt-Jakob disease, is rare but can be fatal.
When the disease swept across Britain's farms in the 1980s and '90s, an estimated 3.7 million animals were slaughtered. The disease was linked to the deaths of 180,000 cattle and roughly 150 people.
Many 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 officials issued a statement Tuesday saying the discovery of the animal was an indication of the success of the state's food-safety system.