'Mad cow' disease found in Californian cow © Reuters/Jeff Haynes A dairy cow in California has been found to have "mad cow" disease, the US Department of Agriculture has confirmed. The diagnosis is the first in America since 2006 but experts rallied to assure the public that it is an "isolated" case and is not likely to enter the food chain. Agriculture Secretary Tom Vilsack said the finding posed "no risk to the food supply or to human health." Two major South Korean retailers instantly cancelled orders of all US beef, although Mexico, Korea and Japan, three of the top markets for overseas US beef sales, will continue imports. Known officially as bovine spongiform encephalopathy, "mad cow" disease is fatal in cattle, and is believed to lead to a similar deadly brain disease in humans, called vCJD, by eating meat from infected animals. First discovered in Britain in 1986, the disease has killed more than 150 people and 184,000 cows globally.