Image caption The USDA says there were 29 cases of mad cow worldwide last year

A dairy cow in California has bovine spongiform encephalopathy, or "mad cow" disease, the US Department of Agriculture (USDA) has confirmed.

The diagnosis, the fourth in the US and its first since 2006, would not affect beef exports, said the department's veterinary chief.

"Both human health and animal health are protected," John Clifford said.

The first outbreak of mad cow disease in the United States occurred in 2003, and hurt global trade in beef.

In a statement, Mr Clifford said that the recently diagnosed dairy cow in central California had not entered the food supply.

"There is really no cause for alarm here with regard to this animal," he said.

The disease was detected as part of a screening programme that tests an estimated 40,000 animals every year, the USDA said.

Canada said it did not expect the diagnosis to affect food trade between the two nations.

Fourth case

Two of South Korea's biggest retailers have halted sales of US beef.

The EU said it was satisfied that the appropriate steps had been taken to prevent the disease entering the food chain, and would not be imposing a ban.

Mad cow disease, or BSE, is always fatal in cattle, and research suggests humans can contract a similar deadly brain disease, vCJD, by eating infected meat from infected animals.

The disease causes personality change, loss of body function, and eventually death.

Although infected meat is no longer widely viewed as a risk, there has been continuing concern in the UK over human-to-human transmission through blood transfusions.

The USDA noted that the disease is not transmitted via cow's milk.

According to the USDA, there were only 29 worldwide cases of mad cow disease in 2011.

BSE rose to global prominence in the 1990s with a major outbreak in the UK that prompted fears of large-scale transmission to humans.

The disease has killed more than 150 people and 184,000 cows globally, mainly in Britain and Europe.

The US and many other nations took steps to reduce the spread of the disease, changing animal feed practices seen as enabling transmission among herds.

The California diagnosis is the fourth case of BSE found in the US.

The first was a Canadian-born cow diagnosed in 2003 in Washington state; the others were detected in 2005 in Texas and in 2006 in Alabama.

News of the diagnosis appeared to have an short-term impact on cattle prices: live cattle futures on one US exchange dropped by the daily limit of three cents per pound on reports of the confirmed case, but quickly rebounded.