HANFORD, Calif. - Food safety experts played down the risk of mad cow disease entering the U.S. food supply Wednesday, a day after a government agency detected the first new case of the disease in the U.S. since 2006.

It was the fourth case discovered in the country, and no human version of the illness has ever been linked to eating U.S. beef.

"What we know is that 3,000 Americans die every year from preventable food-borne illnesses that are not linked" to mad cow disease, said Sarah Klein of the consumer advocacy group Center for Science in the Public Interest. "Things like E. coli, salmonella -- that's where we should be focusing our attention, outrage and policy."

Two major South Korean retailers suspended sales of U.S. beef in response. Reaction elsewhere in Asia was muted, with Japan saying there's no reason to restrict imports.

"From simply a public health issue, I put it very, very low," Cornell University food safety expert Martin Wiedmann said of the level of concern about mad cow disease.

Maintaining confidence in exports fuels the nation's monitoring of the beef supply as much as continuing safety concerns, he said.

Tuesday's news came from that monitoring: Routine testing of a dead dairy cow from central California showed the animal had bovine spongiform encephalopathy, or BSE, a disease that gradually eats holes in the animal's brain. U.S. health officials were adamant that there was no risk to the food supply -- the cow never was destined for the meat market, and the World Health Organization says humans can't be infected by drinking milk from animals with BSE.

The U.S. has been guarding against BSE for years, since a massive outbreak in Britain that not only decimated that country's cattle but showed that eating BSE-contaminated meat could trigger a human version of the disease. A key part of the safety net: The animal tissues that can carry the BSE -- including the brain and spinal cord -- are removed from cattle before they're processed for food.

Tests are performed on only a small portion of dead animals brought to the transfer facility in central California.

The cow had died at one of the region's hundreds of dairies, but hadn't exhibited outward symptoms of the disease: unsteadiness, incoordination, a drastic change in behaviour or low milk production, officials said. But when the animal arrived at the facility with a truckload of other dead cows on April 18, its 30-month-plus age and fresh corpse made her eligible for USDA testing. Experts say it takes at least that long for the disease to develop.

"We randomly pick a number of samples throughout the year, and this just happened to be one that we randomly sampled," Baker Commodities executive vice-president Dennis Luckey said. "It showed no signs" of disease.

The samples went to the food safety lab at the University of California, Davis on April 18. By April 19, markers indicated the cow could have bovine spongiform encephalopathy (BSE), a disease that is fatal to cows and can cause a deadly human brain disease in people who eat tainted meat. It was sent to the USDA lab in Iowa for further testing.

On Tuesday, federal agriculture officials announced the findings: the animal had atypical BSE. That means it didn't get the disease from eating infected cattle feed, said John Clifford, the Agriculture Department's chief veterinary officer.

It was "just a random mutation that can happen every once in a great while in an animal," said Bruce Akey, director of the New York State Veterinary Diagnostic Laboratory at Cornell University. "Random mutations go on in nature all the time."

In humans, experts say the disease can occur in one in 1 million people, causing sponge-like holes in the brain. But they say not enough is known about how and how often the disease strikes cattle.

The disease cannot be transmitted by contact among cows, and experts say it's unclear whether this rare type of BSE ever has been transmitted from a cow to a human by eating meat.

The California Department of Public Health and the state Department of Food and Agriculture quickly worked to assure consumers that the food supply is safe -- and that the cow hadn't been destined for human consumption. The building where the cow was selected to be tested sends animals to a rendering plants, which process animal parts for products not going into the human food chain, such as animal food, soap, chemicals or other household products.

Among the unknowns about the current case is whether the animal died of the disease and whether other cattle in its herd are similarly infected. The name of the dairy where the cow died hasn't been released, and officials haven't said where the cow was born.

"It's appropriate to be cautious, it's appropriate to pay attention and it's appropriate to ask questions, but now let's watch and see what the researchers find out in the next couple of days," said James Culler, director of the UC Davis dairy food safety laboratory and an authority on BSE.

The National Cattlemen's Beef Association said in a statement that "U.S. regulatory controls are effective, and that U.S fresh beef and beef products from cattle of all ages are safe and can be safely traded due to our interlocking safeguards."

The infected cow was identified through an Agriculture Department surveillance program that tests about 40,000 cows a year for the fatal brain disease.

There have been three confirmed cases of BSE in cows in the United States -- in a Canadian-born cow in 2003 in Washington state, in 2005 in Texas and in 2006 in Alabama.

Both the 2005 and 2006 cases were also atypical varieties of the disease, USDA officials said.

The mad cow cases that plagued England in the early 1990s were caused when livestock routinely were fed protein supplements that included ground cow spinal columns and brain tissue, which can harbour the disease.

The Agriculture Department is sharing its lab results with international animal health officials in Canada and England who will review the test results, Clifford said. Federal and California officials will further investigate the case. He said he did not expect the latest discovery to affect beef exports.

State and federal agriculture officials plan to test other cows that lived in the same feeding herd as the infected bovine, said Michael Marsh, chief executive of Western United Dairymen, who was briefed on the plan. They also plan to test cows born at around the same time the diseased cow was.