The discovery of mad cow disease in a dairy cow in California is being described as a combination of the U.S.'s targeted surveillance of its beef supply and random chance.
The Department of Agriculture confirmed Tuesday that the disease had been discovered in a dead cow.
In a press briefing, John Clifford, the USDA's chief veterinary officer, said the cow's meat did not enter the food supply and the carcass will be destroyed.
The animal was found at a rendering facility run by Baker Commodities in Hanford, Calif. The disease was discovered when the company selected the cow for random sampling, Baker Commodities executive vice president Dennis Luckey told The Associated Press. The cow had not shown any other signs of the disease, such as unsteadiness or low milk production.
The Department of Agriculture performs tests for mad cow disease on about 40,000 cows each year. Ronnie Silcox, a professor of animal science and a beef cattle specialist at the University of Georgia, said cows that are at high-risk of the disease, such as those that can't walk or show signs of neurological disease, are the ones routinely tested. He called the discovery of mad cow disease "incredibly, incredibly rare." "This is one out of a million, or something like that," he said. "It's probably inevitable that we find it eventually because we're actively looking for it."
The Agriculture Department confirmed Tuesday that the cow is the fourth discovered in the United States to test positive for the disease, but stressed that consumers were not at risk because the animal was not bound for the food supply.
"There is really no concern for alarm here with regards to this animal," Clifford said. "Both human health and animal health are protected with regards to this issue."
Nevertheless, two retailers in South Korea pulled U.S. beef from their stores Wednesday in response to the report of mad cow disease.
"We stopped sales from today," Chung Won-hun, a spokesman for Lotte Mart, one of the retailers, told the Associated Press. "Not that there were any quality issues in the meat but because consumers were worried."
Home Plus, the other retailer, resumed sales and cited a government announcement of increased inspection.
U.S. government officials were quick to reassure the public that the finding posed no threat to humans.
"The beef and dairy in the American food supply is safe and USDA remains confident in the health of U.S. cattle," Agriculture Secretary Tom Vilsack said in a statement. "The systems and safeguards in place to protect animal and human health worked as planned to identify this case quickly, and will ensure that it presents no risk to the food supply or to human health. USDA has no reason to believe that any other U.S. animals are currently affected, but we will remain vigilant and committed to the safeguards in place."
The Food and Drug Administration pledged to "work with the USDA to fully investigate the feed supply as part of the epidemiological investigation." And the USDA said in a statement that milk does not transmit mad cow disease.
Silcox noted that the cows processed for consumption in the U.S. are usually too young to have developed mad cow disease.
Mad cow disease, otherwise known as bovine spongiform encephalopathy or BSE, affects the brain and spine of an animal. Those body parts are kept out of grocery stores and restaurants and have no contact with the meat that does make its way to consumers. Eating contaminated meat can cause Cruetzfeld Jakob (vCJD) disease in humans.
While many diseases are caused by bacteria and viruses, mad cow disease is caused by a transmissible protein called a prion, according to the U.S Centers for Disease Control and Prevention. While the function of these proteins in nature is unknown, what is known is that they act on the human nervous system to cause rapid brain degeneration.
An outbreak of mad cow disease infected thousands of cows in Britain in the 1990s and led to the deaths of nearly 150 British people who ate tainted meat. The outbreak was the result of tainted cattle feed, which at the time routinely included protein supplements made of ground spinal column material and brain tissue. That practice is currently prohibited by the FDA.
On Tuesday, U.S. officials announced that the cow had atypical BSE, meaning that the disease did not come from tainted cattle feed. Instead, scientists think atypical BSE is the result of a rare random mutation.
This is the fourth case of mad cow disease in the U.S. cattle supply since December 2003. The first U.S. BSE case was in 2003, found in a Washington state dairy cow that had been slaughtered. The second case was found in June 2005 in a cow in Texas, and the third was found in a cow in Alabama in 2006.
Officials are uncertain if the cow died of the disease and if others in the herd were infected. Officials are working now to identify where the cow came from and to test other cows in the same feeding herd, the Associated Press reports.
Dr. William Schaffner, chair of preventive medicine at Vanderbilt University, said it's unlikely that more cows will be infected.
"Mad cow occurs in animals as it does in humans -- rarely and sporadically. At this point, I would not expect there to be another cow to be found," he said.
Humans who have the disease tend to experience a progressive neurological or psychiatric disorder that lasts longer than six months with symptoms including dementia, seizure, unusual sensory symptoms, dizziness, or progressive unusual mood changes. The illness usually lasts 14 months, and the disease is always fatal.
As of last year, 221 cases of probable vCJD had been reported. This includes 172 in the United Kingdom, 25 in France, five in Spain, four in Ireland and three in the United States, with a smattering of cases in other countries around the world. The patients that were diagnosed in the United States were thought to have been infected while they were residing in the UK or living abroad in Saudi Arabia.
ABC News' Dr. Christopher Tokin contributed to this report.