Public health officials in the United States said the risk to the public was extremely low, and that residents do not need to take any specific precautions.
But in South Korea, one of the largest importers of U.S. beef, the discovery was enough to prompt retailer LotteMart to remove American beef from store shelves.
"Currently, the sale of U.S. beef is temporarily suspended to ease our customers from anxiety," LotteMart said.
For its part, South Korean government said it will step up checks on U.S. beef imports -- but not halt it for now.
In 2010, South Korea imported 125,000 tons of U.S. beef, a 97% increase from the year before, the United States Department of Agriculture said.
The carcass was at a Baker Commodities Inc. rendering facility in Hanford, California, according to Executive Vice President Dennis Luckey.
The company renders animal byproducts and had randomly selected the animal for testing last Wednesday, he said.
"We are in the business of removing dead animals from dairies in the Central Valley," he told CNN in a telephone interview. "As part of that program, we participate in the BSE surveillance program."
The sample was sent to UC Davis for initial testing, which came back inconclusive. It was then sent to the U.S. Department of Agriculture's laboratory in Ames, Iowa, where it tested positive, the agency said.
The carcass was in quarantine Tuesday night. "We're waiting now for USDA to tell us how to dispose of it," Luckey said.
The fatal disease was blamed for the deaths of 150 people in Britain, where there was an outbreak in the 1980s and 1990s.
In people, symptoms of the disease include psychiatric and behavioral changes, movement deficits, memory disturbances and cognitive impairments.
BSE can cause infected animals to display nervousness or aggression, difficulty in coordination and standing up, decreased milk production or loss of body weight, according to the agency.
It is usually transmitted between cows through the practice of recycling bovine carcasses for meat and bone meal protein, which is fed to other cattle. In this case, the USDA reports that it was a rare form of BSE not likely carried by contaminated feed.
The head of the USDA said he remains confident in the health of the national herd and the safety of beef and dairy products.
"This is an atypical case, which means it's not connected in any way, shape or form to feed ... A very rare circumstance and situation," said Agriculture Secretary Tom Vilsack told CNN's "John King, USA." "It can just sort of come up and pop up -- sometimes it's genetic."
Since 2004, the USDA has removed the brain and the spinal column -- the parts suspected of causing mad cow disease in humans -- from the food system.
The Centers for Disease Control and Prevention reports that the odds of a person contracting mad cow disease, even after consuming contaminated products, are less than one in 10 billion.
Unlike most other meat-borne illnesses, such as those caused by E.-coli bacteria, cooking does not kill the infectious agent that causes mad cow disease.
Consumers who wish to exercise extra caution can follow the advice presented by the Web-based consumer advocacy group Consumeraffairs.com, which advises the avoidance of brains, neck bones and beef cheeks, bone marrow and cuts of beef that are sold on the bone. The group also says to choose boneless cuts of meat and ground beef only if it has been ground in the store.
"Evidence shows that our systems and safeguards to prevent BSE are working, as are similar actions taken by countries around the world," said John Clifford, the USDA's chief veterinary officer.
Last year, 29 cases of BSE were reported worldwide, down 99% since the peak of 37,311 cases in 1992. "This is directly attributable to the impact and effectiveness of feed bans as a primary control measure for the disease," he said.
"A case of a single cow with bovine spongiform encephalopathy is not a reason for significant concern on the part of consumers, and there is no reason to believe the beef or milk supply is unsafe," said Sarah Klein, food safety attorney for the Center for Science in the Public Interest.
"If the cow were exposed to the typical strain of BSE via animal feed -- and the government says that's not the case here -- that would have represented a significant failure."
But she said the government would have had a difficult time tracking down other cattle that may have been eaten the same feed because the nation lacks an effective animal ID program.