The first American case of mad cow disease since 2006 was found this week in a dairy cow in California, but the animal had not been slaughtered for food, government officials said.

Known formally as bovine spongiform encephalopathy, or BSE, the infection decimated English cattle herds in the 1980s and 1990s. It was also linked to about 225 cases worldwide of a fatal human brain aliment known as variant Creutzfeldt-Jakob disease.

Only three previous cases of BSE have been found in cattle in the United States, and no cases of the human version of the disease have been linked to U.S. beef.

The new case will spark a search for animals born on the same farm about the same time as the stricken animal. If found, those animals almost certainly will be killed.

Agriculture Department officials would not provide the location of the farm or the size of its herd. Government officials, however, were quick to reassure the public. “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. “USDA has no reason to believe any other U.S. animals are currently affected, but we will remain vigilant and committed to the safeguards in place.”

Cows suffering from BSE are believed to be infected soon after birth, although symptoms do not appear for years. The animal’s age is being investigated. “There is a lot of information we need to get,” said John Clifford, the USDA’s chief veterinarian.

Old dairy cows are frequently used to make hamburger, but this cow had been collected by a renderer, and the disease was discovered in random testing. Baker Commodities picked up the cow from the dairy after it died, said Dennis Luckey, the company’s executive vice president. Luckey said samples were taken from the cow last Wednesday.  They were sent to UC Davis for testing, and results were inconclusive. The samples were then sent to a USDA lab in Iowa for further analysis. The company learned on Tuesday that the final results were positive when the USDA made the announcement.

The meat industry pointed to the rarity of BSE — only four American animals diagnosed since 2003, including the new case — as evidence that control measures have worked well. “That translates into one of the lowest rates of BSE in any nation that has ever diagnosed a case,” said James H. Hodges, executive vice president of the American Meat Institute.

But one consumer group immediately voiced alarm.

Of the millions of cows slaughtered each year, the government tests only 40,000 for the disease, said Michael Hansen, a scientist at Consumers Union. “So we really don’t know if this is an isolated unusual event, or whether there are more cases in U.S. beef,” he said. “Our monitoring program is just too small.”

In South Korea, the country’s No. 2 and No. 3 supermarket chains said they have “temporarily” halted sales of U.S. beef, the Associated Press reported.

The European BSE epidemic is believed to have started when cattle ate feed containing brain and nerve tissues from animals with BSE. Feed supplemented with meat and bones from specific animals is now banned.

The chance of human infection is further lessened by a ban on cattle brain and spinal cord as food for humans and the prohibition of butchering practices that might inadvertently contaminate beef with nerve tissue.

How the California cow got the disease remains unknown. Government officials expressed confidence that contaminated food was not the source, saying the animal had atypical L-type BSE, a rare variant not generally associated with an animal consuming infected feed.

However, a BSE expert said that consumption of infected material is the only known way that cattle get the disease under natural conditons.

“In view of what we know about BSE after almost 20 years experience, contaminated feed has been the source of the epidemic,” said Paul Brown, a scientist retired from the National Institute of Neurological Diseases and Stroke.

BSE is not caused by a microbe. It is caused by the misfolding of the so-called “prion protein” that is a normal constituent of brain and other tissues. If a diseased version of the protein enters the brain somehow, it can slowly cause all the normal versions to become misfolded. It is possible the disease could arise spontaneously, though such an event has never been recorded, Brown said.