The first new case of mad cow disease in the United States since 2006 has been discovered in a dairy cow in California, but health authorities said Tuesday the animal never was a threat to the nation's food supply.
The infected cow, the fourth ever discovered in the United States, was found as part of an Agriculture Department surveillance program that tests about 40,000 cows a year for the fatal brain disease.
No meat from the cow was bound for the food supply, said John Clifford, the department's chief veterinary officer.
"There is really no cause for alarm here with regard to this animal," Clifford told reporters at a hastily convened news conference.
Mad cow disease, or bovine spongiform encephalopathy (BSE), is fatal to cows and can cause a fatal human brain disease in people who eat tainted beef. The World Health Organization has said that tests show that humans cannot be infected by drinking milk from BSE-infected animals.
In the wake of a massive outbreak in Britain that peaked in 1993, the U.S. intensified precautions to keep BSE out of U.S. cattle and the food supply. In other countries, the infection's spread was blamed on farmers adding recycled meat and bone meal from infected cows into cattle feed, so a key U.S. step has been to ban feed containing such material.
Tuesday, Clifford said the California cow is what scientists call an atypical case of BSE, meaning that it didn't get the disease from eating infected cattle feed, which is important.
That means it's "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."
The testing system worked because it caught what is a really rare event, added Mike Doyle, director of the University of Georgia's Center for Food Safety.
Dennis Luckey, executive vice president of Baker Commodities, told the Associated Press that the disease was discovered at its Hanford, Calif., transfer station when the company selected the cow for random sampling. Luckey said the cow died at the dairy and was randomly tagged for the surveillance program.
Michael Marsh, chief executive of Western United Dairymen, said it was an adult cow over 30 months old, not a downed or sick animal, and it appeared normal when it was last observed. He said the cow was first tested on April 18.
Rendering plants process animal parts for products not going into the human food chain, such as animal food, soap, chemicals or other household products.
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.
The Canadian Food Inspection Agency said Tuesday that the latest finding would not affect trade between the United States and Canada.