Tests have confirmed mad cow disease (search ) in what appears to be the first case in a U.S.-born animal, the Agriculture Department said Friday. Officials would not specify where the case turned up, but Agriculture Secretary Mike Johanns said there is no evidence the cow was imported.
An internationally recognized laboratory in Weybridge, England, confirmed the case after U.S. tests produced conflicting results, Johanns said. The animal had been tested last year and cleared of having the brain-wasting illness.
Read Web MD's "Mad Cow Disease: Know the Basics"
New tests were ordered two weeks ago. Those results came back positive, leading officials to seek confirmation from the Weybridge lab. The department also performed more tests at its lab in Ames, Iowa.
The first case of mad cow disease in the United States was confirmed in December 2003. It turned up in Washington state in a dairy cow imported from Canada. Like the first case, the new case was in an animal at least 8 years old, meaning it was born before the United States banned cattle parts in cattle feed, which is how the disease is believed to spread.
Read Web MD's "USDA Beefs Up Meat Safety Precautions"
Human health is not at risk in the new case, Johanns said. The animal was a "downer," meaning it was unable to walk. Such animals are banned from the food supply.
"I am encouraged that our interlocking safeguards are working exactly as intended," Johanns said at a news conference. "This animal was blocked from entering the food supply because of the firewalls we have in place. Americans have every reason to continue to be confident in the safety of our beef."
Officials said the brain tissue samples appeared different from the classical form of mad cow disease seen in Britain, where there was an outbreak in the 1990s, but they are classifying it as mad cow disease, anyway.
Johanns said his department will start conducting more sensitive tests as a matter of routine. The department has come under fire from both consumer groups and cattlemen for not resolving conflicting test results on this animal last November.
The department did initial screening using a "rapid test," which was positive. A more detailed immunohistochemistry, or IHC test (search ), was negative. But the department did not conduct a third round, using a test called the Western blot, until the department's inspector general ordered it to do so two weeks ago, said USDA officials, including the inspector general.
Now the department will use both IHC and Western blot when rapid tests indicate the presence of the disease, Johanns said.
"By adding the second confirmatory test, we boost that confidence and bring our testing in line with the evolving worldwide trend," he said.
U.S. officials escalated testing for the disease after the first U.S. case. More than 388,000 dead cattle have been screened in the past 18 months, compared with about 2,000 screenings annually before then.
Mad cow disease — officially called bovine spongiform encephalopathy (search ), or BSE — occurs when proteins called prions bend into misfolded shapes. They deposit plaque that kills brain cells and leaves behind spongy holes.
A form of the disease in people is variant Creutzfeldt-Jakob disease (search ). It has been linked to the consumption of contaminated meat. The disease has killed about 150 people worldwide, mostly in Britain.