Tests have confirmed mad cow disease in a U.S. cow previously cleared of having the brain wasting illness, the Agriculture Department said today. It is the second case of mad cow disease in the United States.

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WASHINGTON —
Tests have confirmed mad cow disease in a U.S. cow previously cleared of having the brain wasting illness, the Agriculture Department said today. It is the second case of mad cow disease in the United States.

An internationally recognized laboratory in Weybridge, England, confirmed the case of mad cow disease after U.S. tests produced conflicting results, Agriculture Secretary Mike Johanns said.

Human health was not at risk, Johanns said. The animal was a “downer,” meaning it was unable to walk. Such animals are banned from the food supply.

The department has said there was no reason to believe the animal was imported.

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 confirmed in the U.S. was in December 2003, a dairy cow imported from Canada.

“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,” he said.

Johanns also announced the department will do more sensitive tests as a matter of routine. The department has come under fire for not resolving conflicting test results on this animal in November.

The department did initial screening using a “rapid test,” which was positive. A more detailed immunohistochemistry, or IHC test, was negative.

But the department did not conduct a third round, using a test called the Western blot, until ordered by the department’s inspector general.

Now the department will use both IHC and Western blot when rapid tests indicate the presence of the disease, Johanns said.

“I want to make sure we continue to give consumers every reason to be confident in the health of our cattle herd,” Johanns said.

“By adding the second confirmatory test, we boost that confidence and bring our testing in line with the evolving worldwide trend,” the secretary said.

Mad cow disease, or bovine spongiform encephalopathy, 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. It has been linked to the consumption of contaminated meat. The disease has killed about 150 people worldwide, mostly in Britain, where there was an outbreak in the 1990s.