updated 6/25/2005 12:43:27 PM ET 2005-06-25T16:43:27

The government hopes DNA tests to find the herd where the cow with mad cow disease came from can lead to the source of the infection, an Agriculture Department official said Saturday.

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The United States confirmed what may be its first homegrown case of mad cow disease on Friday, seven months after officials first suspected the animal might be infected.

Pinpointing the cow's herd will help track the animal's feed and explow it became infected. The only known way the disease spreads is through feeding infected cattle remains to other cattle, which the U.S. banned in 1997.

"We're pretty confident that we have the herd, but we want to make sure," John Clifford, the department's chief veterinarian, said in an interview with The Associated Press. "Testing is being done now on tissue from cows that may have been herdmates."

Mistakes made
The effort is complicated by mistakes made after the animal was killed. The cow's type of breed was accidentally mislabeled, and its tissues were mixed with tissues from other cows, Clifford said.

Despite the delay in reliable results, the government says the food safeguards are working well.

"The fact that this animal was blocked from entering the food supply tells us that our safeguards are working exactly as they should," Agriculture Secretary Mike Johanns said at a news conference Friday.

Taiwan renews ban
Still, the emergence of a native-born case could cast a shadow over the nation's 96 million cattle, the largest herd in the world. Taiwan, which imported more than $76 million in U.S. beef in 2003, announced Saturday it would immediately reimpose its ban on U.S. beef. Japan, once the largest importer of U.S. beef, still has not lifted its ban.

The only previous U.S. case, confirmed in December 2003, was in a dairy cow that had been imported from Canada, where three other cases have been found. Even that 2003 case involving an imported animal prompted some 50 nations to ban U.S. beef imports.

While Johanns would not say where the cow turned up, he said there was no evidence it was imported.

No surprise
Johanns said the new case was no surprise, given that the department is testing about 1,000 cattle a day. Since escalating its testing after the 2003 case, the government has screened about 388,000 animals.

An internationally recognized laboratory in Weybridge, England, confirmed the new case Friday after U.S. tests produced conflicting results.

The animal was a "downer" that could not walk and was delivered to a rendering plant for animals unfit for human consumption. The government banned downer cows from the food supply just days after the 2003 case.

The ban on downer cows is one of many safeguards aimed at keeping the disease from getting into the food or feed supply.

Also banned are tissues, including the brain, skull and spinal cord, from older cows believed to carry the disease. Those materials must be removed from slaughtered cows older than 30 months because it is believed that infection levels increase with age.

In addition, the U.S. and Canada banned the use of cattle parts in cattle feed in 1997 following the mad cow disease outbreak in Britain.

Series of conflicting results
Officials have not said how old the infected U.S. cow was but said it was born before the feed ban.

The feed ban has loopholes allowing cattle to be fed poultry litter, blood and restaurant leftovers, all potential pathways for mad cow disease.

The new case was confirmed after a series of conflicting test results.

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 the Western blot, until the department's inspector general, Phyllis Fong, ordered it to do so two weeks ago. Fong has not explained why she ordered new tests.

Mad cow disease -- medically known as bovine spongiform encephalopathy, or BSE -- kills brain cells and leaves spongy holes behind. 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.

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