The United States has what may be its first home-grown case of mad cow disease, confirmed a full seven months after officials first suspected the animal might be infected.
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 on Friday.
While Johanns would not say where the cow turned up, he said there was no evidence it was imported.
Johanns said the new case was no surprise, given that the department is testing about 1000 cattle a day.
Since escalating its testing after the 2003 case, the government has screened about 388,000 animals.
"Frankly, we have said all along that we expected additional positive test results," Johanns said. "One positive result out of 388,000 tests in our enhanced surveillance programme indicates that the presence of the disease is extremely, extremely low."
An internationally recognised laboratory in Weybridge, England, confirmed the new case on Friday after US tests produced conflicting results.
The department did initial screening using a "rapid test", which was positive.
"There's a better chance you'll get hurt crossing the street to get to the grocery store than by the beef you buy in the grocery store"
US Agriculture Secretary
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.
Results from those tests came back positive, leading officials to seek confirmation from the Weybridge lab.
The department also performed more tests at its lab in Ames, Iowa. Now the department will use both IHC and Western blot when rapid tests indicate the presence of the disease, Johanns said.
Consumer groups and cattle farmers have criticised the department for not using the test to resolve the conflicting results.
Still, the emergence of a native-born case could cast a shadow over the nation's 96 million cattle, said to be the largest herd in the world.
The only previous US case, confirmed in December 2003, was in a dairy cow that had been imported from Canada, where three other cases have been found.
Officials have not revealed the infected US cow's age but said it was born before the feed ban.
Confidence in US beef
"There's a better chance you'll get hurt crossing the street to get to the grocery store than by the beef you buy in the grocery store," Johanns said. "There is absolutely no question in my mind that Americans can and should continue to be very confident in our beef supply."
The US has 96 million cattle,
more than any other country
Even the 2003 case involving an imported animal prompted some 50 nations to ban US beef imports.
Japan, once the largest importer of US beef, and some other countries have yet to lift those bans.
Taiwan on Saturday reimposed its ban on US beef.
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 humans 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.
The only known way the disease spreads is through feeding infected cattle remains to other cattle.