Confirmation that a Holstein cow in Washington State had the deadly disease came Thursday from the Veterinary Laboratories Agency in Weybridge, England. British researchers agreed with the reading of U.S. tests on the stricken cow.
Those tests led U.S. officials to preliminarily conclude that the animal had the brain-wasting disease.
"We are considering this confirmation," said USDA spokeswoman Alisa Harrison, adding that the English lab still would conduct its own test using another sample from the cow's brain. Final test results on the cow from Washington State are expected by the end of the week, she said.
Professor Steven Edwards, chief of the British lab, said those results already have been given to USDA. But Edwards refused to disclose whether the tests showed that the animal had mad cow disease, officially known as bovine spongiform encephalopathy or BSE.
CBS News Correspondent Jerry Bowen reports Federal officials are now trying to find the herd the cow was raised with since it was likely sickened several years ago while with a different group of animals.
Eating feed that includes parts from an infected cow spreads the disease. The incubation period in cattle is four to five years, said Dr. Stephen Sundlof of the Food and Drug Administration.
Authorities also want to know where the animals were transported and have narrowed their search to two unidentified livestock markets in Washington State, where the sick cow could have been purchased.
The cow had lived since 2001 at the Sunny Dene Ranch in Mabton, Wash., a town 40 miles south of Yakima, according to government sources speaking on condition of anonymity. Officials have said a dairy farm near Mabton is under quarantine and that its herd would be slaughtered if the mad cow diagnosis were confirmed.
Investigators worked through the Christmas holiday to prevent a potential outbreak of the deadly disease and to calm public fears about the food supply, Harrison said.
Despite bans of imports of U.S. beef around the world, government officials have said there was no threat to the food supply because the cow's brain, the spinal cord and the lower part of the small intestine — where scientists say the disease is found — were removed before it was sent on for processing.
Humans can contract a fatal variant of mad cow disease by eating infected beef products, but experts say muscle cuts of beef — including steaks and roasts — are safe.
Still, authorities scrambled to find where the meat cut from the animal was sent. The Agriculture Department already has issued a recall for beef slaughtered along with the infected cow Dec. 9 at a meat company in Moses Lake, Wash.
Agriculture Secretary Ann Veneman said the recall was an extra precaution.
But the government came under criticism from John Stauber, the author of "Mad Cow U.S.A." He said the U.S. hasn't done enough to keep BSE out of the country.
Cattle get sick by eating feed that contains tissue from the brain and spine of infected animals. The United States has banned such feed since 1997.
"Here's the problem. The feed ban has been grossly violated by feed mills," Stauber said in a telephone interview from his home in Madison, Wis.
In one such case, X-Cel Feeds Inc. of Tacoma, Wash., admitted in a consent decree in July that it violated FDA regulations designed to prevent the possible spread of the disease.
The Food and Drug Administration says only two companies have serious violations of the 1997 regulations.
Stauber also said he believed the ban has been ineffective because it exempts blood from cattle, which he said could transmit mad-cow type diseases. Government officials and industry executives have said there was no evidence that animals could be infected from the blood of other animals.
A misshapen protein —a prion — that eats holes in a cow's brain causes BSE. A total of 153 people worldwide have been reported to have contracted the human form of the illness, according to the federal Centers for Disease Control and Prevention.