Canadian officials said no part of the cow - the third case of the disease ever found in Canada - has entered the human or animal feed system.
But the announcement may strengthen the position of a group of U.S. cattlemen who have sued to block the lifting of the ban. The cattlemen say allowing the trade will hurt U.S. producers and put consumers at risk.
U.S. Department of Agriculture officials said Tuesday they are sending a team to Canada to evaluate the latest mad cow case before deciding whether to change their plan to resume imports.
Ron DeHaven, administrator of the Agriculture Department's Animal and Plant Health Inspection Service, said the agency is confident that protections in place in Canada and the United States will safeguard U.S. consumers and livestock.
The latest case of bovine spongiform encephalopathy (BSE) was discovered in an animal under 7 years old. Canadian Agriculture Minister Andy Mitchell said the case was unrelated to the second case of the disease, which was confirmed Jan. 2 and also involved an Alberta cow.
"A third case of BSE in a cow in Canada has been confirmed today through our surveillance system," Mitchell said in a news conference in Ottawa, the nation's capital. "This is not unexpected, as we have already acknowledged that there is a low level of incidence of BSE in North America."
The latest sick cow was born after a 1997 feed ban in Canada removed the use of animal remains in feed, commonly believed to be the cause of the disease. This makes this third Canadian cow with BSE more serious than the others because the cause could be much more difficult to trace.
However, Dr. Brian Evans of the Canadian Food Inspection Agency said the cow was most likely exposed to feed before the ban came into effect in the fall of 1997 and that his agency was investigating.
Nevertheless, Mitchell stressed the new case was not a cause for alarm.
"The rules upon which beef should move between countries should be based on science, and we believe that Canada has clearly followed a scientific approach," he said. Mitchell pledged that Canada would toughen measures to stem the disease and that the Canadian Food Inspection Agency, with an independent third party, would conduct an audit of the country's animal feed safety.
The Jan. 2 case was announced after the disease was detected in an 8-year-old cow from Alberta. It was born in the same herd - within one year - of a cow shipped to the United States in February 2002 for immediate slaughter, the USDA said.
Canada's first case of mad cow surfaced in May 2003, prompting the U.S. government to close the border to Canadian beef imports. Concerns persisted after a Canadian-born cow in Washington state was found in December 2003 to have the disease, which attacks the animal's nervous system.
All three cases have involved animals from Alberta.
On Dec. 29, the Bush administration announced plans for lifting the ban.
After learning of the second case days later, the administration had said it would stand by its decision, expressing confidence that public health measures in both countries will protect U.S. livestock and consumers.
Mad cow disease eats holes in the brains of cattle. Food contaminated with BSE can afflict humans with a variant of Creutzfeldt-Jakob disease, which is usually fatal.
The border closure by the United States has cost the Canadian beef industry at least $3 billion. Before the trade ban, animals regularly crossed the border and Canada sold more than 70 percent of its live cattle to the United States.
Imports of some packaged beef resumed in the fall of 2003, but it was not until this month that Washington said it would resume trade in live animals on March 7.
The USDA ruling declared Canada a "minimal-risk region" so that cattle could be shipped into the United States under certain restrictions. The cattle must be slaughtered by the age of 30 months, which scientists say is too young to contract mad cow disease, and they must also be transported in sealed containers to a feedlot or slaughter house.
Under World Health Organization guidelines, a country may have up to 11 cases of mad cow disease in a year and still be considered a minimal-risk country.
It appeared in Britain in 1986 and spread through Europe and Asia, prompting massive destruction of herds and devastating the European beef industry. So far 147 people in Britain, and another 10 elsewhere, are known to have contracted Creutzfeldt-Jakob disease since BSE was first identified.
By Colin McClelland