Dr. Claude Lavigne, a top official at Canada's animal products directorate, said officials are searching for other cows infected with the disease, also known as bovine spongiform encephalopathy.
"At this point, it's a precautionary measure," he said Wednesday in a teleconference with U.S. and Canadian reporters.
On Tuesday, Canada said it found a cow had been sick with mad cow disease, a brain-wasting illness. Officials got the test results four months after taking samples from the cow at a rendering plant.
U.S. and Canadian officials have said none of the infected animal went into the food supply.
Canada still is looking for where the 8-year-old cow was born and other cattle, including newborn calves, with which it may have been in contact. The disease can incubate in a cow for up to eight years.
Sen. Byron Dorgan of North Dakota said Canadian officials should have promptly checked samples and released the results.
"Somehow it took four months to have it tested and to tell the people in this country, and also in Canada there was a cow with mad cow disease killed in January," said Dorgan. "That's absurd."
Although Canadian and U.S. officials have declared that the food supply is safe, Consumers Union said the two governments cannot guarantee that.
"That's exactly what they said in Britain, and now nobody trusts the government there," said Mike Hansen, a senior research associate at the U.S. consumers group.
Mad cow disease sickened herds in Britain in 1986, killing more than 2 million cows. It also was linked to 130 human cases of Creutzfeldt-Jakob disease, the human form which causes paralysis and death. Scientists believe humans get it by eating meat from infected cattle.
Canadian veterinarians took samples of the infected brood cow on Jan. 31 at a rendering plant. They suspected the cow had pneumonia
a belief that led to the long delay in discovering that the animal had mad cow disease, said Dr. Gerald Ollis, Canada's chief provincial veterinarian.
Samples were sent to the Edmonton laboratory on Feb. 8.
"They sat there until we had time to process them," Ollis said.
The lab checks samples from livestock destined to be made into food and then tests those taken from euthanized and downed animals.
Canadian officials do not know how the cow got sick with the fatal disease. Cattle can get mad cow by eating feed that contains infected animal protein products, such as bone meal. Both Canada and the United States outlawed such feed for cattle, sheep and goats in 1997 to guard against the disease.
The discovery led to bans on Canadian beef in the United States, Australia, South Korea, Singapore, New Zealand and Japan, which had an outbreak last year.
The United States imported nearly 1 billion pounds of Canadian beef and 1 million head of cattle last year, according to the National Cattlemen's Beef Association.
The group's chief executive, Terry Stokes, said he believed American consumers should be confident that their food is safe because of the ban and because the U.S. government routinely tests for the disease.