Canada has said it is too soon to say the US' first mad cow case began in an animal from one of its herds.
The US farm where BSE was discovered has been quarantined
US agriculture officials reported that the diseased cow came from Alberta in August 2001.
But Ottawa's chief veterinarian Dr Brian Evans said any conclusion on the cow's origin was "premature".
Two US cattle herds are now in quarantine after the BSE discovery, which could cost the American beef industry billions of dollars.
US Chief Veterinary Officer Dr Ron DeHaven said the dairy cow found to have BSE was believed to have been one of 74 imported from Canada into Idaho.
The cow recently gave birth to a bull calf which was sold to another farm in Washington state. None of the calves on that farm have been tagged, officials said, so all males under 30 days old are likely to be killed as a precaution.
Dr DeHaven said it was not yet known where the other 73 animals imported from Canada were, adding that they may or may not be infected.
But his Canadian counterpart, Dr Evans, said the details on the cow's records in the US do not match those kept in Canada.
"As yet, there is no definitive evidence that confirms that the BSE-infected cow originated in Canada," Dr Evans said.
Canada reported its own first case of mad cow disease - or bovine spongiform encephalopathy - in Alberta last May. It is not known if the US and Canadian cases are connected.
Correspondents say the Canadian beef industry would suffer again if the Canadian link is confirmed, just as it was beginning to recover after exports fell in the wake of the may case.
In the US, experts have predicted this week's news of the BSE case will cost the US cattle industry billions of dollars. At least 26 countries have now banned imports of American beef because of BSE fears.
On Saturday, government officials left the US for Japan, the top buyer of US beef, to discuss how trade could be maintained.
Human disease link
US officials say American beef is still safe because the infected cow's brain and spinal cord - which is the only part normally tainted by the disease - was removed before the meat was processed.
The US Department of Agriculture said meat from the infected cow was sold in four western states - Washington, Oregon, California and Nevada.
BSE has been linked to new variant Creutzfeldt-Jakob Disease (vCJD), a human brain-wasting disease.
First diagnosed in Britain in 1986, BSE affected 178,000 British cattle and resulted in the eventual destruction of 3.7 million animals.
It cost British farming billions of pounds as countries around the world banned British beef.