Canadian authorities have confirmed a new case of mad cow disease - in the western province of British Columbia.
The US is to send an expert to help with the investigation
It is the country's fifth native-born case and the second this year.
Food inspection authorities said the finding in the Fraser Valley dairy cow did not endanger the public as no infected part entered the food chain.
Scientists believe humans can contract a deadly brain-wasting disease by consuming beef products from cows with bovine spongiform encephalopathy (BSE).
Canadian health authorities said in a statement: "This finding does not affect the safety of Canadian beef.
"Tissues in which BSE is known to concentrate in infected animals are removed from all cattle slaughtered in Canada for domestic and international human consumption."
The US closed its border to Canadian beef in May 2003 after a cow in Alberta tested positive for BSE.
But this year it resumed conditional imports of beef from Canadian cattle aged under 30 months and of young live cattle for slaughter.
The US responded to the latest case by saying it would not block such imports.
US Agriculture Secretary Mike Johanns said his department would send an expert to join the Canadian investigation into the latest case.
"Based on the information currently available, I do not anticipate a change in the status of our trade," Mr Johanns said.
Experts believe more than 150 people, mostly in Britain, have died from variant Creutzfeldt-Jakob Disease as a result of eating BSE-infected beef.