A dairy cow in California has bovine spongiform encephalopathy (BSE), or "mad cow" disease, the United States Department of Agriculture has confirmed.
The diagnosis is only the fourth in US history and the first since 2006, the BBC reports.
The department's veterinary chief John Clifford said the recently diagnosed dairy cow in central California had not entered the food supply. The disease is not transmitted through milk.
"There is really no cause for alarm here with regard to this animal."
Mr Clifford said the discovery would not affect beef exports.
The disease was detected as part of a screening programme that tests an estimated 40,000 animals every year, the USDA said.
Canada said it did not expect the diagnosis to affect food trade between the two nations. The first outbreak of mad cow in the US occurred in 2003 and hurt global trade in beef.
Mad cow disease is always fatal in cattle, and research suggests humans can contract a similar deadly brain disease, vCJD, by eating infected meat from infected animals.
That disease causes personality change, loss of body function, and eventually death.