A dead cow in Alabama state has tested positive for BSE, or "mad cow disease", the third such case in the US.
Confirming the result, the US authorities stressed the cow had not entered the human food supply chain.
The infected cow had been in the cattle herd in Alabama for less than a year, officials said. An investigation into its origins is under way.
The first US case was found in the state of Washington in 2003. A second was discovered last June.
The latest case was discovered in a routine test last week.
'Safe beef'
The US Agriculture Department's chief veterinarian, John Clifford, said they were working with Alabama officials "to gather any further information we can on the herd of origin of this animal".
He went on to emphasise that "a system of interlocking safeguards" protected human and animal health, and "we remain very confident in the safety of US beef".
"Again, this animal did not enter the human food or animal feed chains," he added.
The first US case in 2003 - in a cow imported from Canada - halted billions of dollars worth of American beef exports and raised questions about the safety of the US food supply.
A form of the disease in people, called variant Creutzfeldt-Jakob disease, has been linked to eating contaminated meat.
The disease has killed about 150 people worldwide, mostly in Britain, the epicentre of an outbreak in the 1990s.
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