The animal was tested as part of a regular inspection programme organised by the US Department of Agriculture that examines tissue from 40,000 slaughtered cattle each year.
Dr John Clifford, the USDA's chief veterinary officer, said the department was continuing its investigation of the case, but added "there should be no concern … about the safety of our food supply."
Despite assertions from Clifford that the case should not affect trade in meat, two of South Korea's largest importers of US beef halted sales.
The disease, bovine spongiform encephalopathy (BSE), is caused by misfolded proteins called prions, which build up in the brain with fatal consequences.
Unlike the viruses that cause many other diseases in farm animals, prions do not spread through casual contact or by coughs and sneezes. The infectious prions are concentrated in certain tissues of the animals, such as the nervous system.
The huge outbreak of BSE in Britain that peaked in the 1990s was driven by cattle feed that contained infectious prions from slaughtered animals that had the disease. The tainted feed spread the prions through the national herd and into the human food chain, which led to cases of a related disease in humans, variant Creutzfeldt–Jakob disease (vCJD).
Random genetic mutations happen all the time in nature, so once in a while a cow will be born with a mutation that makes the BSE prion. In the latest case - the fourth in US cattle and the first since 2006 - the animal showed no symptoms and no meat entered the human food supply, according to the USDA.
Since the 1990s, Britain, the US and many other countries have removed brain and spinal cord from animals at slaughterhouses to prevent these tissues from entering the food supply. Other measures are aimed at preventing animal remains from being recycled into animal feed.
Research into BSE and vCJD has tailed off with a decline in fresh cases of both diseases, but there is still much that scientists do not understand about the infections. Most, if not all, cases of vCJD have been in the 40% of the population who have what is called the MM form of the human prion protein. When BSE infects a person, it is this protein that becomes misfolded and drives the disease.
The remaining 60% of the population, who have one of two other variants of the human prion (MV or VV) may harbour the disease but take longer for symptoms to show. Future cases might arise in these people up to 50 years after consuming infected beef.
Last year, Britain's Health Protection Agency published the results of tests on tissue taken from people's appendixes and found the disease-causing prion in around one in 4,000 cases. The figures suggest there could be between 15,000 and 20,000 people in the UK who are incubating the disease.
