The US Department of Agriculture said it was investigating the herd of the infected beef cow, which was described as at least eight years old. Meat from the cow was not sold to consumers or as animal feed.
US Agriculture Secretary Mike Johanns said the USDA would change some testing procedures, but defended current safeguards as more than adequate to protect consumers.
“The BSE threat to humans in this country is so remote that there’s a better chance you’ll get hurt crossing the street to get to the grocery store than by the beef you buy in the grocery store,” Mr Johanns said at a news conference.
The announcement came a week before the US Independence Day celebration on July 4, where Americans traditionally barbecue hamburgers at a rate that makes it one of the year’s biggest days of beef consumption.
The first case of the disease 18 months ago prompted Japan and Korea to halt imports of billions of dollars of US beef.
Neither has yet resumed American purchases, but Mr Johanns said he did not expect the new case to hurt negotiations.
Officials said the infected animal was born before a 1997 ban on recycling cattle remains into cattle rations. There was no evidence the infected animal was imported, said the USDA, which was doing DNA testing to confirm its herd of origin.
The previous US confirmed case of mad cow disease, or bovine spongiform encephalopathy (BSE), was found in December 2003 in a Washington state dairy cow imported from Canada.
Officials said two confirmed infections was a tiny number considering the 30 million cattle slaughtered annually in the United States.
Ron DeHaven, head of the USDA’s Animal and Plant Health Inspection, said the infected animal was slaughtered at a pet food plant.
Neither official would identify the state where the cow came from. Published reports have said it was sent to slaughter last November in Texas, the nation’s biggest cattle state.
Mad cow disease is believed to be spread through infected livestock feed.
Consumer groups and some lawmakers called for tougher feed rules and BSE testing on all high-risk cattle.
Taiwan recently said it would reinstate a ban on US beef if a second case was confirmed. But a Japanese farm ministry official said on Friday that even if the United States confirmed a second case, the ministry would still take steps to resume some imports of US beef.
USDA officials said the new BSE case was a different strain from the first US case and the outbreak that occurred in Britain in the 1980s. Some experts had speculated the conflicting test results were due to an unusual type of BSE.