But officials with the Florida Department of Health emphasized that there is no reason to suspect cattle in the United States have the cow version of the disease, known as bovine spongiform encephalopathy, or BSE.
BSE is a brain-destroying illness that first surfaced in British cattle but now has spread to cattle in much of Europe. A human form, referred to as "new variant Creutzfeldt-Jakob disease," apparently spread by eating infected beef, has claimed more than 90 lives in Britain and parts of Europe.
The woman in Florida is believed to have caught the disease by eating beef products in Britain at the height of that country's cattle epidemic in the late 1980s or early 1990s, said Dr. Steve Ostroff of the Centers for Disease Control and Prevention.
"There's every reason to suspect that she acquired her illness there," he said Thursday.
"All evidence indicates her illness poses no threat to anyone else or the agriculture industry," said state Health Department spokesman Bill Parizek.
Ostroff agreed there was no risk to Americans from the case.
"Although experience with the disease is pretty limited, there is no evidence to suggest that cases are transmitted from person to person," he said.
Mad cow disease has never been found in U.S. cattle. Nor had the human variant, called vCJD for short, ever been diagnosed in anyone living here — although Americans can get a similar disease, regular CJD.
The woman was born and raised in Britain and lived there at the height of that country's BSE epidemic. She was diagnosed in Britain recently, but is living in Florida with her family now, the CDC said.
British health officials informed their U.S. counterparts of her illness Thursday.
Health officials wouldn't give her whereabouts or her condition. But the prognosis is grim: Symptoms can include dementia and loss of motor skills, and the disease is almost always fatal, Florida Health Secretary John Agwunobi said.