A British scientific team says it has strong evidence linking so-called mad-cow disease, a deadly brain disease of cattle, to a similar disease in humans.
The finding, researchers say, bolsters the idea that the brain-destroying agent is passed from cattle to humans, a case that so far has been based on strong inference but not proof. The disease has raised concerns in many countries where beef is eaten but so far no cases in humans or even in cows have been detected in the United States.
The new study, by Dr. John Collinge and his colleagues of the Imperial College School of Medicine, at St. Mary's Hospital in London, focused on a characteristic protein that accumulates in the brains of people who die of the disease. In a paper published on Thursday in Nature, a British science journal, they report that the human protein closely resembles one found in cows with mad-cow disease. The implication is that the two diseases are likely to be one and the same, Collinge concluded.
They also note that the protein, called a prion, differs from the one found in people with Creutzfeldt-Jakob disease, a human brain disease of unknown cause. This suggests that people suspected of contracting mad-cow disease from cows have a different form of the disease from those with Creutzfeldt-Jakob.
Dr. Michael Harrington, an expert on mad-cow disease at the California Institute of Technology, said the new finding is "the first direct evidence" linking the cow disease with the human one.
"Most of us felt that the connection was most likely," Harrington said. "But this gives us much stronger evidence," he said. "It strengthens the presumption." And as a result, he said, "it changes the nature of the debate."
The brain disease has so far struck a dozen or so people in Britain, most of whom are much younger than the typical patient with Creutzfeldt-Jakob disease. This and other dissimilarities suggested that the victims had a different disease.
Some researchers immediately attributed the outbreak to the transmission of disease from mad cows but others sought different explanations.
The only way to know for sure is to wait, said Dr. Paul Brown, the medical director of the laboratory of central nervous system studies at the National Institute of Neurological Disorders and Stroke, in Bethesda, Md.
"The cases started two years ago," Brown said. If they do represent the start of an epidemic caused when people ate infected cattle, the number of cases should soon explode, Brown said. "I would expect it to take off pretty darn soon," he said.
Dr. Laura Manuelidis, the head of neuropathology at Yale University School of Medicine, said that she was inclined from the start to believe that the new disease was transmitted from contaminated beef. She was further persuaded last spring, she said, when researchers infected monkeys by injecting extract of mad-cow brains.
"I think it obviously shows that something is going on that is worrisome," Dr. Manuelidis said.