The study, appearing Tuesday in the Proceedings of the National Academy of Sciences, makes clear that people in Britain who developed a new type of Creutzfeldt-Jakob disease could have gotten it from eating meat from cattle infected with bovine spongiform encephalopathy, the so-called mad cow disease. The brain disease has not been found in America.
Experts said the study also suggests the infectious protein, called prion, that causes the diseases can move more easily between species than once believed.
"These findings argue unequivocally that BSE and the new variant CJD are the same strain of prion," said Dr. Stephen DeArmond, a researcher at the University of California, San Francisco, and a senior author of the study.
An outbreak of BSE occurred in British cattle early this decade. Later, some people in Britain developed a new, more deadly type of the Creutzfeldt-Jakob disease. In 1996, experts suggested mad cow disease possibly could be linked to the brain-wasting human disease.
The European Union banned imports of British beef for three years until last August. France still blocks such imports. Officials in Britain banned sale of bone-in domestic beef in 1997 and lifted the sanction last Thursday.
More than 175,000 British cattle died of BSE during the 1990s. At least 48 people in Britain have died of the new form of CJD, along with two in France and one in Ireland. Neither disease has been detected in the United States.
Although a link between mad cow disease and the new type of CJD had been suggested, there were experts who questioned the connection and contended a true scientific link was lacking. The new San Francisco study virtually removes doubt, said Dr. Donald Price, a neurological disease researcher at Johns Hopkins University in Baltimore.
"This lends new credence to the idea that the new variant CJD is a direct result of consuming products from animals with BSE," Price said Monday. "It is something the European community should be concerned about."
Officials for the British health and agricultural departments in London declined comment Monday, saying they have not yet read the new study.
Since the new form of CJD is thought to have an incubation period of at least 10 years, experts say it is impossible to know now how many people have been infected.
Co-authors of the study include Dr. Stanley Prusinger, a UCSF professor who won a Nobel Prize in 1997 for his discovery that the protein prion caused BSE.
In a study conducted by scientists at the University of California at San Francisco and Western General Hospital in Edinburgh, Scotland, the mice became ill in exactly the same way when they were injected with diseased material from cattle and humans who had died of the illness.
The finding, the scientists said, provides very strong evidence that bovine spongiform encephalopathy (mad cow disease) and a new form of its human cousin, Creutzfeldt-Jakob disease, are the same.
Mad cow swept through British herds in the 1980's, killing more than 175,000 cows, most of them dairy cattle.
British officials became especially concerned when some people came down with a new variant of Creutzfeldt-Jakob, a fatal brain disease usually seen in just one person in a million.
The researchers called the finding unsettling, "because it undermines the comforting presumption that a 'species barrier' dramatically lessened the likelihood that people exposed to mad cow disease through meats, cosmetics and medicinal supplies would be infected."
Since 1995 the human form of the disease, called new-variant Creutzfeldt-Jakob or nvCJD, has killed more than 50 teenagers and young adults, nearly all of them in Britain. Nine of the victims died in the last quarter of 1998.
"Although it is not yet known whether this trend will continue," the scientists wrote in their report, published today in The Proceedings of the National Academy of Sciences, "the possibility that a large section of the population is at high risk must be seriously entertained."
Experts say the disease has such a long incubation period in humans that an epidemic could be seen among people who ate infected British beef decades from now.
Mad cow and nvCJD have been strongly linked before, but little experimental evidence proves the two are the same. The researchers, led by Dr. Michael Scott of University of California at San Francisco, showed this link by creating mice carrying a bovine version of the prion, or brain protein, that is believed to cause both the human and animal forms.
When these mice were injected with material from diseased cattle, they developed mad cow disease in 250 days.
A second group of mice was then injected with prions from the first group of mice and also fell ill, demonstrating that the disorder could spread unchanged within a noncattle species.
The researchers then injected another group of mice with prions from diseased human brains. The results were identical, suggesting that mad cow and its human version are interchangeable.
Because different species handle the same proteins in different ways, an illness cannot generally spread from species to species.
But in the case of the prion diseases, the species barrier seems to be leakier.
Scientists believe that British cattle caught the disease in the first place from eating feed made out of sheep that were infected with scrapie, another version of mad cow.
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