Professor Stanley Prusiner, who won the Nobel Prize for Medicine last year, told the public inquiry into the handling of the BSE crisis that he was not convinced that there was any link between BSE and new-variant CJD, from which 26 people in Britain have died so far.
Professor Prusiner, who won the Nobel Prize for his work on prions, the infectious agents that are thought to cause BSE and CJD, told the inquiry in London that research linking BSE in cows and CJD was interesting but that he was sceptical of such a link: "I don't agree that these findings close the circle on this issue. To say we know this is the cause of CJD may be slightly overstating what we know."
It was the possible link between the two diseases that led to orders for the destruction of cattle over 30 months of age - it is expected that the slaughter scheme and compensation to farmers will cost as much as pounds 3.4bn by the end of the century.
Professor Prusiner, Professor of Neurology and Biochemistry at the University of California School of Medicine in San Francisco, told the inquiry that he couldn't predict the number of future deaths from new- variant CJD, and he added that the disease's cause was still not known.
Speaking of work that has linked mad cow disease and CJD, he said: "This work is interesting and provocative and I'm not sure how to interpret it. It may in the end be seminal in what's occurring."
Professor Prusiner did admit, however, that in the late 1980s, the prospect of eating British beef was "not a very appetising thought". Yet he told the inquiry there was "no scientific basis for eating beef or not eating beef."
In his own research, Professor Prusiner has isolated prions and demonstrated how they can be transformed from benign proteins present in every brain to cell-destroying agents that leave brain tissue dead.
The complex nature of the debate concerning BSE and CJD was highlighted by Professor Prusiner. He said the age of the sufferers was "very unusual" and that there was no clear reason why such a small number of people had developed it while 10 million others with the same environmental history had not.
"There's no unique behaviour trait," he said. "These people didn't go out and crave cow brains. Why should it be these young people? I don't understand that at all."
Professor Prusiner said he would be more convinced of a link between mad cow disease and the new variant of CJD if greater numbers of people had died of it or were to die in the near future. "The one thing that would convince me is a large number of cases, which we haven't had," he said.
The BSE inquiry opened in March and is hearing evidence from victims' families, farmers, beef-industry representatives and scientists.
Last week, members of the CJD surveillance unit admitted that they were "in the dark" about the effects of the new form of the disease, and a leading government adviser, Professor John Collinge, said the long incubation period was making him "more worried about the fact we are going to see a lot more cases".
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