Nobel Scientist Says Mad
Cow Disease May Infect
Millions Of Sheep
By Jonathon Leake - Science Editor
The Sunday Times
A Nobel prize-winning scientist has warned that there is increasing evidence that BSE, so-called "mad cow" disease, is endemic in British sheep.
Research by Stanley Prusiner suggests that the infective prion agent that causes BSE is found in sheep but at levels which have until now been undetectable.
Prusiner, associate professor of neurology at the University of California in San Francisco (UCSF), won the Nobel prize in 1997 for discovering prions. Last week he said: "The implication of our latest work is that BSE is endemic throughout the British national sheep flock."
Prusiner's original work described how prions form when proteins that occur naturally in the brains of all mammals become deformed. The altered protein then acts as a template, changing other molecules in a chain reaction that devastates the brain.
If an infected beast is eaten, the prions start a similar chain reaction in the animal that ate it.
However, although this mechanism has been well described, the BSE prion has never been found in sheep.
Last week Prusiner said he and Professor Mike Scott, a Scottish researcher, appeared to have produced BSE in mice by infecting them with material from sheep suffering from scrapie, which is another prion disease.
Prusiner said: "Our initial data suggests that these sheep were producing more than one type of prion. One was the scrapie prion that killed them, but we believe some were also making the BSE prion."
The research uses "bovinised" mice, in which the gene that makes prion proteins is replaced by the same gene from a cow. Such mice react to BSE prions just like cows, but take less than 10 months to develop the disease, compared with more than three years for cows.
When such mice were inoculated with BSE prions the resulting disease was identical to that caused by variant CJD prions from humans - evidence that the prions are the same.
The mice were then injected with material from sheep with scrapie and, again, the incubation and symptoms were close to those of BSE. This was a powerful indicator that sheep can produce BSE prions.
Is it dangerous? Consumer confidence in lamb could be hit by the warning
Fred Cohen, professor of pharmacology at UCSF, said there was strong evidence that cattle developed BSE because of changes in the way sheep carcasses were rendered into animal feed.
In their latest work, Cohen, Prusiner and Scott replicated the changes in the rendering process in the laboratory and injected the resulting material into mice. Cohen said early results suggested that the theory was correct.
"When scrapie-infected sheep were slaughtered, the rendering process destroyed the scrapie prions but left behind the tougher BSE prions - to which cattle were vulnerable," he said.
If the unpublished results are confirmed, it could have a serious impact on the sheep industry. There is no evidence that people can catch BSE from eating sheep, but most research has focused on cattle, so the possibility cannot be ruled out.
The main damage to the industry would come from a loss of consumer confidence in sheep products. The government has already prepared contingency plans for testing and slaughtering if BSE should be found in sheep.
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