Scientists refuse to rule out epidemic of CJD from sheep

January 10, 2002 The Daily Telegraph(London) by Roger Highfield
THE risk of contracting variant Creutzfeldt-Jakob Disease - "human bovine spongiform encephalopathy" - could now be much greater from eating lamb than beef, if sheep have also become infected with the disease, scientists say today.

If BSE has passed into the national flock, the number of animals infected up until now would have been at least 100 times lower than the number of cattle infected during the disease epidemic, an Imperial College team concludes.

But Prof Neil Ferguson's group says in Nature that, because BSE in sheep may spread from animal to animal, it cannot rule out the possibility that a much larger epidemic may develop.

If nothing is done, BSE in sheep could triple vCJD's future human death toll to up to 150,000. This theoretical risk could be cut by 90 per cent if comparable control measures to those used on cattle were also used on sheep, it adds. The Human BSE Foundation, which represents families bereaved by vCJD, yesterday called for immediate action, even though the risks posed by BSE in sheep remain theoretical.

But the Food Standards Agency said the Nature paper was only "one piece of a jigsaw" of research that it was weighing up to see if new safeguards were necessary. "The risk of BSE in sheep remains theoretical and the agency is not advising against the consumption of lamb," said a spokesman. Writing in Nature, Sir John Krebs, chairman of the agency, and Lord May, a former Government chief scientist, say: "In the light of such scientific uncertainty, there is no easy formula for deciding on the right level of precautionary risk management."

BSE was first diagnosed in 1986 and its impact on sheep was described by the October 2000 BSE inquiry report as "perhaps the most important unanswered question about the BSE epidemic".

Because only 180 sheep have been tested for BSE and because of the recent fiasco in which scientists looking for BSE in the national flock spent four years studying cows' brains, the extent of its spread among sheep is unknown.

The research in Nature predicts that the future number of deaths from vCJD due to exposure from BSE in cattle alone lies between 50 and 50,000.

The upper estimate is lower than previous figures provided by the team but greater than estimates from other scientific groups, said Dr Azra Ghani, a team member. "The other groups were over optimistic in terms of the assumptions that they made."

The additional risk from sheep has a limited impact on these figures except in the "worst-case" scenario of a growing sheep epidemic, in which case the range of possible future numbers of deaths increases to between 110 and 150,000.

Prof Ferguson, the leader of the team, said: "Our latest analysis shows that the current risk from sheep could be greater than that from cattle, due to the more intensive controls in place to protect human health from exposure to infected cattle, as compared with sheep. However, more optimistically, our analysis shows that the overall historical risk from sheep was much less than that from cattle."

He added: "We were not trying to evaluate the probability that BSE has entered the sheep flock, but rather, given the pessimistic assumption that infection has occurred, to explore its potential extent and pattern of spread."

In the paper, the authors explore potential risk-reduction strategies similar to those in place for cattle, including restrictions on the age of sheep slaughtered for consumption and removing certain offal and tissues from the food supply.

Combining these controls is estimated to reduce risk by up to 90 per cent for all three sheep BSE scenarios they considered.

"We find that if tissue- and age-based risk-reduction measurements, such as those put in place for cattle, were adopted, the current and future risk could be reduced by up to 90 per cent," said Prof Ferguson.

He said there are many uncertainties in performing this type of study, due to the limited data available on BSE in sheep.

"Our estimates are ultimately dependent on the quality and quantity of information that is gathered by other researchers and we feel that large-scale testing of the national flock, and additional experimental research are urgent priorities," he said.

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