Mad mutton could be as big a threat as mad cows

January 12, 2002 New Scientist by Debora MacKenzie
THOUSANDS more people could die from the human equivalent of mad cow disease if Britain's sheep turn out to be infected with the disease, epidemiologists warn this week. They say that sheep offal, and sheep older than six months, should be banned from human consumption to ensure that any such infection cannot pass to people in the future.

But British authorities have still not managed to establish whether sheep are carrying BSE. Until they do, any further restrictions on an already stricken meat industry seem unlikely. Sheep can catch BSE by eating infected feed, and they carry the infection in more of the edible tissues than cattle do. This makes sheep potentially much riskier for human consumers. Worse, while cattle get BSE almost exclusively from feed, sheep can spread such prion diseases to each other, by nibbling on afterbirths for instance. That means BSE could still be spreading among sheep despite controls on feed.

British sheep are known to have eaten infected feed, but no one knows whether they actually got BSE because its symptoms look exactly like those of scrapie, an existing disease of sheep thought not to affect humans. Government efforts to detect BSE in sheep that apparently died of scrapie were ignominiously abandoned last year, when the sheep brains being tested were found to have been contaminated with cattle tissue (New Scientist, 27 October 2001, p 14).

So Neil Ferguson and a team at Imperial College, London, modelled what would happen if BSE did infect British sheep. So far, 180 sheep that showed symptoms of scrapie have been tested for BSE in Britain, and all were BSE-free. This means that the prevalence of BSE in sheep is not likely to be more than 2 per cent that of scrapie, which is endemic, the researchers at Imperial calculated. But this would still mean that sheep are now the main source of human exposure to BSE in Britain.

If some sheep do have BSE, the degree of risk to humans depends on how readily it spreads. If it is not infectious enough in sheep to pass from flock to flock, the team predicts that the numbers of infected carcasses entering the human food supply should now be falling. But if BSE spreads between flocks, the problems would just be beginning. The team's model shows that the number of infected sheep entering the British food supply would start rising steeply. This explosion of infected mutton would start about now, and increase fivefold by 2020.

Ferguson and his colleagues estimate that, at worst, 40,000 to 100,000 people could die by 2080 of vCJD acquired from cattle. If BSE is in sheep, and spreading between flocks, the maximum number of deaths could rise to 150,000. Yet the team found that nearly all these extra cases would be prevented if more precautions are taken with slaughtered sheep now.

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