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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
(¿ìè¶ÌÊÓƵ, 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.