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Estimating the human health risk from possible BSE infection of the British sheep flock


Following the controversial failure of a recent study1 and the small numbers of animals yet screened for infection2, it remains uncertain whether bovine spongiform encephalopathy (BSE) was transmitted to sheep in the past via feed supplements and whether it is still present. Well grounded mathematical and statistical models are therefore essential to integrate the limited and disparate data, to explore uncertainty, and to define data-collection priorities. We analysed the implications of different scenarios of BSE spread in sheep for relative human exposure levels and variant Creutzfeldt–Jakob disease (vCJD) incidence. Here we show that, if BSE entered the sheep population and a degree of transmission occurred, then ongoing public health risks from ovine BSE are likely to be greater than those from cattle, but that any such risk could be reduced by up to 90% through additional restrictions on sheep products entering the food supply. Extending the analysis to consider absolute risk, we estimate the 95% confidence interval for future vCJD mortality to be 50 to 50,000 human deaths considering exposure to bovine BSE alone, with the upper bound increasing to 150,000 once we include exposure from the worst-case ovine BSE scenario examined.

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Figure 1: Epidemiological inputs to transmission model.
Figure 2: Epidemiological characteristics of BSE transmission scenarios in sheep.
Figure 3: Impact of risk-reduction measures.


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This work was funded by the Food Standards Agency. N.M.F. and A.C.G. acknowledge funding from The Royal Society. C.A.D., T.J.H. and R.M.A. acknowledge funding from the Wellcome Trust. We are very grateful to M. Bruce, S. Bellworthy and M. Jeffrey for access to pre-publication data on infectivity. We also thank L. Hoinville, J. Wilesmith and R. Will for access to data. We thank N. Hunter, L. Green, J. Anderson, A. James, A. Bromley, H. Mason, P. Comer and members of the Spongiform Encephalopathy Advisory Committee for discussions.

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Correspondence to N. M. Ferguson.

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The authors declare no competing financial interests.

Supplementary information


Supplementary chapters: Sheep demography, Infectivity of infected animals, Epidemiology of TSEs in sheep, Human consumption of ovine material, Epidemiological model of sheep TSEs, Human exposure and impact of risk reduction strategies, Absolute risk estimates: vCJD epidemic projection. Contains 13 supplementary tables, 4 supplementary figures and references. (PDF 432 kb)

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Ferguson, N., Ghani, A., Donnelly, C. et al. Estimating the human health risk from possible BSE infection of the British sheep flock. Nature 415, 420–424 (2002).

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