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How much sheep-TSE infectivity enters the human food chain? An assessment of risk reduction by removal of scrapie-flocks

Project Code: M03027

31/08/2006

Department of Zoology, Oxford University
Fryer, H ;
Institute for Animal Health, Compton Laboratory
Baylis, M;
Veterinary Laboratories Agency, Addlestone
Sivam, K;
Department of Zoology, Oxford University
McLean, A

Although no naturally infected sheep with bovine spongiform encephalopathy (BSE)has ever been discovered, it remains possible that BSE once infected the UK sheep population, has been transmitted between sheep, and is still present today. We constructed a mathematical model to assess the current maximum theoretical risk to consumers from BSE-infected ovine material and to estimate the risk reduction that could achieved by abattoir-based control options. We predict that the risk to consumers from a single BSE-infected sheep is high; one sheep, close to the end of its incubation period, is likely to contribute 10-1000 times more infectious material than a fully infectious cow. Furthermore, 30% of this risk comes from infectivity residing in lymphatic and peripheral tissue that cannot be completely removed from a carcass.

We are 95% confident that throughout Great Britain no more than four sheep flocks might currently harbour an ongoing BSE epidemic. However, because the exposure from a single infected sheep is high, the annual human exposure from four ‘typical’ BSE-infected flocks could be considerable. Small reductions in exposure could be achieved by strategies based on tissue testing, a 12-month age restriction or expanded definitions of high-risk tissues. A 6-month age restriction is likely to be more effective and genotype-based strategies, the most effective.

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