BSE controls draft report, 6 November 2000, section 7
21. There is a theoretical possibility that BSE is present in sheep in the UK although, so far, it has not been found to have occurred naturally. Previous and ongoing studies have shown in a number of cases that it is possible to infect genetically predisposed sheep with BSE by dosing them orally, usually at around six months of age, with brain from clinically infected cattle. Some sheep may have consumed the same type of feed that is believed to have infected cattle. However, any sheep infected in this way in the 1980s and early 1990s would no longer be alive. Therefore, if BSE were to have persisted, it would have to have been maintained by sheep to sheep transmission. An additional complication is that the agent may have changed its nature during the transmission from sheep to sheep if this, indeed, has taken place.
22. Sheep have been known to suffer from another TSE, scrapie, since the early 18th century. This disease, unlike BSE in cattle, does not appear to pose any direct risk to human health. Scrapie also differs from BSE in that, for some strains, infectivity is found in lymphoid tissue as well as in the nervous system. Scrapie appears to be maintained by "horizontal" transmission between sheep but how this happens is not clear. This mode of transmission has not been demonstrated for BSE in cattle. If it occurs at all, it is likely to be at a low level because the pattern of decline of the epidemic is inconsistent with the levels of horizontal transmission that would be required to maintain BSE in cattle indefinitely18.
23. Scrapie infectivity is more widely distributed in the body of sheep than is the infectivity of BSE in cattle. Experiments in which mice are injected with tissues from BSE infected sheep, show that the distribution of BSE infectivity in sheep is more similar to that of scrapie in sheep than to that of BSE in cattle (see Annex H).
24. Although only a few hundred cases are reported each year, recent research19 has suggested that 5-10 thousand of the 40 million sheep in the UK suffer from scrapie each year. Because the external signs of scrapie and experimentally induced BSE are similar, if BSE is present, scrapie may be "masking" it in commercial flocks. Scrapie was made a notifiable disease in 1993 and, since 1998, confirmed cases have been compulsorily slaughtered.
25. Current research to look for BSE in sheep involves injecting mice with material from TSE-infected sheep (see paragraph 10). These experiments are costly (more than £20,000 per sheep) and slow (they take up to two years). Hence only a very small number of TSE-infected sheep (up to 200) are being screened. Therefore, as a matter of great urgency, there is a need to develop and apply a rapid screening method so that large numbers of sheep can be tested to reduce the uncertainty of whether or not BSE occurs (see also 52(iv), 87 and 88).
26. MAFF is currently considering measures to reduce or eliminate scrapie (and TSEs in general) from sheep. A key element of the MAFF plan is a breeding programme using rams with a dominant genotype thought to confer resistance to scrapie and BSE. As proposed, the plan will take 10 years or more to complete. Whilst we fully support this plan, and would like to see it accelerated if possible, there is also a need for more immediate steps both to reduce uncertainty about BSE and to develop options for risk management. These are currently being developed by MAFF (see paragraph 49). The risk management options should consider what might be done whilst BSE in sheep remains a theoretical possibility and a contingency plan if BSE were to be discovered. Also, at SEAC’s request, the FSA is commissioning an assessment of foodborne risk to human health should BSE infection be confirmed in the national flock.
27. There have been reports of possible BSE in sheep but these are unproven20.
28. Further data published by Gravenor et al21 in August 2000 showed, on the basis of a postal survey, no evidence in the UK for a peak in scrapie incidence before, during or after the BSE outbreak. They conclude that it is unlikely that a substantial BSE epidemic had occurred in the sheep population, although the data on which this conclusion is based are necessarily imprecise.
29. Recent research22 has indicated that some species of animal can be carriers of the TSEs without showing clinical signs. The possibility that sheep may be carriers of BSE without developing the disease is being researched. Healthy cattle are also being surveyed. We recommend that further research in this area is carried out.
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18 - Ferguson NM, Donnelly CA, Woolhouse MEJ and Anderson RM. Estimation of the basic reproduction number of BSE: the intensity of transmission in British cattle. Proceedings of the Royal Society London B 266, 23-32, 1999.
19 - 'Scrapie occurrence in Great Britain', The Veterinary Record, 2.10.99 (vol.145, no 14)
20 - These relate to discovery of a possible TSE in sheep imported from Belgium to Vermont, USA, and data from research by Professor Prusiner and colleagues at the University of California, San Francisco which indicates that sheep may harbour BSE prions as well as scrapie prions.
21 - Gravenor et al ‘Scrapie in Britain during the BSE years’ (Nature, 406, 584-585)
22 - Hill et al, Proceedings of the National Academy of Sciences of the USA, 97, 10248-10253, 29 August 2000