THE EPIDEMIOLOGY OF BSE IN CATTLE HERDS IN GREAT-BRITAIN .1. EPIDEMIOLOGIC PROCESSES, DEMOGRAPHY OF CATTLE AND APPROACHES TO CONTROL BY CULLING

This paper explores the key epidemiological processes and demographic factors that determined the pattern of transmission of the aetiologica l agent of bovine spongiform encephalopathy (BSE) in cattle herds in G reat Britain (GB). The analyses presented utilize data from published and unpublished experimental studies and from the GB central database of confirmed BSE cases. We review the experimental and epidemiological evidence that has both confirmed indirect horizontal transmission via the consumption of infectious material as the major transmission rout e and provided information on the duration and variability of the dose -dependent incubation period of BSE in cattle. The epidemiological and genetic data pertaining to the possible existence of maternal transmi ssion and/or genetically variable susceptibility to infection is discu ssed. The demography of British cattle is characterized and the impact s of key demographic features on the observed epidemic profile are dis cussed. In the main BSE case database, analyses reveal that BSE cases cluster significantly at both the holding and county scale. Furthermor e, analysis of longitudinal patterns reveal substantial temporal withi n-holding correlation. Such clustering of cases suggests a highly hete rogeneous infection process. The paper ends with a discussion of how a nalyses of spatio-temporal clustering inform the design of targeted cu lling programmes aimed at reducing future disease incidence. We show h ow the retrospective implementation of culling policies on the BSE cas e database allows the qualitative evaluation of policy performance! bu t that model predictions of future trends in case incidence are requir ed to estimate the precise impact of any current. or future programme.