Geographical clustering of cases of bovine spongiform encephalopathy (BSE) born in Switzerland after the feed ban

Over one-third of the cases of BSE in Switzerland have been born after the feed ban of December 1, 1990. Evidence for the geographical clustering of these cases emerged in two distinct regions. All the 354 BSE cases recorded until June 30, 2000 (set A), and the 376 cases recorded up to May 14, 2001 (set B), were georeferenced to the centres of the communities in which the herds of origin were located, and control populations were georeferenced to the centres of the communities in which these herds were located at the time of the census. The latitudes and longitudes of these centres were used in the statistical analysis of the spatial clustering. The Cuzick-Edwards test and the spatial scan statistics were applied to assess the significance of the clusters, while controlling for the spatial distribution of the underlying cattle population. There was global clustering of the cases born after the ban, and distinct and significant (P<0.05) spatial clusters were repeatedly identified in the two case datasets, and in several control populations (all cases born before the feed ban on a random sample of control farms) in terms of cattle density by region or cow density by region. Differential reporting was excluded as the underlying reason for the observed clusters.