Discrepant epidemiological patterns between classical and atypical scrapie in sheep flocks under French TSE control measures.

The occurrence of secondary cases of atypical and classical scrapie was examined in 340 outbreaks of atypical and 296 of classical sheep scrapie detected in France during active surveillance programmes between 2002 and 2007. The prevalence of atypical scrapie in these flocks was 0.05% under selective culling and 0.07% under intensified monitoring i.e. not significantly different from that detected during active surveillance of the general population (P>0.5), whereas these figures were much higher for classical scrapie (3.67% and 0.25%, respectively, P<10(-5)). In addition the number of atypical scrapie cases per outbreak did not indicate clustering. The results suggest that atypical scrapie occurs spontaneously or is not particularly contagious, and that the control measures in force allowed appropriate control of classical scrapie but were not more efficient than active surveillance in detecting cases of atypical scrapie.