Modelling the expected numbers of preclinical and clinical cases of bovinespongiform encephalopathy in Switzerland

The objective of this study was to model the expected numbers of cattle inc ubating bovine spongiform encephalopathy (BSE) and the numbers of clinical cases of BSE in the Swiss cattle population between 1984 and 2005. The resu lts were compared with the observed number of clinical BSE cases and with t he results of a culling and testing scheme on herdmates of cattle with ass. The age distribution of the Swiss cattle population, the age-at-death dist ribution of the first 235 BSE cases and exposure information were used to c alculate the expected number of infected cattle in each birth cohort and th e resulting numbers of clinical cases and survivors incubating the disease for each year. The model which did not assume any underreporting of cases f itted the observed epidemic curve of clinical cases reasonably well, and pr edicted that the Swiss BSE epidemic would come to an end between 2003 and 2 005. The age of survivors incubating BSE is increasing. The higher than exp ected incidence of subclinical cases observed in animals from the culling s cheme is most probably the result of the heterogeneous distribution of infe cted animals and affected herds in the population, The results of the model need to be taken into account when designing surveillance and testing sche mes for BSE.