The unrecognised French BSE epidemic.

In France, implementation of systematic screening programs in 2000, as a complement to the mandatory reporting of animals with clinical signs of BSE (passive surveillance), revealed certain limitations of the mandatory system. Indeed, systematic screening showed that some BSE cases were not detected by the clinical surveillance system, implying considerable BSE case under-reporting throughout the epidemic. As the most likely explanation for variant Creutzfeldt-Jakob disease (vCJD) is exposure to the aetiologic agent of BSE, it is essential to reconstruct the French BSE epidemic pattern accounting for this under-reporting. We estimated age- and year-specific incidence rates of BSE by using a back-calculation method. This approach relies on the principle that the number of clinical BSE cases is the consequence of the number of BSE-infected animals after a known incubation time, defined as the time between infection and clinical onset. We generalized this model to take into account epidemiological characteristics of BSE, such as French cattle mortality, BSE case reporting probability, and age-dependent susceptibility and/or exposure to the BSE agent. We confirmed that the average BSE incubation period is five years and that the peak risk of bovine infection occurs between 6 and 12 months of age. The results also showed that the proportion of underreporting is the most influential parameter in the model, and that BSE was substantially underreported until rapid tests were introduced. Indeed, only 103 BSE cases were detected by passive surveillance up to June 2000, while we estimated that there was 301 200 (95% confidence interval (CI) [27 600-837 600]) cattle infected by the BSE agent. Despite uncertainty over the beginning of the epidemic, we showed that the French BSE epidemic in the late 1980s was completely undetected, and only the second wave, after 1990, was observed.