Using the National Cattle Register to estimate the excess mortality during an epidemic: application to an outbreak of Bluetongue serotype 8.

National Cattle Registers have been widely used to examine animal movements and their role in disease transmission, but less frequently for other epidemiological applications. Our study shows how routinely collected identification data can be used to evaluate the population impact of an epidemic in cattle and to derive an indirect estimate of the associated mortality. We adapted a method developed by Human health agencies, based on the modelling of historical mortality fluctuations, to analyze the evolution of mortality in a cattle population subjected to a Bluetongue serotype 8 (BT8) outbreak. Between 01/07/2007 and 01/07/2008, 21,017 cattle died in the considered population whereas 16,691 deaths were expected according to the model. 43% of the 4326 extra deaths were found in calves less than 7 days of age, but excess mortality was found in each age group. The temporal distribution of extra-deaths, described at a weekly scale, suggests that they were related to the BT8 epidemic. The presented method could be an appreciable tool for estimating the global burden of epidemics since it is based on data already routinely collected in each European Member State. This study was conducted retrospectively but considering the promptness of the notification system, the method could be used to monitor the evolution of epidemics in near-real time.