Implications of BSE infection screening data for the scale of the British BSE epidemic and current European infection levels

Abstract

The incidence of confirmed clinical cases of bovine spongiform encephalopathy (BSE) in Great Britain continues to decline, but the recent discovery of cases in previously unaffected countries (including Israel, Japan, Poland, Slovenia and Spain) has heightened concerns that BSE transmission was more intense and widespread than previously thought. We use back-calculation methods to undertake an integrated analysis of data on infection prevalence in apparently healthy cattle and the incidence of confirmed clinical disease. The results indicate substantial underascertainment of clinical cases over the course of the British epidemic, and consequently that two- to fourfold more animals were infected than previously estimated. Upper bounds on the predicted size of the new variant Creutzfeldt-Jakob Disease (vCJD) epidemic are unaffected, as the prediction methods employed fit to observed vCJD mortality data, and are not sensitive to estimates of the absolute magnitude of past human exposure to BSE-infected cattle, only to relative changes in exposure through time. We also estimate the per-head incidence of infection in cattle born between 1993 and 1997 in other European Union countries, using data on the testing of apparently healthy cattle slaughtered for consumption. Infection incidence for cattle born after mid-1996 was highest in Greece, Italy and Belgium, with Spain and The Netherlands having intermediate levels, and estimates for Great Britain, Germany and France being comparably low.