Factors that influence the age distribution of BSE cases: potentials for age targeting in surveillance

Recently, due to consumers fears concerning BSE and vCJD, the need arose for methods to detect BSE, to estimate the present prevalence of BSE among cattle and to predict future BSE prevalence. As a part of that set of urgent questions, it has become important to indicate groups in which BSE risk is higher or lower. One of the well-known risk factors for BSE is age: very young animals do not develop the disease, and very old animals are less likely to develop the disease. Using age-structured modelling, three factors influencing the age distribution of BSE were found to be important: (1) the incubation period of BSE, (2) age structure of the cattle population, and (3) the local risk history (methods of rendering, feeding of compound feed containing Meat and Bone Meal (MBM), and the development of BSE control). The EU has considered these three risk factors to be the most important for BSE risk assessment. So far, this EU risk assessment method has been proven right by several countries detecting BSE after being classified as ‘BSE is most likely present here’. The age distribution of BSE seems to vary a lot between countries and regions. When information on these three factors is available, the expected age distribution of BSE in different countries can be calculated. Our calculations show that in countries where, until very recently, the reproduction ratio was high, (i.e., BSE risk factors were high), the BSE prevalence is expected to be highest in 4-year-old cattle. In countries with low reproduction ratio for BSE, (i.e., BSE control at a very high level) for more than 5 years, the prevalence will be highest in the 6–8-year-old cattle. Thus, surveillance could be targeted specifically at the age groups with the highest BSE risk. For each country, a short assessment shows in which age group BSE is most likely to be found