Estimation of the age-dependent risk of infection to BSE of dairy cattle in Great Britain.

An important aspect of the bovine spongiform encephalopathy (BSE) epidemic has been an apparent age-dependent risk of infection, with younger cattle being more likely to become infected than older cattle. Our objective was to determine the age-dependent risk of infection of dairy cattle. We first reviewed unpublished data on the feeding patterns of proprietary concentrates for dairy-replacement cattle. These data showed that autumn- and spring-born cattle would receive different feeding patterns of proprietary concentrates, and so age-dependent risk of infection profiles were obtained separately for autumn- and spring-born cattle. We used back-calculation methods to analyse BSE-epidemic data collected in Great Britain between 1984 and 1996. Dairy cattle were most at risk in the first 6 months of life; adult cattle were at relatively low risk of infection. Between 6 and 24 months of age, risk profiles reflected feeding patterns of proprietary concentrates in each of the autumn- and spring-born cohorts.