BSE in Portugal: anticipating the decline of an epidemic

BACKGROUND: Attention throughout Europe continues to focus on bovine spongiform encephalopathy (BSE), with increasing evidence linking it to the new variant of Creutzfeldt-Jakob disease in humans. In particular, recent attention has been directed at Portugal, where the incidence of confirmed BSE cases continues to rise. METHODS: We modelled the age-specific incidence of BSE in Portuguese-born cattle by birth cohort as a function of: the survival distribution; the cohort-specific incidence of BSE infection; the age-specific probability, conditional on survival, that an infected animal will experience clinical onset; and the under-reporting rate of BSE cases prior to 1998. RESULTS: We obtained good fits to the age-specific incidence of BSE by birth cohort in Portugal. Under a range of assumptions, the estimated incidence of BSE infection was relatively low initially, except possibly in the 1989 cohort, and then rose gradually between the 1992 and 1994 cohorts. The estimated decrease in infection incidence between the 1994 and 1995 cohorts probably reflects the effectiveness of the ban on the use of mammalian meat and bone-meal introduced in Portugal in mid-1994. Assuming no infections in animals born after June 1995, the models predict that the incidence of BSE cases in Portugal will peak in 1999 with BSE case-incidence declining thereafter. DISCUSSION: Our results illustrate the power of epidemiological analysis to detect decreasing trends in infection incidence prior to the resulting decrease in case incidence. The findings should inform the deliberations of the European Commission, which recently reported concerns about the sharp increase in case incidence from 1997 to 1998.