Extending Comparisons Beyond Time and Space: Looking for Similarities Between Diseases

Early detection of new diseases such as bovine spongiform encephalopathy is the subject of great interest (Gibbens et al., 2008). Understanding whether a disease is infectious or sporadic becomes essential for the application of control measures. Consistent and robust ways to the assessment of temporal trends are required to help in the elucidation of this question. Clustering of cases in space, or time and space, is also relevant in the understanding of the aetiology of a new disease. This paper presents a third approach: knowledge by comparison, either of diseases, surveillance sources or both. We applied this approach to the current debate about the nature of atypical scrapie, a fatal neurological animal disease, by comparing the spatial distribution of this form of scrapie with that of classical scrapie. A similar spatio-temporal distribution of these two diseases would indicate shared environmental disease determinants and help in the generation of hypotheses about the aetiology of atypical scrapie.