Spatial comparison of herald and main waves in London's nineteenth-century cholera epidemics.

Nineteenth-century London experienced four extraordinarily severe summertime cholera epidemics. Three were preceded by less severe non-summer outbreaks. Twenty-first-century research hypothesizes them as herald waves of potentially new cholera strains. This study examined the geographical characteristics of these herald waves and compared them to their subsequent main waves to determine if there was a geographical component to the significant difference in wave severity. Cholera mortality data for London's parishes and registration districts were extracted from contemporaneous records. The data were normalized and scaled. Each epidemic wave was divided into two segments for analysis. A Spearman's rank correlation was used to assess the relationship between a herald and its subsequent main wave. Geospatial analytical tools were used to determine and display each segment's geographic distribution pattern using autocorrelation techniques to determine its central point. Results show that the herald wave of each epidemic shared characteristics similar to its following main wave. Central-point locations were similar and Spearman's rank coefficients showed high degrees of correlation. Autocorrelation results were similar, with one exception reflecting an appalling anomalous cholera outbreak at an institution for children. Because of the demonstrated similarity of each epidemic's herald and main waves, this study did not detect a spatial characteristic that could explain the observed difference in severity between the studied heralds and mains.