The mortality rates and the space-time patterns of John Snow's cholera epidemic map.

Narushige Shiode,Elodie Rod-Thatcher,Sanjay Rana,Shino Shiode,Peter Vinten-Johansen

doi:10.1186/s12942-015-0011-y

Abstract

BackgroundSnow’s work on the Broad Street map is widely known as a pioneering example of spatial epidemiology. It lacks, however, two significant attributes required in contemporary analyses of disease incidence: population at risk and the progression of the epidemic over time. Despite this has been repeatedly suggested in the literature, no systematic investigation of these two aspects was previously carried out. Using a series of historical documents, this study constructs own data to revisit Snow’s study to examine the mortality rate at each street location and the space-time pattern of the cholera outbreak.MethodsThis study brings together records from a series of historical documents, and prepares own data on the estimated number of residents at each house location as well as the space-time data of the victims, and these are processed in GIS to facilitate the spatial-temporal analysis. Mortality rates and the space-time pattern in the victims’ records are explored using Kernel Density Estimation and network-based Scan Statistic, a recently developed method that detects significant concentrations of records such as the date and place of victims with respect to their distance from others along the street network. The results are visualised in a map form using a GIS platform.ResultsData on mortality rates and space-time distribution of the victims were collected from various sources and were successfully merged and digitised, thus allowing the production of new map outputs and new interpretation of the 1854 cholera outbreak in London, covering more cases than Snow’s original report and also adding new insights into their space-time distribution. They confirmed that areas in the immediate vicinity of the Broad Street pump indeed suffered from excessively high mortality rates, which has been suspected for the past 160 years but remained unconfirmed. No distinctive pattern was found in the space-time distribution of victims’ locations.ConclusionsThe high mortality rates identified around the Broad Street pump are consistent with Snow’s theory about cholera being transmitted through contaminated water. The absence of a clear space-time pattern also indicates the water-bourne, rather than the then popular belief of air bourne, nature of cholera.The GIS data constructed in this study has an academic value and would cater for further research on Snow’s map.

Highlights

Snow’s work on the Broad Street map is widely known as a pioneering example of spatial epidemiology
Investigating the distribution of population in the area will help confirm whether the concentration of victims around the Broad Street pump (BSP) was caused by high mortality rates, rather than a product of having a high number of residents in the area
This study re-examined Snow’s map by investigating the mortality rates and the space-time pattern of the spreading of the epidemic. They were made possible by constructing datasets that incorporate victim locations from Snow’s and other reports, the number of household residents, and the date of death for each victim. It showed that houses around the BSP’s location suffered from high mortality rates, confirming the water from the BSP to be the source of contamination

Summary

Introduction

Snow’s work on the Broad Street map is widely known as a pioneering example of spatial epidemiology. It lacks, two significant attributes required in contemporary analyses of disease incidence: population at risk and the progression of the epidemic over time. Two significant attributes required in contemporary analyses of disease incidence: population at risk and the progression of the epidemic over time Despite this has been repeatedly suggested in the literature, no systematic investigation of these two aspects was previously carried out. In the fields of epidemiology, public health and medical geography, John Snow is often referred to as ‘hero of cholera’ [1] for his seminal work in creating a map of cholera deaths in Soho, London [2] Investigating the distribution of population in the area will help confirm whether the concentration of victims around the BSP was caused by high mortality rates, rather than a product of having a high number of residents in the area

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Conclusion