Disease Maps: Epidemics On the Ground

  • Tom Koch
University of Chicago Press: 2011. 368 pp. $45, £29 9780226449357 | ISBN: 978-0-2264-4935-7

“Ill-clothed, ill-fed, uncleanly in their persons and dwellings, subsisting chiefly on indigestible and unwholesome foods and in the habit of using pernicious drinks.” So wrote physician John Hamett in the London Medical Gazette — describing not modern teenagers, but rather the people within nineteenth-century society who were thought by many to cause disease. 'Putrid effluvia', 'foul personal habits' and 'noxious miasmas' were held liable for sickness until the late 1800s. And from the seventeenth century, maps of outbreaks were used as tools in debates — with clusters of cases in the poorest, smelliest or lowest parts of town apparently confirming suspicions that the disease in question was caused by poor people, sewage or altitude.

According to London's 1667 bill of mortality, 11 people were killed by itches and one died from fainting in the bath.

In Disease Maps, medical geographer Tom Koch tours the history of disease mapping, focusing on plague, yellow fever, cancer and cholera. He explains how maps of each have both increased our understanding of epidemiology and fuelled now-discredited theories. Mapping techniques have advanced hugely — today's surveys are located using the Global Positioning System and analysed using geographic information systems. Yet Koch's main message is that the politics, use and interpretation of spatial disease data have changed little in the past 500 years.

Red lines on an 1832 map show how cholera spread from India to Europe and North America along major trade routes. Credit: NEW YORK ACADEMY OF MEDICINE/NATIONAL LIBRARY OF MEDICINE

In a world where we can download a mobile-phone application that locates for us the nearest outbreak of Rocky Mountain spotted fever, it is easy to forget the poor understanding of pathogens we once had. In the late seventeenth century, a good 200 years before bacteria and viruses were properly understood, it was thought that there were many 'plagues' — a sickness that appeared only in warm seasons; another that was caused by noxious smells from slums; and others that moved from city to city with ships and travellers. Disease maps were drawn up but generally proved inconclusive.

Such uncertainty lingers on in scientific debates over the socioeconomic drivers of epidemic spread, or the effects of climate change on vector-borne diseases such as malaria. Disease mapping has allowed us to detect potential sociological or environmental reasons for outbreaks through their spatial association. However, maps are easily misinterpreted because of the challenge of untangling correlation and causation. The book is strewn with examples of outbreaks that have been found to cluster with a particular factor, leading to claims of an ultimately unfounded causal link. Such was the case for cholera, until one of the most famous disease-mapping studies was undertaken.

John Snow's plotting of the 1854 Broad Street cholera outbreak in London suggested the waterborne nature of the disease. His halting of the epidemic by removing the handle of the water pump around which the cases clustered is a popular tale, but Koch reveals that the conventional version is oversold. Snow's idea on the waterborne transmission of cholera was later proved correct, but his work, and that of many other mappers of the time, merely revealed the conditions in which cholera thrived, not that it is caused by a bacterium, Vibrio cholerae.

Other cholera case studies in the book include an 1831 continental-scale meta-analysis of the outbreaks that spread across Europe in the early nineteenth century — a forerunner of modern large-scale mapping projects. Koch highlights colonial Indian maps of cholera spread that were of limited use because 'natives' were not included, and London neighbourhood mapping battles that aimed to prove all kinds of transmission mechanisms. Each contains lessons for today.

Disease Maps is well researched, and packed with beautifully reproduced epidemiological maps and colourful tales of the arguments and insights each one sparked. Yet it skirts recent advances in our knowledge of disease spread. There is no mention of the state-of-the-art tools that could have settled some of the historical debates.

Developments include new phylogeographic techniques for reconstructing spatial and temporal evolutionary histories of the spread of pathogens such as influenza, and Bayesian statistical models for generating uncertainty maps to accompany those charting endemic diseases such as malaria. Mobile-phone data are also providing unprecedented opportunities for exploring short-term spatial epidemiological dynamics by tracking human travel patterns.

At a time when resources are flowing into the fields of health metrics and epidemiological mapping, Disease Maps shows that some things never change. The epidemic drivers of increased trade, rapid urbanization, inequalities and civil unrest are as strong today as they were during the seventeenth-century outbreaks of plague. Koch's book takes us back to the dawn of disease statistics and mapping, when — according to London's 1667 bill of mortality — 11 people were killed by itches, one person died from fainting in the bath and many more perished when their stomachs simply 'stopped'.