Disease Mapping

The concept of disease mapping is not new. In his detailed description of the history of disease mapping, Howe identified several American and British studies dating from the beginning of the 1800s in which maps were employed to demonstrate the distribution of disease.8 Mostly, these maps portrayed the distributions of infectious diseases such as yellow fever in the United States and contagious fevers in Ireland.

Possibly the most famous uses of mapping in epidemiology were the studies by John Snow of the cholera epidemics in London during the middle of the nineteenth century.9 At that time, the method of spread and the nature of the cholera vibrio were unknown. Through careful observation of his patients and by plotting where the cases lived, Snow was among the first to show clearly that cholera could be spread through a contaminated water supply. His 'dot map' of the residences of the victims of the 1854 cholera epidemic in the Golden Square area of London demonstrated a distinct cluster of cases around the water pump in Broad Street (Figure 1.3).

Later investigations indicated that the pump had become contaminated by faecal material from a case of cholera. When studying the outbreak of cholera in south London during July to October 1854, Snow also perceived that the dual system of water supply to that district constituted a natural experiment in which the question of the contribution of polluted water to the epidemic could be studied epidemiological^:

'The pipes of each Company go down all the streets and into nearly all the courts and alleys. In many cases a single house has a supply different from that on either side No fewer than three hundred thousand people of both sexes, of every age and occupation, and of every rank and station... were divided into two groups without their knowledge, one group being supplied with water containing the sewage of London and, amongst it, whatever might come from the cholera patients, the other having water quite free from such impurity. No experiment could have been devised which would more thoroughly test the effect of water supply on the progress of cholera.'

Snow obtained the name of the water company supplying each house where a fatal case of cholera had resided. From data held by the water companies, he then calculated the total number of houses supplied by each company in each district. The results showed that the Southwark and Vauxhall Water Company had a death rate from cholera of 315 deaths per 10 000 houses; by contrast, the adjoining districts supplied by the Lambeth Water Company

Figure 1.3. Dot map of deaths from cholera in London (the arrow points to the Broad Street pump). Redrawn from Snow (1936)9 by permission of Oxford University Press

experienced a death rate of only 37 per 10 000 houses, while the rest of London had a rate of 59 per 10 000 houses.9 The value of Snow's mapping exercises is unquestionable; these studies led to the prevention of cholera epidemics in the United Kingdom.

A comparable use of disease mapping, which included also maps of demographic parameters of aetiological relevance, was shown at the time of the Hamburg cholera epidemic of 1892.10 Initially, the physicians constructed a 'dot map' showing the cases of cholera in the city of Hamburg and in the adjoining suburb of Altona. They also constructed several others, such as maps of social class, death rates per 1000 inhabitants, morbidity rates per 1000 inhabitants and population per hectare. By showing clearly the demarcation between a high incidence of cholera in Hamburg and a low incidence in Altona, and by indicating that the cholera epidemic was geographically associated with the heavily contaminated water supplied only to Hamburg, the value of the mapping exercise mirrored Snow's original successful work in London.

A less well-known demonstration of the value of disease mapping appeared in the Mortality and Sanitary Record of Newark, New Jersey, published in 1880 by Edgar Holden.11 It was only after studying that community's distribution of preventable deaths (for example, diarrhoeal diseases, diphtheria, scarlet and yellow fevers, typhoid and smallpox) during the typhoid epidemics of the 1870s that Holden recognized that typhoid was associated more with the absence of sewage systems than with the presence of unavoidable topographical features such as watercourses and elevation above sea level.12

The value of mapping in clarifying the aetiology of an infectious disease was demonstrated again in the early 1920s. In a study of the distribution of typhus in Montgomery, Alabama, 'dot maps' were made of the cases reported during 1922-5.13 By showing that the cases plotted by place of work were more closely clustered than those plotted by residence, the maps indicated that typhus in that locality required close person-to-person contact of people in sizeable groups. Hence, unlike the louse-borne basis of epidemic typhus of the Old World, endemic typhus of the New World depended upon a rodent reservoir, with the rat flea acting as vector for the Rickettsia mooseri. Previously, the natural histories of typhus in the Old World and in the New World had been considered identical. However, this exercise in mapping led directly to the formation of the new hypothesis of a rodent-borne disease, which was subsequently confirmed.13

The relationship between the environment and some of the infectious diseases was becoming generally accepted by the nineteenth century, and 'dot maps' and mapping in general provided valuable assistance in the identification of causal associations. Nevertheless, the connection between the distribution of the non-infectious diseases and the environment received less recognition. In 1875 and 1882, maps were published showing the distribution of non-infectious disease in England and Wales (and particularly the counties of Cumberland, Westmorland and the Lake District).14-17 Haviland plotted crude death rates for the 11 registration divisions and the 44 registration counties of England and Wales for several diseases: heart disease, dropsy, female cancer and female phthisis. In addition, he included a detailed geographical description of the coastal and inland boundaries, hydrography, physical geography and geology, population characteristics, and general and local meteorology and climatology. In doing so, he must have been one of the earliest people in more recent times to appreciate fully the potential impact of the environment upon health.

The mapping of disease became more common at the turn of the twentieth century. In Britain, the next landmark in its development was between the World Wars, when Stocks18-20 produced the first maps of infectious and non-infectious disease in England and Wales which were standardized for differences in age, sex and urbanization. In 1939 Stocks presented maps using the standardized mortality ratio.

Between World War II and the 1960s, the techniques of mapping remained constant, with many of the data being collated, calculated and presented manually. However, the advent of easily operated computers in the late 1960s, with their modern data processing and graphical facilities, allowed the mapping of disease distributions to become almost commonplace. Several atlases of disease have been published worldwide; and, excluding those by Howe, four atlases have now been published covering the United Kingdom.21-24

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