MAPPING DISEASE - part 2

More than 50 years passed before Dr. John Snow produced what is arguably the most famous map of disease ever created. Similarly inspired by an epidemic disease, Snow mapped London’s cholera deaths in relation to water pumps. He used his map to convince authorities to remove the handle to the Broad Street Pump, a likely source of contaminated water, and the epidemic subsided. There is some evidence that the epidemic was “already in rapid decline by the time the handle was removed” suggesting that the epidemic would have subsided without intervention (Tufte 1997) [3]. Modern investigators have also found that a water engineer in London, Walter Cooper, produced a very similar map months before Dr. Snow (Brody et al. 2000). Nevertheless, Snow was a talented cartographer and physician and his map demonstrated to future generations the value of what is now called “spatial epidemiology” [4].

Although epidemic diseases inspired, and still inspire, considerable cartographic innovation, spatial examination of endemic disease data has also led modern epidemiologists and cartographers to some significant discoveries. Examples include: Mapping of cancer mortality in the United States 1950-69 revealed “exceptionally” high rates of lung cancer along the eastern seaboard, particularly in parts of Georgia, Florida and Louisiana (Blot et al. 1978; Devesa et al. 1999). These patterns led to further investigation. Several studies found that exposure to asbestos among employees at shipyards accounted for a significant part of the excess mortality from lung cancer (Blot et al. 1982; Blot et al. 1978; Blot et al. 1980).

Researchers in the 1950s used the geographic patterns in malignant melanoma to identify causes. In his influential 1956 paper, Dr. Henry Lancaster noted that rates of malignant melanoma were higher in the people of Southern Africa and Australia than in the countries from which they originated. He also noted that incidence increased with proximity to the equator in the United States, Australia and New Zealand and proposed that sunlight may be a cause of the increased incidence (Hutt and Burkitt 1986; Lancaser 1956). Prior to Dr. Lancaster’s paper, sunlight had not been considered a source of the disease due to the fact that tumors are common on non sun exposed sites such as the trunk in men and legs in women.

[3] Cholera does not withstand cold weather and it is possible that by mid-September London may have been less hospitable to the disease.

[4] Snow is also attributed with the invention of the Voronoi diagram (also known as Thiessen polygon). In his map of the cholera outbreak he included a continuous broken line (the ‘boundary of equal distance between Broad Street Pump and other Pumps’). Remarkably, the lines do not measure Euclidian distance, but distance along the streets (Okabe et al. 2000).