Friday, October 16, 2020

Mapping Cholera in Amsterdam, Soho & Leeds

John Snow's map of cholera victims in Soho during the 1854 cholera outbreak helped to prove that cholera is spread by contaminated water and not by air. The cholera outbreak in London however was just one outbreak among many across Europe in the 19th Century. In the Netherlands for example there were major outbreaks of cholera in 1859 and again in 1866.

In Amsterdam alone 1,149 people were killed in the 1866 epidemic. In 1860 the Dutch Journal of Medicine published a report on the spread of cholera in the 1859 epidemic in Amsterdam. This report included data on all the cholera victims in the city. The recorded data included information on the addresses where each victim died, the date of death and even in which part of the house each victim died.

Kolerkaart has used the data from the 1860 Dutch Journal of Medicine report to map the 1859 cholera outbreak in Amsterdam using modern mapping technology. Kolerkaart includes a number of interactive maps which show how the outbreak developed over time, the number of cholera deaths in each neighborhood of the city and the number of deaths recorded in different parts of the house. These modern maps of the 1859 cholera outbreak reveal, among other things, that a relatively large number of people died in the outer ring of the city. The lasck of data on the location of pumps and water sources however mean that Kolerkaart is unable to prove a direct connection between contaminated water and the spread of cholera in Amsterdam.

Mapping cholera deaths was common even before John Snow's map of the 1854 Soho outbreak. For example Robert Baker's Sanitary Map of the Town of Leeds plotted the locations of deaths during the 1832 cholera epidemic in the Yorkshire town. Although Baker never made a direct link between cholera and contaminated water in his report to the Leeds Board of Health, Baker noted that "the disease was worst in those parts of the town where there is often an entire want of sewage, drainage and paving". 

In developing his theory that cholera was transmitted by water rather than air Snow was able to use the detailed statistics recorded by Dr William Farr. In 1838 Farr, a qualified doctor, was appointed to the General Register Office. This was the government department responsible for recording births, deaths and marriages in the UK. In his role at the General Register Office Farr was able to introduce a system which recorded causes of death. This data could then be used to look for geographical, environmental and occupational patterns in death rates and different diseases.

It was partly Snow's use of these death rate statistics which led him to believe that cholera was caused by germs which were transmitted by water. William Farr was impressed with Snow's germ theory of cholera being transmitted by water. However Farr himself believed that cholera was more commonly transmitted by air (the miasma theory). He even developed his own theory based on the idea that deadly miasmata are greater at lower than higher elevations. In his 'Report on the mortality of cholera in England 1848-49' Farr's detailed analysis of the distribution of cholera deaths in London actually established an apparent link between the rate of cholera deaths and elevation.

In this map from the report the red numbers 'denote the elevation in feet above the Trinity Highwater Mark' (image from the Wellcome Collection). Farr believed that the link between elevation and cholera was further evidence for the miasma theory. In 1854 Farr was a member of the Scientific Committee for Scientific Enquiries in Relation to the Cholera Epidemic of 1854. A committee which rejected John Snow's Broad Street pump analysis. The report concluded that "on the whole of evidence, it seems impossible to doubt that the influences, which determine in mass the geographical distribution of cholera in London, belong less to the water than to the air."

William Farr however was finally persuaded of Snow's germ theory of cholera and its waterborne transmission. In 1866 Farr himself wrote a report, which included detailed analysis of death statistics, to show that water and not air transmission was the most important cause of cholera. 

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