Mapping an epidemic Science

John Snow, the father of modern epidemiology!

Snow's map that showed the spatial cluster of cases of cholera after the outbreak in the summer of 1854   | Photo Credit: Wikimedia Commons

A handle-less pump and a commemorative plaque stands on Broadwick Street, Soho, London - a monument to Dr. John Snow in recognition of his actions. If Paul Ehrlich lived at a time when they were starting to look for ‘magic bullets’ that would strike parasites, Snow came in much earlier – at a time when even the causes of many of the prevailing diseases weren’t yet known.

Photo: Rsabbatini/ Wikimedia Commons

Photo: Rsabbatini/ Wikimedia Commons  

Cholera is an intestinal disease caused by a strain of the bacterium Vibrio cholerae and can cause death within hours after the first symptoms if there is no treatment. While we now know that it is not contagious from person to person and is in fact spread through unsanitary water and food supply sources, it was a an idea for which Snow had to fight for, while the epidemic struck and killed people around him.

Against popular opinion

The first cases of cholera in England were reported in 1831 and it then struck the country with successive waves. The popular belief at the time was that cholera was caused by breathing vapours or miasma in the atmosphere. So when Snow, who had recently completed his medical studies and was involved in experiments to deliver babies using anaesthesia , proposed that drinking contaminated water was at the root of disease’s spread, it went largely unnoticed.

When Soho, a suburb of London, was hit by an outbreak of cholera in the summer of 1854, Snow, who lived near Soho himself, went to work to prove his theory. Hundreds died within the first three days of September and people began to flee the city.

Trail of an epidemic

Snow produced a map marking the 13 public wells in the neighbourhood and all the known cholera deaths in Soho. He was able to show a spatial clustering of cases around the Broad Street pump which stood at the intersection of Broad (now Broadwick) Street and Cambridge (now Lexington) Street. He went a step further and examined a sample from this source under a microscope and found the presence of an unknown bacterium.

On September 7, 1854, Snow took his findings to the officials, who weren’t entirely convinced. Still, for want of a better option and in their desperation, they decided to heed to his request of removing the handle off the pump. It was done , effectively knocking out the Broad Street pump as a water source.

With the epidemic already on the wane, this move meant that cholera cases dropped immediately and came to a stop. Officials, however, were not willing to give Snow the credit and went on to point the anomalies and that his hypothesis didn’t explain the cause. For months, Snow worked tirelessly and tracked down every case.

The anomalies proved to be a blessing in disguise as they reiterated Snow’s belief. Snow was able to show that those further away from the pump who had contracted cholera corresponded to people who used the Broad Street pump – either because they liked this water or due to the fact that it was on their way.

Similarly, those closer to the pump and yet unaffected by the outbreak were those who had access to a different source of water. When the Soho outbreak was traced down to a single source — a baby who had contracted it from a different water source but whose diapers were washed into a cesspool just three feet from the Broad Street pump — the case was effectively closed in Snow’s favour.

Snow published a number of maps to support his theory, considered as a milestone in studying the geographic distribution of a disease. His persistent efforts and statistical mapping models have made him the father of modern epidemiology.

(The author can be reached at ganesh.a.s@thehindu.co.in)


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Printable version | Sep 23, 2021 3:51:56 PM | https://www.thehindu.com/in-school/sh-science/john-snow-the-father-of-modern-epidemiology/article7622056.ece

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