Revolutions in Epidemiology: Snow Map of Cholera
John Snow was born in 1813 at York, England. He is known for his remarkable work in the field of anesthesia on ether and chloroform, as well as in the field of epidemiology for his work on cholera on which he also published his book “mode of communication of cholera” and wrote 16 papers on prevention of Cholera. He was the pioneer of obstetric anesthesia and known for administering chloroform to Queen Victoria during her delivery.
Snow proposed that “Cholera is a water-borne communicable disease”. His theory gained importance when in 1848, pandemic of cholera reached London from Asia. In mid 1800s it was considered that cholera was air born infectious disease and occurs due to “miasma” (pollution or bad air). Snow, on his knowledge of anesthetic gases denied the hypothesis that if it was true than the workers involved in offensive trades would have suffered more but that was not true.
Snow reached to the conclusion that cholera was transmitted by the ingestion of “morbid matter”. This theory was not accepted until 1854, when an outbreak of cholera attacked Soho section of London. Snow proposed that people who drink water from water pump on Broad street in Golden Square were more affected. He interviewed all the residents of Broad Street who complained of unpleasant odour of water.
He reviewed the records and came to know that 500 cholera cases were reported in three days and total number of deaths reached 616 around the vicinity of Broad street. A lady in the Hampstead who had belief that water from Broad street pump is superior and used to drink water from there, died. Snow plotted the deaths due to cholera on a dot map in the vicinity of Broad street and presented his findings in front of Board of Governors who ordered for the removal of pump handle and so the epidemic abated. Snow proposed that a sewer from Marshall street passes below the pump which was responsible for the contamination of water.
This map changed the perspective of data visualization. Cholera endemic of 1848 and presentation of map at that time make us think about how a map brought attraction towards cholera theory of being transmitted by water. Long charts and data as well as their interpretation sometimes could not convince as maps or diagrams that make conclusion easy.
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4. McLeod KS. Our sense of Snow: the myth of John Snow in medical geography. Social Science & Medicine 2000; 50(7-8): 923-35.
About the Author: Feriha Fatima graduated from LUMHS, Pakistan, in 2007. She is working as a lecturer in biochemistry department since 2010. Feriha is interested in research especially in field of molecular biology and human genetics and can be reached [email protected]
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