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Commentary: Confronting Unexpected Results: Edmund Parkes Reviews John Snow


DOI: 10.1093/ije/dyt195

Eyler, J. “Commentary: Confronting Unexpected Results: Edmund Parkes Reviews John Snow.” International Journal of Epidemiology 42.6 (2014): 1559–1562. Web.

p.1559: John Snow’s On the Mode of Communication of Cholera is one of the most famous works in the history of epidemiology. It first appeared as a modest pamphlet in 1849 in the midst of Britain’s second epidemic of cholera. A second and more substantial edition appeared in 1855 following another but smaller outbreak, and it is this second edition for which Snow is remembered today. Edmund Alexander Parkes was a logical choice to review Snow’s second edition. -- Highlighted mar 15, 2014

p.1559: In short, Parkes was an established authority on epidemic diseases and hygiene. In fact Snow cites Parkes’s pathological work in his second edition. Parkes’s views were certainly informed and mainstream in the mid 1850s. His critique of Snow’s work is consequently useful in helping us understand how Snow’s contemporaries reacted initially to his cholera theory, and perhaps more importantly, it underlines the magnitude of the conceptual changes Snow’s work represents. -- Highlighted mar 15, 2014

p.1560: was an active and important one in the 1840s. Medical opinion was strongly divided on whether diseases such as cholera could be transmitted person to person, with the majority of the profession holding that they were not communicable. -- Highlighted mar 15, 2014

p.1560: Although it is no coincidence that the anti-contagionist sentiment grew with the political influence of merchants and manufacturers who favoured free trade and opposed the quarantines and isolation which a contagionist model would suggest, there were very good critical or observational reasons for holding that contagion, as then understood, could not adequately explain the behaviour of some of the most important epidemic diseases. -- Highlighted mar 15, 2014

p.1560: Parkes was not sympathetic to Snow’s theory and, as a critical reader, he subjected Snow’s volume to very close scrutiny and pointed out weaknesses or lapses which a modern reader, who knows that Snow was right, would be likely to miss or gloss over. -- Highlighted mar 15, 2014

p.1561: The incautious reader today easily concludes that Snow’s evidence was conclusive. Parkes correctly pointed out that, in this second edition, Snow did not have the evidence he needed. Parkes emphasized two great weaknesses. First, as Snow acknowledged, it was difficult to learn the water supply of individual houses. -- Highlighted mar 15, 2014

p.1561: More seriously and less obviously from Snow’s account, Snow did not know the number of people exposed to the two water supplies in the mixed district. In fact he did not even know the number of houses supplied by the two companies in the mixed district. Parkes’s careful reading found that the cholera mortality figures Snow gave were for all customers of the two companies and not only for the customers living in the district served by both companies. This was fatal weakness, because customers of these two companies living in different districts served by only one company might be subject to differences in environmental factors thought to be relevant to cholera’s prevalence: elevation, soil condition, organic waste, housing density, occupation etc. -- Highlighted mar 15, 2014

p.1561: Still, Parkes had to admit that Snow had provided highly suggestive evidence, and he was willing to accept contaminated water as a predisposing cause of cholera, i.e. one of several factors to be considered in explaining cholera outbreaks. This admission brings us to a fundamental difficulty Snow’s contemporaries had in accepting his conclusions on cholera. These contemporaries were used to looking to multiple causes to explain outbreaks of diseases like cholera. -- Highlighted mar 15, 2014

p.1562: it took some time for Snow’s theory to gain full acceptance. Some of the objections to his evidence in the second edition were soon eliminated. Local investigators showed how the well served by the Broad Street pump had been contaminated and identified a probable index case, and Snow acquired the data he needed on the numbers of households served by both water companies in the mixed districts. Then in the next cholera epidemic, the one of 1866, after Snow’s death, Farr himself presented strong statistical evidence that an intense outbreak in east London was caused by sewerage-contaminated water. By this time informed medical opinion was changing. -- Highlighted mar 15, 2014