The Snow series is an educational course. We hope you will recognise our efforts by donating to TTE or becoming a paying subscriber, as writing the series took a lot of time and effort.

Last week, we examined the Broad Street outbreak. Looking at the Broad Street map, we are struck by the relationship between the location of the pump and the cluster of deaths: the closer to the pump, the higher the death toll. 

This fact fits Snow’s theory well. Fetching water is heavy work, and it is only natural that you would draw water from your closest source. There was little chance of ascertainment bias as Snow had managed to identify most of those who died of cholera regardless of where they died, although he was well aware that some might have remained unidentified.

As we shall see further on, Snow was slowly disproving alternative explanations for the communication of cholera through his industry and hard work.

So, the map links exposure (contaminated water) to outcome (cholera death) in a spatial and temporal distribution. However, the map provides far more information than just that. Look carefully, and you will see that some buildings (the brewery) had no deaths. Others, like the large workhouse in Poland Street, had five deaths (out of over 500 inmates).

As usual, Snow was quick to understand the significance. Mr Huggins, the brewery manager, confirmed that no deaths had occurred amongst his 70-plus staff. They drank beer and malt liquor, and the brewery had its own deep well from which all water was drawn.

The same goes for the workhouse with a deep well in its yard.  

So, the absence of exposure meant the absence of disease.

In the following lecture, we will see the outbreak's time distribution and its significance in the field of communicable disease.

Here, again, is a close-up of the Broad Street area:

This is a contemporary picture of the Workhouse on Poland Street (on the left), a large establishment.

Snow remarked: the mortality appears to have fallen pretty equally amongst all classes, in proportion to their numbers. Masters are not distinguished from journeymen in the registration returns of this district, but judging from my own observations, I consider that out rather more than six hundreds deaths, there were about one hundred in families of tradesmen and other resident householders…the greatest portion of the persons who died were tailors and other operatives, who worked for the shops about Bond Street and Regent Street and the wives and children of these operatives. They were living chiefly in rooms which they rented by the week.

Previously, he had explained that the district around Golden Square was inhabited by a mixture of poor and well-off people interspersed with houses “intermediate in point of respectability.” As he had done with the districts of South London, he dismissed social class and wealth as variables linked with the spread. Cholera was democratic.

CONTEMPORARY THEMES

This is the build-up to the refutation of the poverty-non-sanitary and foulness link. A contagious disease that hits everyone regardless of social class must have a different source. Today, respiratory viruses cause the biggest problems in those with low immunity and frailty. This suggests that exposure is widespread, but susceptibility is not.

In turn, this should inform government and societal interventions to target those most in need and leave those who are least likely to suffer consequences - children and healthy adults - alone. 

However, in 1971, Julian Tudor Hart, a Welsh GP, proposed the inverse care law that says the availability of good medical or social care tends to vary inversely with the needs of the population served. 

“The availability of good medical care tends to vary inversely with the need for it in the population served. This inverse care law operates more completely where medical care is most exposed to market forces and less so where such exposure is reduced. The market distribution of medical care is a primitive and historically outdated social form, and any return to it would further exaggerate the maldistribution of medical resources.”

Readings

Koch T, Denike K. Crediting his critics' concerns: remaking John Snow's map of Broad Street cholera, 1854. Soc Sci Med. 2009 Oct;69(8):1246-51. doi: 10.1016/j.socscimed.2009.07.046. 

Shiode N et al. The mortality rates and the space-time patterns of John Snow's cholera epidemic map. Int J Health Geogr. 2015 Jun 17;14:21. doi: 10.1186/s12942-015-0011-y. 

Julian Tudor Hart. THE INVERSE CARE LAW. Lancet Volume 297, Issue 7696, 27 February 1971, Pages 405-412

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