Snow on cholera

Colour map showing districts on the south side of the Thames and their water supplyJohn Snow (1813-58) is famous for two accomplishments. His work in anaesthesiology, in which he helped to make chloroform a safe treatment, may have sparked his interest in cholera, as chloroform was thought at one time to be effective in treating the symptoms of cholera.

Whatever the faults in his methodology, this book is a pioneering work of epidemiology, as it is one of the first treatises to examine systematically the effects of a disease outbreak on a geographically defined population. It is also, as the image here demonstrates, an important work in the history of medical cartography, even though Snow used maps as illustrative enhancements rather than analytical tools.

Snow had one purpose in mind: to demonstrate that cholera was spread by faecal-oral contamination alone. As he was no microscopist, he used pathology and epidemiology as his preferred analytical methods. Cholera’s clinical pathology and symptoms all pointed to a disturbance of the alimentary canal.

Snow’s epidemiological investigation in 1854 centred first on the Broad Street pump, where he demonstrated a correlation between consumption of cholera-infected water from the pump and cholera victims. He showed that the only factor common to all the victims was their consumption of water from that pump.

Second, he endeavoured to show that there was a correlation between consumption of water supplied by the Southwark and Vauxhall Water Company, as shown in the map reproduced here, whose supply from the Thames was contaminated by sewage, and the greater occurrence of cholera in the area which it supplied. Many medical contemporaries, such as Sir Henry Acland, who undertook a similar study of Oxford, were sympathetic to his conclusions but favoured a more multi-causal explanation of cholera.

What did Snow achieve? He intended not just to convince his fellow scientists but to make the world a healthier place. In the short term, the handle was removed from the Broad Street pump (though after the cholera outbreak had passed). From 1852 onwards, water companies were legally obliged to filter water and to cover reservoirs.

However, these duties did not stem from fear of cholera. The problem of the disposal of untreated sewage into rivers was not dealt with satisfactorily until the 20th century. Although cholera did not visit Britain after 1866, this stroke of good fortune had more to do with the imposition of quarantine on shipping from India.

Fear of cholera, with its horrifying symptoms and the terrifyingly swift death which followed from its onset, gripped Victorian Britain. However, many influential Victorians, such as Edwin Chadwick and the sanitarians, thought that susceptibility to cholera stemmed from the poor’s lack of moral fibre, and were not sympathetic to those, like Snow, for whom such explanations were not relevant.

The copy of the book featured here bears Snow’s own inscription.

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