Snow, John (1813–1858)

John Snow has an unusual place in medical history because he is a seminal figure in two medical disciplines—anesthesiology and epidemiology. His contribution to the first field was to establish the chemical and biological principles underlying the administration of consistent dosages of anesthetic gases effectively and with minimal toxicity. In the latter field, he discovered how cholera—and, by extension, every form of intestinal infection—was transmitted. The process by which he discovered the fecal-oral and waterborne routes of disease communication was the first true model of epidemiologic investigation.

Snow's twin accomplishments were not unrelated. As the world's first practicing anesthesiologist, he was intimately familiar with the effects of gases on human physiology. This understanding made him skeptical of the then-reigning dogma that miasmas—hypothesized gaseous emanations from rotting material that were inhaled—could cause disease at a distance. As so often in science, the first step in developing a new hypothesis was recognition of the limitations of the old.

Snow had cared for cholera patients as a teenage apprentice in 1832. When cholera made its second appearance in Europe in 1848, he published a small pamphlet and a two-page paper on cholera transmission. He argued that cholera was fundamentally a disease of the intestinal system and that its major symptoms were the result of fluid loss. This led him to conclude that the ‘agent’ of cholera was ingested.

He further reasoned, from much circumstantial evidence, that the ‘agent’ was transmitted by accidental soiling of the hands by the colorless evacuations of cholera and that this transmission could be greatly multiplied if the evacuations found their way into water supplies.

The work that most epidemiologists recognize as Snow's signature achievement was undertaken during the third European epidemic, from 1854 to 1855. Snow examined mortality from cholera in two regions of London with overlapping water supplies. One water company (Lambeth) used the rural Thames above London as its source, while the other (Southwark and Vauxhall) took its water from the Thames downstream of the city's sewage effluent. Snow visited hundreds of houses in the region to determine their water supplies, enlisting the help of medical colleagues, including the public health official William Farr, and linking the source of water in each house to the number of deaths from cholera among its residents. He found that in the first weeks of the epidemic, death rates were 14 times higher in houses with water from Southwark and Vauxhall than in houses supplied by Lambeth. He also showed that in a major local outbreak in Soho, the source was almost certainly a shallow well supplying a widely used public pump on Broad Street. He persuaded local officials to remove the pump handle, but by then the outbreak was almost over. The well was later shown to have been fecal contamination from a leak from a nearby cesspool. Decades before the work of Pasteur and Koch, Snow speculated that the cholera ‘agent’ could reproduce, that the duration of reproduction accounted for the incubation period, and that the agent probably hadastructurelikeacell.

Although to modern readers Snow seems eminently persuasive, his views on cholera were not widely accepted in his day. Cholera investigations later in the century, however, convinced many British and American physicians that water supplies were a central feature of cholera transmission.

...