Scientific Method John Snow, William Farr: Cholera

Scientific Method

John Snow, William Farr: Cholera and the scientific method

John Snow's thesis

John Snow, a Victorian epidemiologist and physician to Queen Victoria, constructed his (accurate) hypothesis that cholera epidemics were caused by poor sanitation and 'morbid material' in the sufferers' guts based upon his analysis of the disease's physical manifestations and progression. The disease was marked by pain, vomiting, diarrhea, and dehydration and responded to conventional treatments for gut-related disorders. All of this suggested that the sources of cholera entered the body through swallowing. Additionally, examining the water quality of places with known outbreaks of cholera caused Snow to conclude that cholera mortality in these areas was fourteen times higher among with uncontaminated or less contaminated water (Eyler 2001: 225).

Snow's deployment of the scientific method of testing a hypothesis involved a series of experiments, using different types of evidence to support his conclusion. Snow reconstructed how the water supply of a row of 17 houses could have become contaminated from cesspools and surface water drains. Then Snow "examined a specimen of water from an outbreak center" and noted that it "smelled like privy soil and contained bits of undigested food that had clearly passed through the alimentary canal" (Eyler 2001: 226). Combined with the epidemiological evidence compiled by William Farr, Superintendent of the General Register Office, England's center for vital statistics, Snow confirmed his belief that contaminated water was the source of cholera outbreaks.

Farr's epidemiology

William Farr, Snow's contemporary, also attempted to trace the cholera epidemic over time and geographically throughout the city. Farr investigated the roles of sex, age, seasons, day of the week and soil elevation and how they were correlated with cholera transmission. Snow began with a hypothesis and focused upon specific, real world scenarios that could prove or disprove his thesis. Farr was far broader in his outreach, however, and did not focus upon one source of transmission or data: he simply tried to gather as much evidence as possible. Based upon what he learned, Farr proposed a 'law' or formula "that could predict in mathematical terms human mortality from cholera according to soil elevation" (Morabia 2001: 223). Unfortunately, Farr's so-called law was based upon a fundamentally flawed view of disease transmission: the conventional wisdom at the time was that almost all diseases were caused by airborne molecules. While Farr conceded that the water in the areas of cholera outbreaks was quite dirty, he believed this merely confirmed that cholera was "caused by zymotic (that is, produced by fermentation) factors" off of the water (Morabia 2001: 223). Farr thought that the fumes from sewage-treated water were to blame, not ingesting the cholera bacterium. Farr believed that cholera was caused by an as-yet unidentified zymotic material and while he admitted some of the particles might be ingestible he still thought that most of the toxic materials entered the body through the lungs (Eyler 2001: 228).

Snow vs. Farr

Snow found Farr's data valuable but the two men reached entirely different conclusion based upon Farr's information. In "his Tenth Annual Report of 1847, Farr talks of 'the disease mist, arising from the breath of two millions of people, from open sewers and cesspools, graves and slaughter-houses (that) & #8230; like an angel of death has hovered for centuries over London'…Snow had the correct hypothesis, but Farr had built the most advanced public health surveillance system of that time. This system was instrumental in providing the evidence that convinced the public" that Farr was right when he stated that cholera was spread primarily through the air, not contaminated water (Morabia 2001: 223).

In his essay "Snow and Farr: A scientific duet," Alfredo Morabia characterizes Snow's approach as more brilliant and characterized by leaps in logic, while Farr's techniques, he stated were more methodical. John M. Eyler, in contrast, in his article "The changing assessments of John Snow's and William Farr's cholera studies" characterizes Snow as more dogmatic and tenacious in upholding this thesis, while Farr was more willing to entertain alternative possibilities and dissent. On one hand, Farr's thesis upheld common notions of disease transmission. Snow's theory clashed with conventional models that suggested that 'bad air' was behind the spread of all communicable illnesses (Schoenbach 1999). But on the other hand, Farr was interested in accumulating data first, then forming a conclusion while Snow began with a hypothesis, and then sought to…