1854 Broad Street cholera outbreak
The Broad Street cholera outbreak was a severe outbreak of cholera that occurred near Broad Street in the Soho district of London, England in 1854 (in the area now known as Carnaby Street). This outbreak is best known for the physician John Snow's study of the outbreak and his hypothesis that contaminated water, not air, spread cholera. This discovery came to influence public health and the construction of improved sanitation facilities beginning in the 19th century. Later, the term "focus of infection" would be used to describe places like the Broad Street pump in which conditions are good for transmission of an infection.
In the mid-19th century, the Soho district of London had a serious problem with filth due to the large influx of people and a lack of proper sanitary services: the London sewer system had not reached Soho. Many cellars had cesspools underneath their floorboards. Since the cesspools were overrunning, the London government decided to dump the waste into the River Thames. That specific action contaminated the water supply, leading to a cholera outbreak.
On 31 August 1854, after several other outbreaks had occurred elsewhere in the city, a major outbreak of cholera reached Soho. John Snow, the physician who eventually linked the outbreak to contaminated water, later called it "the most terrible outbreak of cholera which ever occurred in this kingdom."
Over the next three days, 127 people on or near Broad Street died. In the next week, three quarters of the residents had fled the area. By 10 September, 500 people had died and the mortality rate was 12.8 percent in some parts of the city. By the end of the outbreak, 616 people had died.
John Snow investigation
Snow was sceptical of the then dominant miasma theory that diseases such as cholera or the Black Death were caused by pollution or a noxious form of "bad air". The germ theory was not established at this point (Louis Pasteur would not propose it until 1861), so Snow was unaware of the mechanism by which the disease was transmitted, but evidence led him to believe that it was not due to breathing foul air. He first publicized his theory in 1849, in an essay titled "On the Mode of Communication of Cholera". In 1855 a second edition was published, with a much more elaborate investigation of the effect of the water supply in the 1854 Soho outbreak.
By talking to local residents (with the help of Reverend Henry Whitehead), he identified the source of the outbreak as the public water pump on Broad Street (now Broadwick Street). Although Snow's chemical and microscope examination of a sample of the Broad Street pump water did not conclusively prove its danger, his studies of the pattern of the disease persuaded the St James parish authorities to disable the well pump by removing its handle. Although this action has been popularly reported as ending the outbreak, the epidemic may have already been in rapid decline, as explained by Snow himself:
There is no doubt that the mortality was much diminished, as I said before, by the flight of the population, which commenced soon after the outbreak; but the attacks had so far diminished before the use of the water was stopped, that it is impossible to decide whether the well still contained the cholera poison in an active state, or whether, from some cause, the water had become free from it.
Snow later used a dot map to illustrate how cases of cholera were centred on the pump. He also used statistics to illustrate the connection between the quality of the source of water and cholera cases. Snow's efforts to connect the incidence of cholera with potential geographic sources centred on creating what is now known as a Voronoi diagram. He mapped the locations of individual water pumps and generated cells which represented all the points on his map which were closest to each pump. The section of Snow's map representing areas in the city where the closest available source of water was the Broad Street pump circumscribed most cases of cholera.
There was one significant anomaly – none of the workers in the nearby Broad Street brewery contracted cholera. They were given a daily allowance of beer, and did not consume water from the nearby well. The water used in the brewing process is boiled during mashing which kills cholera bacteria.
Snow also showed that the Southwark and Vauxhall Waterworks Company was taking water from sewage-polluted sections of the Thames and delivering the water to homes with an increased incidence of cholera. Snow's study is part of the history of public health and health geography, and can be regarded as the founding event of epidemiology.
In Snow's own words:
On proceeding to the spot, I found that nearly all the deaths had taken place within a short distance of the [Broad Street] pump. There were only ten deaths in houses situated decidedly nearer to another street-pump. In five of these cases the families of the deceased persons informed me that they always sent to the pump in Broad Street, as they preferred the water to that of the pumps which were nearer. In three other cases, the deceased were children who went to school near the pump in Broad Street...
With regard to the deaths occurring in the locality belonging to the pump, there were 61 instances in which I was informed that the deceased persons used to drink the pump water from Broad Street, either constantly or occasionally...
The result of the inquiry, then, is, that there has been no particular outbreak or prevalence of cholera in this part of London except among the persons who were in the habit of drinking the water of the above-mentioned pump well.I had an interview with the Board of Guardians of St James's parish, on the evening of the 7th inst [September 7], and represented the above circumstances to them. In consequence of what I said, the handle of the pump was removed on the following day.— John Snow, letter to the editor of the Medical Times and Gazette
It was discovered later that this public well had been dug only 3 feet (0.9 m) from an old cesspit that had begun to leak foecal bacteria. Nappies used by a baby who had contracted cholera from another source were washed into this cesspit, the opening of which was under a nearby house that had been rebuilt further away after a fire and a street widening. It was common at the time to have a cesspit under most homes. Most families tried to have their raw sewage collected and dumped in the Thames to prevent their cesspit from filling faster than the sewage could decompose into the soil.
After the cholera epidemic had subsided, government officials replaced the Broad Street pump handle. They had responded only to the urgent threat posed to the population, and afterwards they rejected Snow's theory. To accept his proposal would have meant indirectly accepting the oral-foecal method of transmission of disease, which was too unpleasant for most of the public to contemplate.
Henry Whitehead involvement
A former believer in the miasma theory of disease, Whitehead worked to disprove false theories, eventually focussing on John Snow's idea that cholera spreads through water contaminated by human waste. Snow's work, particularly his maps of the Soho area cholera victims, convinced Whitehead that the Broad Street pump was the source of the local infections. Whitehead then joined Snow in tracking the contamination to a faulty cesspool and the outbreak's index case.
Board of Health
The Board of Health in London had several committees, of which the Committee for Scientific Inquiries was placed in charge of investigating the cholera outbreak. Their main focus was to be the atmospheric environment in London; however, they were also to examine samples of water from several water companies in London. The committee found that the most contaminated water supply came from the South London water companies, Southwark and Vauxhall.
As part of the Committee for Scientific Inquiries, Richard Dundas Thomson and Arthur Hill Hassall examined what Thomson referred to as "vibriones". Thomson examined the occurrence of vibriones in air samples from various cholera wards and Hassall observed vibriones in water samples. Neither identified vibriones as the cause of cholera.
As part of their investigation of the cholera epidemic, the Board of Health sent physicians to examine in detail the conditions of the Golden Square neighbourhood and its inhabitants. The Board of Health ultimately attributed the 1854 epidemic to miasma.
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