A severe outbreak of cholera occurred in 1854 near Broad Street (now Broadwick Street) and Golden Square in Soho, London, England, during the worldwide 1846–1860 cholera pandemic. The outbreak (also known as Golden Square outbreak), which killed 420 people, is best known for the physician John Snow's study of its causes and his hypothesis that germ-contaminated water was the cause, rather than something in the air called "miasma". This discovery influenced public health and the construction of improved sanitation facilities beginning in the mid-19th century. Later, the term "focus of infection" was used to describe sites, such as the Broad Street pump, where conditions are favourable for transmission of infection. Snow unknowingly took advantage of a natural experiment during his endeavours to identify the cause of cholera transmission.
Background
In the mid-19th century, Soho in London had a serious problem with filth due to a large influx of people and lack of proper sanitary services: the London sewer system had not reached Soho. Cowsheds, slaughterhouses and grease-boiling dens lined the streets and contributed animal droppings, rotting fluids and other contaminants to Soho's primitive sewer system. Many cellars had cesspools beneath their floorboards, which formed from the sewers and filth seeping in from outside. With the cesspools overrunning, London's government decided to dump the waste into the River Thames, contaminating the water supply. London had already had a "series of debilitating cholera outbreaks". These included outbreaks in 1832 and 1849, which killed 14,137 people.
Competing theories of cholera
At the time of the Broad Street outbreak, physicians and scientists held two competing theories on the causes of cholera in the human body: miasma theory and germ theory. The London medical community debated the cause of the city's persistent cholera outbreaks. The cholera-causing bacterium Vibrio cholerae was isolated in 1854, but the finding became well known and accepted only decades later.
Miasma theory
Miasma theorists concluded that cholera was caused by particles in the air, or "miasmata", which arose from decomposing matter or other dirty organic sources. "Miasma" particles were thought to travel through the air and infect individuals, and thus cause cholera. Dr William Farr, the commissioner for the 1851 London census and a member of the General Register's Office, believed that miasma arose from the soil surrounding the River Thames. It contained decaying organic matter which contained miasmatic particles and was released into the London air. Miasma theorists believed in "cleansing and scouring, rather than through the purer scientific approach of microbiology". Farr later agreed with Snow's germ theory following Snow's publications.
Germ theory
In contrast, the germ theory held that the principal cause of cholera was a germ cell that had not yet been identified. Snow theorised that this unknown germ was transmitted from person to person by individuals ingesting water. John Simon, a pathologist and London's lead medical officer, called Snow's germ theory "peculiar".
Excerpt from John Simon:
This doctrine is, that cholera propagates itself by a 'morbid matter' which, passing from one patient in his evacuations, is accidentally swallowed by other persons as a pollution of food or water; that an increase of the swallowed germ of the disease takes place in the interior of the stomach and bowels, giving rise to the essential actions of cholera, as at first a local derangement; and that 'the morbid matter of cholera having the property of reproducing its own kind must necessarily have some sort of structure, most likely that of a cell.
Even though Simon understood Snow's theory, he questioned its relation to the cause of cholera.
Broad Street outbreak
On 31 August 1854, after several other outbreaks elsewhere in the city, there was a major outbreak of cholera in Soho. Snow 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. During the next week, three quarters of the residents had fled the area. By 10 September, more than 300 people had died and the mortality rate was 12.8 per thousand inhabitants in some parts of the city. By the end of the outbreak, 420 people had died.
Many of the victims were taken to the Middlesex Hospital, where their treatment was superintended by Florence Nightingale, who briefly joined the hospital in early September in order to help with the outbreak. According to a letter by Elizabeth Gaskell, Nightingale "was up night and day from Friday afternoon (Sept. 1) to Sunday afternoon, receiving the poor creatures (chiefly fallen women of that neighbourhood - they had it the worst) who were being constantly brought in - - undressing them - putting on turpentine stupes, et cetera, doing it herself to as many as she could manage".
By talking to local residents (with the help of Reverend Henry Whitehead), Snow identified the outbreak's source as the public water pump on Broad Street (now Broadwick Street) at Cambridge Street. Although Snow's chemical and microscope examination of a sample of the water from this Broad Street pump water did not conclusively prove its danger, the patterns of illness and death among residents in Soho persuaded the St James parish authorities to disable the 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:
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 occurred around this pump. His efforts to connect the incidence of cholera with potential geographic sources was based on what is now known as a Voronoi diagram. He mapped the individual water pumps and generated cells representing all the points on his map that 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 had the highest incidence of cholera. Snow also performed a statistical comparison between the Southwark and Vauxhall Waterworks Company and a waterworks at Seething Wells (owned by the Lambeth Waterworks Company) that was further upriver and hence had cleaner water; he showed that houses supplied by the former had a cholera mortality rate 14 times that of those supplied by the latter.
Of the decline in cases related to the Broad Street pump, Snow said:
It will be observed that the deaths either very much diminished, or ceased altogether, at every point where it becomes decidedly nearer to send to another pump than to the one in Broad street. It may also be noticed that the deaths are most numerous near to the pump where the water could be more readily obtained.
There was one significant anomaly: none of the workers in the nearby Broad Street brewery contracted cholera. As they were given a daily allowance of beer, they did not consume water from the nearby well. During the brewing process, the wort (un-fermented beer) is boiled in part so that hops can be added. This step killed the cholera bacteria in the water they used to brew with, making it safe to drink. Snow showed that the Southwark and Vauxhall Waterworks Company was taking water from sewage-polluted sections of the Thames and delivering it to homes, resulting in an increased incidence of cholera among its customers. Snow's study is part of the history of public health and health geography. It is 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 was, that there had 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 Thursday, the 7th September, and represented the above circumstances to them. In consequence of what I said, the handle of the pump was removed on the following day.
It was discovered later that this public well had been dug 3 feet (0.9 m) from an old cesspit that had begun to leak faecal bacteria. Waste water from washing nappies used by a baby who had contracted cholera from another source drained into this cesspit. Its opening was under a nearby house that had been rebuilt further away after a fire and a street widening. At the time there were cesspits 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.
At the same time, an investigation of cholera transmission was being conducted in Deptford. Around 90 people died within a few days in that town, where the water was known to be clean, and there had been no previous outbreaks of cholera. Snow was informed that the water had recently turned impure. Before using it, residents had to let it run until the sudsy, sewer-like water turned clear. Snow, finding that the water the residents were using was no different from the usual water from their pump, determined that the outbreak must be caused by a leak in the pipes that allowed surrounding sewage and its contaminants to seep in to the water supply. This was similar to the Broad Street outbreak. The incoming water was being contaminated by the increasing levels of sewage, coupled with lack of safe plumbing.
After the cholera epidemic 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-faecal method of transmission of disease, which was too unpleasant for most of the public to contemplate.
Investigation by John Snow
The Broad Street outbreak was an effect rather than a cause of the epidemic. Snow's conclusions were not predominantly based on the Broad Street outbreak, as he noted that he hesitated to come to a conclusion based on a population that had predominantly fled the neighbourhood and redistributed itself. He feared throwing off results of the study.
From a mathematical perspective, Snow's innovation was focusing on death rates in areas served by two water companies that drew water from the Thames, rather than basing it on data from victims of the Broad Street pump (which drew water from a well). Snow's work also led to a far greater health and safety impact than the removal of the Broad Street pump handle. Deactivating the pump "hardly made a dent in the citywide cholera epidemic, which went on to claim nearly 3,000 lives".
Snow was sceptical of the prevailing miasma theory, which held 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 yet established (Louis Pasteur proposed it in 1861). Snow did not understand the mechanism by which disease was transmitted, but the evidence led him to believe that it was not foul air. Based on the pattern of illness among residents, he hypothesized that cholera was spread by an agent in contaminated water. He first published his theory in 1849, in an essay titled "On the Mode of Communication of Cholera". In 1855 he published a second edition, including a more elaborate investigation of the effect of the water supply in the 1854 Soho outbreak.