John Snow and epidemiology
As we move from the “basics” of Microbiology into more applied topics, we’ll begin to address the ways that microorganisms cause disease in individuals and populations, and the ways that we can begin to prevent the transmission of infectious disease. Our first topic from Chapter 14 (and the topic of last week’s blog posting) is that of symbiosis; where we describe the relationship between a microorganism and a host. The last topic to be covered in this chapter is that of epidemiology, the study of how infectious diseases spread through a population, and how public health policy can block the spread of disease.
Prior to the 19th century and the development of the germ theory of disease, it was believed that diseases were the result of misdeeds of the afflicted individual. This perception originated with the Greeks or earlier, and persisted through the development of the Scientific method, where is was supplanted by the thought that diseases were transmitted via “bad air.” A variety of this latter belief persists today, with the common perception that colds can be caused by drafts, a belief with little scientific support.
John Snow (1813-1858) was an English physician, who gained recognition for his pioneering work into the use of general anesthetics. Wikipedia notes that he personally administered chloroform to Queen Victoria during the birth of the last two of her 9 children, and that the use of surgical anesthetics gained widespread use at this time. Fantastic as this development was, I feel that Snow’s contribution to the basis of infectious disease was more significant.
Cholera outbreaks plagued cities of the time, and were attributed to pollution. Snow was skeptical of this mechanism, and believed that cholera patients (persons with the disease) were somehow the cause of new cases, and published an essay detailing this in 1849. When a serious outbreak of the disease afflicted London in 1854, Snow investigated the appearance of new cases, and correlated the spread of the epidemic with a map of the city. The graphic is a copy of Snow’s original map, detailing the occurrence of cases, and Snow quickly realized that the cases appeared to increase in frequency to the center of the map. When Snow went to investigate what was at that location, he found a water pump that supplied local residents with drinking water. The water supplied to that location came from the Thames River, directly below a sewage discharge station. Snow was convinced that this was the cause of the epidemic, and brought his findings to the authorities. Snow himself said:
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.
The pump handle was removed, and the outbreak was rapidly brought under control. Interestingly, after the outbreak subsided, government officials replaced the handle, and in essence rejected Snow’s theory for the transmission of cholera. An author on Wikipedia felt that this was because the public found the theory of fecal-oral transmission too unpleasant of a concept, however citations for this premise are not given and so must be taken with at least a grain of salt. This article in Pubmed however, gives an extremely detailed picture of the local politics of the time. Many people found it difficult to believe that the rapidity and highly localized spread of cholera through neighborhoods could be caused by something so many people depended on.
Snow’s other significant contribution to this process was the introduction of statistics to strengthen his conclusions. This had the effect of making the study of disease and human health into a field where concrete outcomes could be measured and compared, and be used to make informed public health decisions.
BONUS: There has been a cholera epidemic in Haiti following the earthquake just over a year ago. The epidemic came as a surprise to epidemiologists, even though the earthquake led to a significant breakdown in public infrastructure. Why were epidemiologists surprised?