Using GIS To Replicate John Snow's Cholera Research

Cholera is an acute bacterial intestinal infection caused by ingesting contaminated water or food and is one of the most feared infectious diseases in public health [1]. Symptoms of cholera include muscular cramps, vomiting, diarrhea, and rapid collapse, with infection generally occurring in areas with poor sanitation and where untreated sewage has contaminated the drinking water [2]. In extreme cases, cholera is one of the most rapidly fatal diseases due to severe water loss. A healthy person may become hypotensive within an hour of symptom onset and can die within 2-3 hours, although death typically occurs within a day or so. Fluid and electrolyte replacement can reduce mortality to less than 1% [1].

Until 1817, cholera largely remained in South Asia until its spread worldwide. Medical historians attribute this to European powers' colonization of the region, resulting in intensified migration, alterations to the natural environment, and socio-economic restructuring of local communities [2]. Major welfare reforms and improvements in basic living conditions, sanitation, and dietary standards led to the virtual disappearance of cholera from most industrialized countries by the mid-1920s [1]. Cholera lends itself to historical analyses, as it was the first disease for which modern public health surveillance, reporting, and monitoring were undertaken in an organized way [1].

In the 1850s, people believed cholera spread by miasma in the air - germs were not understood, and the sudden and severe cholera outbreak in London’s Soho was a mystery. So Dr. John Snow did something data journalists do today: he mapped the cases [3]. Snow suspected that the agent of cholera was an agent too small to be seen with the naked eye, capable of reproduction, and likely constructed like a cell. Instead of focusing on a fruitless search for the microscopic agent like his contemporaries, Snow focused his science on establishing cholera transmission from person to person [4].

Snow’s South London study established the mode of communication of cholera by comparing cholera death rates in an area of London with two different water suppliers intermingled in the same neighborhoods. One supplier took its water from the Thames upstream of London and the other from the heart of the city, where the sewers poured in. In studying the distribution of cholera, he could only count deaths because no systematic information was available on all cases [4]. It became apparent that the cases were clustered around the pump in Broad (now Broadwick) street. There were some outliers, though, and Snow wrote: “In some of the instance, where the deaths are scattered a little further from the rest on the map, the malady was probably contracted at a nearer point to the pump” [3 para 4]. Snow’s work on cholera demonstrates a fundamental epidemiologic principle: that the most important information about any infectious disease is its mode of communication [4]. 

The purpose of this project is to recreate John Snow’s 1854 cholera research using modern GIS technology. This process uses data of cholera deaths, then compares them to the locations of nearby water pumps to reach conclusions about the root cause. First, I observed data to find clusters of cholera deaths. Next, I located local pumps and found trade areas. By overlapping pump data with cholera deaths, I deciphered the pump’s roles in the spread of cholera. Processes used to understand the data were:

• Cholera death points
• Standard deviational ellipse of cholera deaths 
• Cholera death point density
• Pump points
• Thiessen Polygons of pump data showing pump trade areas 
• Zonal statistics showing the relationship between cholera deaths and trade areas

Thanks to John Snow, among others, today the biology of cholera is among the best understood infectious diseases, but there is still a lot that we do not know. Now recognized as more durable and complex than previously thought, cholera has the potential to exist permanently within the environment rather than living for a few days outside the human intestine [1]. Cholera outbreaks today primarily occur in overcrowded and poorly developed areas without proper sanitation. The correlation between outbreak and economic decline has indicated how urban environmental health is a function of economic performance [2]. Cities in developing countries, particularly Africa, Latin America, and Asia, often have inadequate funding for development programs, poor infrastructure support systems, and the environments in which the urban poor live suffer from ‘the tragedy of the commons [2].

Examples of cholera epidemics today include the 2008-2009 outbreak in Harare, Zimbabwe, and the 2010 outbreak in Haiti. In the case of Harare, the legacy of apartheid meant that many houses in urban areas - which rose rapidly from 23% in 1982 to 30% in the early 1990s - did not have toilet facilities [2]. Current water facilities and sewer infrastructure are several decades old, and there has not been any expansion of infrastructure to account for the rapid increase in the population [2]. Outbreaks in Zimbabwe have been increasing in frequency and severity, becoming more difficult to control. Finance shortages, declining infrastructure, and the departure of demoralized health personnel who faced poor pay and difficulty in practicing due to shortages of drugs, diagnostics, and support systems compromised Zimbabwe's healthcare system[8]. The population of Zimbabwe has become increasingly vulnerable through the economic crisis, an unemployment rate of 94%, and shortages of necessities, including food and other commodities [8].

The cholera outbreak in Haiti began on October 22, 2010. Haiti, the least developed nation in the Western hemisphere, was devastated by a massive earthquake in January of 2010 that left at least 200,000 dead, 1.3 million unhoused, and damaged the nation’s health and sanitation infrastructure. The combination of poor sanitary conditions, lack of immunity, and weak health infrastructure was the catalyst for an extensive epidemic [9]. This outbreak was “due to contamination of the Meye tributary of the Artibonite River with a pathogenic strain of current South Asian type Vibrio cholerae.” An expert panel concluded that a UN peacekeeping camp’s poor sanitary conditions were the source [6 para 4]. The UN ultimately denied responsibility, and Haiti did not receive sufficient funding to treat the outbreak. However, since the end of 2016, cholera cases have been decreasing, with the effects of action on protection and surveillance for the population, vaccinations, and WASH (water, sanitation, and health systems) becoming prevalent [5].

Because of the severe loss of fluid cholera causes, it is responsible for a significant proportion of life-threatening infectious diseases and death during the cholera season in endemic areas [6]. While clean water supplies and the safe disposal of human waste dramatically decrease cholera cases, these two goals are unlikely to be achieved in many areas for decades [6]. Cities in developing countries, particularly Africa, Latin America, and Asia, often have inadequate funding for development programs, poor infrastructure support systems, and the environments in which the urban poor live suffer from ‘the tragedy of the commons' [2].

We have John Snow, in part, to thank for today’s more nuanced understanding of the disease. Snow’s basic theory, formulated in the early 1848 outbreak, demonstrated that the cause of cholera was the ingestion of a morbid material or poison, as it began with local abdominal symptoms [7]. Because it was unlike other diseases that began with general symptoms such as fever, Snow hypothesized that the cause of cholera’s trademark symptoms of vomiting, dehydration, and diarrhea was an irritant on the surface of the stomach and intestines [7]. A majority of Snow’s evidence in the Broad Street case came from his investigation into the circumstances of those who died of cholera in the vicinity - he could show that the Broad Street pump was the source of water consumed in most cases [7].