2 Seeking the Cause

Reading 3

John Snow and the Scourge of London, 1831 to 1854

Finding the Source

This reading introduces students to the first big idea of epidemiology, finding the source of the disease, with the story of John Snow and his investigations of cholera in 19th century London. Snow’s hypothesis was that cholera was a waterborne disease and the source of the epidemic was a pump somewhere in the city. Except for the very wealthy, most Londoners during this time period obtained their water for drinking, washing, and food preparation from a town pump closest to where they lived. Snow’s goal was to prove his hypothesis by collecting evidence.

You may want to begin by discussing the idea that the first step in most investigations is the formulation of a hypothesis in which the researcher poses an explanation for an observed phenomenon. The next step is to devise a method for gathering evidence that will prove or disprove the hypothesis. Snow’s methods involved interviewing victims, their families and associates about their lives and habits and then looking for patterns in his data.

Finding the Source

This reading introduces the first big idea of epidemiology, finding the source of the disease, with the story of John Snow and his investigations of cholera in 19th century London. Snow’s hypothesis was that cholera was a waterborne disease and the source of the epidemic was a pump somewhere in the city. Except for the very wealthy, most Londoners during this time period obtained their water for drinking, washing, and food preparation from a town pump closest to where they lived. Snow’s goal was to prove his hypothesis by collecting evidence.

Introduction

Prior to understanding that parasites, bacteria and viruses were the causative agents of infectious diseases, humans attributed devastating epidemics to various causes such as unknown poisons, bad air, evil spirits, or divine retribution for sins and aberrant behavior. Even without understanding the role of microbes in disease, careful observation of the patterns of disease spread led to the understanding that certain diseases were communicable; that is, that they could spread by contact with infected humans or animals or through food and water. Even with this knowledge how to prevent infections was a mystery. Diseases ran rampant through big cities and small villages. London in the 19^th century endured the ravages of infectious diseases including cholera, influenza and typhus.

19th Century London

Between 1831 and 1854 the city of London suffered three major outbreaks of cholera, which resulted in the death of 32,000 Londoners. London during this time was ripe for outbreaks of disease. The industrial revolution brought thousands of laborers to the city from the countryside seeking work. Millions of people lived in overcrowded, unsanitary conditions in ramshackle buildings that accommodated the rising worker population, with large families often occupying a single room. Garbage piled up in the streets providing food and shelter for vermin. Tons of raw sewage was routinely dumped into the Thames River.

Overflowing drains, sewage leaking through defective pipes, gasses emanating from toilets, baths, and sinks, coal fires, unwashed bodies, and decaying animal and human excrement in basements and streets all contributed to the foul-smelling air that hung heavy over the city, adding credence to the theory that “miasma”, some kind of deadly vapor, caused disease when inhaled. Without understanding the real cause of cholera it seemed impossible to stop its spread.

Check timeline for a review of the factors involved in the London cholera outbreak.

Launch Timeline

Cholera

Left untreated, cholera leads to a gruesome death. Often, the victim feels absolutely healthy one minute, and extremely nauseated the next. This nausea is followed by a sudden and total evacuation of the bowels. Cold, clammy sweats begin, together with a pounding headache. The infected person’s bowel movements then produce an odorless liquid, filled with rice-like pellets that are actually fragments of the victim’s intestinal lining. Soon, the patient’s body shrinks and shrivels and skin turns blue or black. Death generally occurs within two to seven days. The disease terrified rich and poor alike since there seemed to be no way to prevent it and no way to treat it.

John Snow, first epidemiologist

John Snow, a physician practicing in London, had three opportunities to study cholera; first in the pandemic of 1832, again in the outbreak of 1848–1849, and finally in the 1853–1854 outbreak. Believing that if cholera were caused by miasma everyone in London would have contracted cholera , he sought a different source. Snow’s hypothesis was that contaminated water was the source of the disease, but the challenge was how to prove this to others who were skeptical. To find the evidence he needed, Snow interviewed families that had lost members to cholera, trying to find some common thread that would link the victims. When he mapped the locations of the victims to locations in London, he observed a pattern.

Figure 1: Death at the Pump

Figure 2 shows the original map Snow created from his data. (Today this kind of map is called a spot map.) Most of the victims lived near a water pump on Broad St. from which they drew their water for cooking, drinking, and washing. Subsequent analysis of water from the Broad Street pump indicated that it was contaminated with fecal matter. Using his data and map, Snow convinced the city council to remove the handle from the pump and the number of cholera cases plummeted.

Figure 2: Mapping cases of cholera

Source: Centers for Disease Control and Prevention

It is now known that cholera is a waterborne disease, spread when water used for drinking or washing food has been contaminated with feces from infected individuals. The causative agent, the bacteria Vibrio cholera, grows in the intestine of the infected person and passes out in the copious diarrhea that is symptomatic of the disease.

Snow’s work combined scientific methodology with statistical analysis and is one of the first recorded investigations in the field of epidemiology. His work was the catalyst for changing the infrastructure of cities and towns. During Snow’s time cities lacked ways of dealing with sewage and ensuring clean water for all of its citizens. Once there was scientific evidence linking disease to dirty water and untreated sewage, cities had to build sewage disposal and treatment systems and find ways to provide clean water for all. Snow’s methods opened whole new approaches for dealing with infectious disease.

Additional Resources for Module 2 Reading 3

This engaging docudrama shows how John Snow, a London physician, traced a major outbreak of cholera in the 1850s to its source. Using logic, statistics, and mapping, Snow determined that contaminated water was responsible for spreading the disease among the local population. Snow’s techniques and approaches became the foundation of modern epidemiology—the science of public health that is built on probability, statistics, and sound research methods.

The site Cholera and Thames provides additional information about the outbreaks of cholera in London in the 1800s in downloadable set of teacher notes, which provides primary source material related to the outbreaks and John Snow’s investigations. This site also provides an interactive game in which students can take on the role of John Snow as he collects data to track down the source of the disease.

Students can work with John Snow’s data to determine for themselves the source of the cholera outbreak in London in 1854. Students develop a working hypothesis about the source of the disease and then use data as evidence to prove or disprove their hypothesis. This activity can be found in Insights in Biology: A Journey of Discovery Kendall/Hunt publisher 2007 pp. 305 - 317