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| Cholera: finding a cause | |
| There were several detailed investigations of
the 1854 cholera epidemic in London. The most famous account is that by John Snow who
studied the pattern of disease outbreaks and clinical symptoms and concluded that cholera
was caused by a specific ‘agent’ (remember, we knew nothing about the role of
bacteria in disease at this time) which was excreted in the stools of infected people. He
claimed people acquired the disease by coming into contact with either raw sewage which
contained infected faeces or contaminated drinking water. Snow’s conclusions differed dramatically from those of the official enquiry by the Board of Health. They conducted an exhaustive study which included quantitative analyses of the relationship between the incidence of cholera and changes in air and water temperatures, rainfall, barometric pressure, humidity, wind pressure and direction, cloud cover and altitude. The also analysed air samples from cholera wards and water and sewage samples from many sites and, in a massive 4-volume report, they concluded:
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The Board of Health’s report reflected the prevailing view of the transmission of disease, i.e. that infectious diseases were not caused by a specific agent, but were the result of the ‘vaporous emanations’ of decaying organic matter which, under the right circumstances, could lead to a range of diseases such as cholera or typhoid. On the other hand, Snow argued that cholera was caused by a specific agent, most likely a living organism, which infected through ingestion. Based on his hypothesis, the isolation of both infected patients and contaminated water and sewage would be a logical course of action to prevent spread of the disease. He took the dramatic step of turning off the water pump in a district of London where the epidemic was raging to show that the outbreak of the disease was radically changed by cutting off access to contaminated water.
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| However, the verification of Snow’s theories of the mechanism and transmission of the disease did not come about until later that century, through the work of Pasteur and Koch. In the mid 1860’s, Koch identified the organism responsible for causing cholera. By developing in vitro methods, bacteria could be cultured; the crucial step was then to demonstrate that the isolated bacteria would produce the disease when injected into an animal. Today, ‘Koch’s Principles’, formulated from this approach, underpin diagnostic microbiology. |
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