John Snow, Asiatic Cholera and the inductive-deductive method - republished
Lecture 9: the Horsleydown and Battersea outbreaks and the importance of an accurate investigation with an open mind
The Snow series is an educational course. We hope you will recognise our efforts by donating to TTE or becoming a paying subscriber, as writing the series took a lot of time and effort.
Trust the Evidence is a reader-supported publication. To receive new posts and support our work, consider becoming a free or paid subscriber. Fitting in with the obsessive and determined way Snow investigated cholera outbreaks is his examination of the 1849 outbreak in Albion Terrace, Wandsworth, Deptford and Thomas Street, all in South London. Horselydown was where the 1848 index case found lodgings - see Lecture 2. The index case is the first documented case in any outbreak or case of special interest.
All these outbreaks occurred near the Thames.
As an example, it is worth summing up Snow’s investigation of the Albion Terrace outbreak. First and foremost, he ascertained the geography of the houses, drains and water source. The following diagram is taken from Peter Vinten Johansen et al.’s masterly biography of Snow:
The 17 houses in Albion Terrace drew water from an abundant spring, which got channelled to fill the water tanks behind each house (black line).
The tanks were built of stone and cement and communicated with each other. From these, a lead pipe took the water to the back kitchen of each house. Four feet away from each tank, a cesspool with a privy was on top. When examined by the Board of Health, some cesspools were found to be overflowing into the water tank, situated 15 inches below it.
Overflows were leaky, and there was communication between pits. This precarious situation was made worse by the heavy storms of 26 July and 2 August 1849, when sewage from the river, drinking water and overflow from cesspools mixed and even inundated some houses. Two days later (28 July), the index case in house 13 became ill. The outbreak occurred in dwellings of the “genteel class”.
Snow even examined the offensive drinking water under a microscope and found undigested remnants of foodstuffs from the digestive canal, like stones and husks of currants, in the deposits of the water tanks.
Dr Gavin Milroy, on behalf of the General Board of Health, investigated the Albion Terrace outbreak and attributed it to miasmata arising from house refuse, flooding, and the nearby river sewer. At this stage, the Board had set up its own investigation to collect as much data as possible to prove that miasmata caused cholera (see Lecture 18).
Snow politely pointed out that offensive smells, rotting refuse, and open sewers were to be found all over London, not just in Albion Terrace.
The Thames at Battersea.
CONTEMPORARY THEMES
We have seen a few maps of the spread of acute respiratory infections, especially in confined spaces such as hospital wards.
However, unless the investigation is carried out by people or a body with no personal agendas, the results will be worthless at best and misleading at worst, regardless of the availability of maps and diagrams.
The key in any such epidemiological map is to relate exposure to outcome (in this case, water supply and cholera cases) - see also Lecture 11.
We have also seen difficulty pinning down the index case without field investigations. Advances in genetic analysis permit tracing back the lineage of a virus through those it infected.
Readings
Vinten-Johansen, Peter, Howard Brody, Nigel Paneth, Stephen Rachman, and Michael Russell Rip. 2003. Cholera, Chloroform and the Science of Medicine: A Life of John Snow. Oxford; New York: Oxford University Press.
'Open mind', 'unbiased': the fundamental necessities for any investigator. IOW: one must look at what is actually there rather than see what one wants to see because one's already made up one's mind. This is when "what is" becomes 'what ought to be' because that's what one needs to find to make one's case.