John Snow and the importance of contaminated water

In the first half of the nineteenth century a cholera epidemic occurred in London. The physician John Snow, already well-known in 1853 as anaesthetist to Queen Victoria (chloroform during childbirth), had a special interest in cholera. In 1854 he examined the various families presenting cases and calculated that the mortality in the houses which were supplied with water by the Southwark and Vauxhall Water Company was 31/1000 houses. This was 8.5 times higher than in houses supplied by the rival Lambeth Company. Although neither of the two companies offered purified water, the first company took its water from the Thames near London Bridge, downstream from the city sewage outlets while the second company pumped its water upstream from the city at Thames Ditton. In 1849 there had been no difference in mortality between the families which received water from Lambeth or Southwark. Before 1851 the Lambeth Company drew its water from a highly contaminated stretch of the Thames near Hungerford Market. It was this spectacular change (1849 compared to 1854) which made Snow conclude that contaminated water had a causal connection with cholera. Although both companies delivered water in the same streets, the water used in any particular house could be identified by its salt content (London Bridge is closer to the sea and its water is more salty than that at Thames Ditton). Adding silver nitrate leads to precipitation of silver chloride, proportionate to the amount of salt in the water. This was the basis of a simple test which could be carried out in every house.

During a particular outbreak he mapped the houses where cholera occurred. On the map spatial clustering around the Broad Street pump could be seen. The houses where cases occurred were closer to the Broad Street pump than to other water pumps. People usually took their water from the nearest pump. There was no similar clustering in front of the sewage outlets, which argued against the miasma theory which was popular at that time (fumes from the sewers). He noticed that families where cholera occurred, used water from a particular public water pump in Broad Street, St James’s parish, Soho. Families which were spared, obtained their water elsewhere (there were five water pumps in the area). After the pump had been turned off (the handle was removed), the epidemic stopped (although the epidemic was already diminishing before the pump was made unusable). Careful inspection showed broken masonry in the well. A few metres further up was a sewer from which water was overflowing into the well. It was quite a new concept to use maps for epidemiological purposes.

Similar findings were made in Hamburg in 1890. The incidence of cholera was 34/1000 in Hamburg, where the drinking water was drawn from the river Elbe, and 3.9/1000 for the surrounding areas where other sources were used. In Altona, downstream from Hamburg, contaminated water was also taken from the Elbe, but there was less cholera. How could this be explained? If anything, more cholera would be expected in Altona. The difference was that in Altona the water was first filtered slowly through sand before being supplied for consumption. These observations led to attempts to provide cities with clean drinking water and to construct adequate sewers. Cholera had an enormous effect on the development of the principles of public health and was the basis for the so-called “sanitary revolution” at the end of the 19^th century. Cholera was the first disease for which surveillance was set up and because of this the disease has code number 001 in the international classification list of diseases.

Category: Medical Subject Notes , Microbiology Notes