The Map that Changed Medicine

For thousands of years the commonly accepted medical theory regarding contageous diseases such as Black Plague and cholera was known as Miasma Theory. This incuded a phenomenon known as "bad air" or night air. The theory held that  epidemics  were caused by miasma, emanating from rotting organic matter.

Not all scientist of the 19th century agreed however.

The English physician and anasthesia researcher, John Snow, doubted the miasma as a cause of infectious disease outbreaks. Snow, with the help of a local priest Henry Whitehead, conducted meticulous interviews of residents of London's Soho neighborhood during the Cholera Outbreak of 1854.

Today, cholera is treated with rehydration therapy. The vast majority who contract it survive. This has not always been the case. There have been seven recorded cholera pandemics. The third pandemic began in the Lower Bengal region of India in 1837. Over successive waves travelling with trade it spread throughought Asia first (green dots on map), then through the Middle East by 1845 (yellow dots) and eventually through Europe, Africa and North America. The most famous events in this pandmeic took place in London's Soho neighborhood in 1854.

Cholera is a bacteria which is ingested through consuming water or food infected with human waste of someone with cholera. It passes to the small intestine where it attaches and causes the infected person to have watery diharrea, thus propogating more potential infection via waste contaminated water. In 1854 in Soho, London sesspools which held human waste underneath houses allowed for cholera to seep into groundwater. While Snow did not completely understand this process, he was suspicious of it.

How did Snow uncover the source of the outbreak?

He began with a map. This pane shows the modern streets of his Soho neighborhood (at the time, the poorest in London).

He spent weeks interviewing residents. Zoom in on the map overlay (the actual image of Snow's map). Notice the black bars at each address. Each one represents a death from cholera at that location. Snow lived in this neighborhood and had relationships with its residents. He understood where the people got their water for cooking and drinking which allowed him to begin to zoom in on his culprit.

Modern GIS technology allows us to place each death on Snow's original map to make the patterns more clear. In addition, all of the public water pumps (the primary source of water for the urban poor of the time) are found here. If one were to point to a possible source of infection what would it be?

Snow was also able to confirm this by comparing the death near the pump at 10 Ramillies Place. This woman had previously lived near the Broadstreet pump and preferred to have servants bring water from there instead of the pump outside her door.

In addition, no deaths were recorded among the residents who worked at the brewery. They would have received beer and cider as a part of their wages and thus drunk low alcohol fermented beverages (which had been sterilized by the production of alcohol) instead of the contaminated water.

Modern geostatistics also allows us to plot the spatial mean of the outbreak. Snow would not have had access to this modern mathematical tool. Still, his analysis allowed him to form a solid hypothesis as to the cause of the outbreak.

Of his results, Snow said the following:

On proceeding to the spot, I found that nearly all the deaths had taken place within a short distance of the [Broad Street] pump. There were only ten deaths in houses situated decidedly nearer to another street-pump. In five of these cases the families of the deceased persons informed me that they always sent to the pump in Broad Street, as they preferred the water to that of the pumps which were nearer. In three other cases, the deceased were children who went to school near the pump in Broad Street...

With regard to the deaths occurring in the locality belonging to the pump, there were 61 instances in which I was informed that the deceased persons used to drink the pump water from Broad Street, either constantly or occasionally...

The result of the inquiry, then, is, that there has been no particular outbreak or prevalence of cholera in this part of London except among the persons who were in the habit of drinking the water of the above-mentioned pump well.

-John Snow, letter to the editor of the Medical Times and Gazette (1854)

On September 7th, 1854 Snow went to the local board of guardians and successfully argued to have the pump handle removed. As a result, the outbreak subsided. For the first time a government body used scientific evidence to support policy backed up by germ theory, not miasma.

The modern field of epidemiology was born in Soho in 1854 with the famous map made by John Snow. Today a memorial of Snow's work is found near the spot of the Broad Street pump.