John Snow, Asiatic Cholera and the inductive-deductive method - republished
The person-to-person spread of cholera: Lecture 2.
In the last post, we summarised Snow’s life.
In today’s post, we follow Snow’s description of the cholera epidemics of his time. Subsequently, we describe Snow’s deduction on the incubation period and the problems defining stable numerators and denominators.
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.
The attention of John Snow was attracted by the 1848 outbreak of cholera, which he says first appeared in London in the autumn of 1848. He starts off On the Mode of Communication of Cholera, 2nd edition, 1855 (MCC2) with a history of what was then known of Asiatic cholera. Snow charted its migration first in the Indian subcontinent and its subsequent spread to China, the Philippines and other parts of South East Asia.
Snow points out that it is impossible to give a detailed account without describing what we would now call a setting, but its spread was suggestive of human-to-human contact by road or ship ”never going faster than people travel”.
Snow ascribes the arrival of cholera in London to a seaman who had arrived from Hamburg on the steamer Elbe who took up lodgings in a house in Horsleydown, south London, where he died on September 22. The subsequent occupant of the sailor’s room developed symptoms of cholera on September 30 and died shortly afterwards.
The physician who attended both cases had considerable experience with cholera from the 1832 pandemic and was adamant that this was the same disease. In the following pages, Snow gives a detailed account of the outbreaks that occurred in England in 1848 and 1849, focussing on the apparent link between poverty and unsanitary conditions (as in the coal mines, workhouses, and lodgings of the poor).
Here is what he had to say about conditions in the crowded dwellings of the poor (from page 17 of MCC2):
The great prevalence of cholera in institutions for pauper children and pauper lunatics, whenever it has gained access to these buildings, meets with a satisfactory explanation according to the principles here laid down. In the asylum for pauper children at Tooting, one hundred and forty deaths from cholera occurred amongst a thousand inmates, and the disease did not cease till the remaining children had been removed. The children were placed two or three in a bed, and vomited over each other when they had the cholera. Under these circumstances, and when it is remembered that children get their hands into everything, and are constantly putting their fingers in their mouths, it is not surprising that the malady spread in this manner, although I believe as much attention was paid to cleanliness as is possible in a building crowded with children. Pauper lunatics are generally a good deal crowded together, especially in their sleeping wards, and as the greater number of them are in a state of imbecility, they are no more careful than children in the use of their hands. It is with the greatest difficulty that they can be kept even moderately clean. As might be expected, according to the views here explained, the lunatic patients generally suffered in a much greater proportion than the keepers and other attendants.
Deficiency of light is a great obstacle to cleanliness, as it prevents dirt from being seen, and it must aid very much the contamination of the food with the cholera evacuations. Now the want of light, in some of the dwellings of the poor, in large towns, is one of the circumstances that has often been commented on as increasing the prevalence of cholera.
Snow wrote several letters to his contemporaries nationwide to elicit data from local outbreaks. He remarked on the living conditions and local epidemics during the 1848-49 and 1854 outbreaks. For example, he described and reported outbreak data from Deptford and other parts of South London, Newcastle, Merthyr Tydfil in Wales, Scotland, Paris, Germany, Moscow, the Baltic Fleet, the British and French fleets during the Crimean War, French troops in Algeria, and India, where cholera supposedly originated.
For each outbreak, Snow tried to ascertain the timings and evolution, the circumstances and the outcomes. Given the characteristics of the disease, the diagnosis of cholera in each outbreak was not in question, as we shall see in our next lecture.
CONTEMPORARY THEMES
Snow starts his work with a narrative review and an accurate description of what was known about the epidemiology of cholera. In the electronic age, such an endeavour would be a systematic or at least a structured review. Approaching an analysis by preceding it with a good description is considered good practice.
In particular, the cholera speed of spread pointed to person-to-person transmission, at least in certain cases.
Person-to-person spread was inconsistent with the miasmatic origin of disease, which was the prevailing paradigm at the time (see Lecture 4). In contemporary literature, there is abundant reasonable quality evidence that acute respiratory infections (ARIs) are spread, at least in part by person-to-person spread. Those studies that claim long-distance spread through aerosol have never produced replication-competent microorganisms in sufficient quantities to cause disease.
This is a recurring problem with those who do not distinguish between identifying single antigens by PCR and isolation of replication-competent microorganisms in sufficient quantities to cause disease. “Replication-competent” is a term used to describe the capability of viruses to replicate inside a cell. The term avoids using the adjective “live”, as viruses are not considered living microorganisms.
Snow’s observation put him on a collision course with the establishment. The current debate on the transmission of SARS-CoV-2 is strikingly similar, with the absence of clear rules of adjudication and polarisation of views. Bad science based on firmly held credos today is the norm.
Such a situation does not benefit scientific discourse when abuse and subterfuge take the place of scientific discourse.
Readings:
Thanks for the comment Vivian, later on in the series we will meet the sanitarians. We will also delve a bit on Edmund Parkes, a close colleague of Nightingale. His Manual of Practical Hygiene lists all the principles that they both re-leaners at Scutari and Rankiol in Turkey. I bought a copy of the Manual for 5 GBP off a market stall and am thinking of serialising in a synthetic format for TTE.
Any thoughts?
Best, Tom.
Interesting coincidence of Dr Snow mentioning cleanliness and light: at the time of writing his book (1855) the Crimean war (1853-56) was in full swing and we recall Florence Nightingale's endeavours regarding the treatment of soldiers in the Scutari hospital ...