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We have seen the spatial distribution of deaths in the area of the Broad Street pump. Snow adds to this with a time distribution showing that the peak of the outbreak occurred in the first two days of September 1854. There had been sporadic cases up to then, but all of a sudden, 143 new cases arose on 1 September, followed almost immediately by a peak in deaths.

The common explanation for the downward curve is that Snow removed the pump handle, and the outbreak soon subsided. This is sheer nonsense for a variety of reasons.

The only authority for any such action was the St James’s Parish Council. To this body, Snow delivered an explanation and a request for the removal of the handle on the evening of 7 September when he knew the worst was already over. 

“I had an interview with the Board of Guardians of St. James's parish, on the evening of Thursday, 7th September, and represented the above circumstances to them. In consequence of what I said, the handle of the pump was removed on the following day”.

He was listened to with scepticism by the members of the council except for its vice-chair, the Reverend Whitehead. The Board gave orders for the removal of the handle the next day.

Whitehead went further and ordered an excavation of the pump well all the way down to the subsoil (see Lecture 18). The well had already been examined, but this time the examination was more thorough and revealed decayed brickwork and communication with a vaulted sewer.

William Farr, Snow’s peer, was well aware of this up-and-down trend in acute disease. He defined it in what became known as Farr’s Law of Epidemics, which stated that what goes up must come down, irrespective of what we do.

Even then, the temptation to tinker with what we now call infectious disease was overwhelming.

William Farr (1807-1883) (image courtesy of Wikimedia)

The problem was and remains our incomplete understanding of the underlying causes for the rise, which can be exponential, while the “burn out” causes of the fall are easier to understand (see Lecture 6). Snow referred to these when he discussed point source outbreaks. Eventually, the “poison” runs out of susceptible folk: they either die, build immunity, or they avoid contact with the presumed source, leading the agent to stop transmitting and infect and cases to fall. 

CONTEMPORARY THEMES

In Lecture 6, we showed a table of Farr’s Law of Epidemics in action in the Lombardy data - March 11th 2020, saw the introduction of restrictions in the whole of Italy, based mainly on the perception of what was happening in Lombardy and Veneto. But March 9th is the day when the epidemic reached its zenith with a case increment of 31% compared to the previous day, thereafter rapidly diminishing. The analogy with the Broad Street outbreak is notable. 

In both cases, the interventions (even if effective) took place too late to influence anything, but in both cases, bias and ignorance assign effectiveness to the removal of the pump handle or total lockdown.

However, in the context of Farr's Law, the unresolved problems are understanding the ecology of the microorganisms, where they reside when not active, if and how they vanish (like SARS-CoV-1) and what activates them. The origin of SARS-CoV-2 has left us with one of the most acrimonious debates in scientific history, which does nothing to advance our knowledge. 

As we said, Farr showed that epidemics rise and fall in roughly a bell-shaped curve (a normal distribution) shape.

Once peak infection has been reached, then it will roughly follow the same symmetrical pattern on the downward slope. However, under-testing and variations in testing regimes often mean we cannot ascertain when the peak of infections occurred. 

In Lecture 10, we introduced the concepts of interaction and complexity, a still understudied area in need of clarification and understanding in any realistic future plan to tackle epidemic disease.

For example, according to Colwell and Morabia:

We know today that cholera epidemics result from interactions between the germ and environmental, biologic, and social factors. Intense episodes of warm precipitation, particularly on the Indian subcontinent, transform the temperature and salinity profiles of estuaries. The influx of large quantities of freshwater mobilizes stored nutrients in the bottom sediments and gives the dormant cholera bacterium a head start in its growth cycle. With the help of copepods, it reaches the gut tracts, first of crabs, clams, and oysters and ultimately of humans who ingest the contaminated seafood raw and disseminate it. The discovery of the comma bacillus could, therefore, not suffice to prevent cholera epidemics from recurring in communities.

So if respiratory viruses worry us so much, why are studies of their ecology and interactions not number one on the list of every government’s scientific enquiries?

Eye witness accounts tell us that the Broad Street fire was extinguished for want of fuel: everyone was either dead, had moved or was recovering, but does the same happen with respiratory viruses?

Learning point

This is an example of a contemporary lecture attempting to apply classic epidemiology to the Snow story and then to respiratory viruses. Can you spot the factual mistakes?

The quote is from JS Memo https://www.johnsnowmemo.com/john-snow-memo.html

Readings

Morabia A. Epidemiologic interactions, complexity, and the lonesome death of Max von Pettenkofer. Am J Epidemiol. 2007 Dec 1;166(11):1233-8. doi: 10.1093/aje/kwm279. 

Colwell RR. Global climate and infectious disease: the cholera paradigm. Science 1996;274:2025–31.

Westminster St. James, Cholera Inquiry Comm. Report on the Cholera Outbreak in the Parish of St. James, Westminster, during the Autumn of 1854. Lond, 1855. Available here.

Susser M, Adelstein A. An introduction to the work of William Farr. Am J Epidemiol. 1975 Jun;101(6):469-76. doi: 10.1093/oxfordjournals.aje.a112117.

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