Miasma, germ theory, and the search of the causes of disease

PUBHLTH 405
Social Epidemiology of Infectious Disease
University of Michigan School of Public Health

Jon Zelner
[email protected]
epibayes.io

Getting to know John Snow, pioneering anesthesiologist

“He sits alone in his cluttered flat,🐸 frogs croaking around him, illuminated only by candlelight. After a few minutes tinkering…he fastens the mouthpiece over his face and releases the gas. Within seconds, his head hits the desk. Then minutes later, he wakes, consults his watch through blurred vision. He reaches for his pen, and starts recording the data.”

Agenda

  • What does it mean to identify the cause(s) of a disease?

  • Unpacking how Snow combined information from different levels of scale to push back on the ☁️miasma☁️ theory of cholera infection?

  • Project time

On your own: What does it mean to identify the cause of a disease?

You can tell the story of the Broad Street outbreak on the scale of a few hundred human lives, people drinking water from a pump, getting sick and dying over a few weeks, but in telling the story that way, you limit its perspective, limit its ability to convey a fair account of what really happened, and, more important—why it happened. Once you get to why, the story has to widen and tighten at the same time: to the long durée of urban development, or the microscopic tight focus of bacterial life cycles. These are causes, too. (Johnson 2007, 96)

Snow’s work is about using data across scales to argue for a specific mode of disease spread.

How many outbreaks we see is really a function of what scale we’re looking at or care about.

Koch’s postulates are a logical framework for attributing infection to a specific pathogen that flowed from Snow’s work

In the decades following Snow’s work, Robert Koch developed a set of criteria for determining whether a given organism - specifically V. Cholera or M. Tuberculosis - was the cause of a disease.

Where are we in the outbreak?

Snow used a systems perspective to identify a transmissible cause of cholera

“The bird’s-eye view of the city, the sense of the urban universe as a system, as a mass phenomenon—this imaginative breakthrough is as crucial to the eventual outcome of the Broad Street epidemic as any other factor.” (Johnson 2007, 97)

What is another term that describes this approach?

(Hint: 🧩)

What are the elements of a transmission system?

Components

  • Pathogens 🦠

  • People 🧑‍🤝‍🧑

  • Infrastructure 🏚️

Mechanisms

  • Pathogen reproduction 🔄

  • Short-scale movement 🚌

  • Long-distance movement ✈️

  • Person-to-person contact 👩‍👩‍👦‍👦

Snow’s sensitivity to patterns across scales allowed him to see things others didn’t

Another word for this is consilience

“In science and history, consilience…is the principle that evidence from independent, unrelated sources can”converge” on strong conclusions. That is, when multiple sources of evidence are in agreement, the conclusion can be very strong even when none of the individual sources of evidence is significantly so on its own.”(Wikipedia definition of consilence)

How did Snow integrate information across scales to make his case?

In groups of 4-5:

  • Illustrate the logical evolution of Snow’s thinking about Cholera leading up to the 1854 Golden Square outbreak.

  • Include the temporal ordering of these insights and anything else you think is important to highlight (i.e. different opinions/ideas from thinkers like Farr & Whitehead)

  • The style is totally up to you: Flow diagram, map, interpretive drawning, some combination of these. Just be prepared to briefly share/explain your illustration with everyone!

  • Take ~15m to do this and when you are done, take a photo or screenshot of your group’s drawing and email or slack message it to me.

Some elements of Snow’s explanations

  • Personal experience

  • 1848 roominghouse outbreak

  • 1849 Thomas St. flats outbreak

  • William Farr’s tables of cholera deaths

  • Anything else?

Some questions that might be helpful to chew over in your groups

  1. What made miasma a compelling explanation for outbreaks of Cholera in South London?

  2. What aspects of Snow’s life made him an ideal investigator of the cause of Cholera?

  3. What are some modern parallels to the strong hold that miasma theory had on 19th century Londoners?

  4. How did Snow start to use evidence at different scales to push back against the miasmatists?

Snow’s Road to the 1854 Outbreak

Rewind to 1848

Rewind to 1848

Rewind to 1848

I smell a rat…

“[I]t was stretching matters beyond belief to suggest that the room should suddenly become prone to those poisonous vapors the very day it was occupied by a sailor traveling from a city beseiged by the disease.”

In Snow’s words

“Who can doubt that the case of John Harnold…was the true cause of the malady in Blenkinsopp…And if cholera be communicated in some instances, is there not the strongest possibility that it is so in the others - in short, that similar effects depend on similar causes.”

What happened in the 1849 Thomas street outbreak that drove these ideas home for Snow?

“Whether you looked at the evidence on the scale of an urban courtyard or on the scale of entire city neighborhoods, the same pattern repeated itself: the cholera seemed to segment itself around shared water supplies. If the miasma theory were right, why would it draw such arbitrary distinctions? Why would the cholera devastate one building but leave the one next door unscathed? Why would one slum suffer twice the losses as a slum with arguably worse sanitary conditions?” (Johnson, p.74)

The Grand Experiment

What was the experimentum crucis?

A response to Snow’s 1849 article in the London Medical Gazette suggested that:

“[T]he experimentum crucis would be, that the water conveyed to a distant locality, where cholera had been hitherto unknown, produced the disease in all who used it, while those who did not use it, escaped.”

  • In a note in the November 26, 1852 Weekly Returns, Farr wrote that in three districts of South London strongly impacted by Cholera, service was mixed between Southwark and Vauxhall and the Lambeth water comanies.

What made this a compelling opportunity for Snow?

  • Around 1850, parliament mandated that all water companies move their intake beyond the outflow of the Thames by 1855
  • In south London there were two major water suppliers: Southwark & Vauxhall (S&V) and Lambeth.
  • Lambeth moved the intake up in 1852, but S&V waited until 1855.

    • Snow’s Water Supply Map

    Snow’s map of water suppliers

    Conducting the grand experiment

    • Snow divided data into sub-districts of South London organized by water supplier.

    • 12 were served by only S&V, 3 by Lambeth alone

    • 1/100 people died of Cholera in S&V only districts, but none among the > 14K living in Lambeth districts

    What made the Grand Experiment inconclusive?

    • 16 sub-districts were served by both suppliers

    • Within these districts, water service was overlapping at a fine scale.

    • Snow went door-to-door…but people living in these districts often had no idea where their water came from.

    • He collected water from them and tried to find evidence of the causal agent of Cholera in the sample, but couldn’t find any.

    Next Time

    More miasma, and other persistent bad ideas

    References

    Johnson, Steven. 2007. The Ghost Map: The Story of London’s Most Terrifying Epidemic–and How It Changed Science, Cities, and the Modern World. Reprint edition. London: Riverhead Books.