Week 8.2: Control of Cholera

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Do curso por The University of Hong Kong

Epidemics

23 classificações

The University of Hong Kong

Epidemics

23 classificações

“If history is our guide, we can assume that the battle between the intellect and will of the human species and the extraordinary adaptability of microbes will be never-ending.” (1) Despite all the remarkable technological breakthroughs that we have made over the past few decades, the threat from infectious diseases has significantly accelerated. In this course, we will learn why this is the case by looking at the fundamental scientific principles underlying epidemics and the public health actions behind their prevention and control in the 21st century. This course covers the following four topics: 1. Origins of novel pathogens; 2. Analysis of the spread of infectious diseases; 3. Medical and public health countermeasures to prevent and control epidemics; 4. Panel discussions involving leading public health experts with deep frontline experiences to share their views on risk communication, crisis management, ethics and public trust in the context of infectious disease control. In addition to the original introductory sessions on epidemics, we revamped the course by adding: - new panel discussions with world-leading experts; and - supplementary modules on next generation informatics for combating epidemics. -------------------------------------------------------- (1) Fauci AS, Touchette NA, Folkers GK. Emerging Infectious Diseases: a 10-Year Perspective from the National Institute of Allergy and Infectious Diseases. Emerg Infect Dis 2005 Apr; 11(4):519-25. What you'll learn - Demonstrate knowledge of the origins, spread and control of infectious disease epidemics - Demonstrate understanding of the importance of effective communication about epidemics - Demonstrate understanding of key contemporary issues relating to epidemics from a global perspective

Na lição

Theme Three: Control (Non-pharmaceutical intervention (NPI))

Theme Three Summary

Week 8 Introduction1:42

Week 8.1: Non-pharmaceutical interventions overview and John Snow4:34

Week 8.2: Control of Cholera8:37

Week 8.3: Hygiene and other personal interventions6:23

Video 8.4: Isolation and quarantine and contact tracing6:02

Video 8.5: School closure7:49

Video 8.6: International measures including travel restrictions and entry screening4:40

Video 8.7: Zoonoses - market closure6:18

Conheça os instrutores

Gabriel M. Leung (HKU)
Dean
Li Ka Shing Faculty of Medicine
Joseph T. Wu (HKU)
Associate Professor
Li Ka Shing Faculty of Medicine
Kwok-Yung Yuen (HKU)
Professor
Li Ka Shing Faculty of Medicine
Tommy Lam (HKU)
Assistant Professor
Li Ka Shing Faculty of Medicine
Maria Huachen Zhu (HKU)
Associate Professor
Li Ka Shing Faculty of Medicine
Guan Yi (HKU)
Chair Professor
Li Ka Shing Faculty of Medicine
Benjamin Cowling (HKU)
Professor
Li Ka Shing Faculty of Medicine
Thomas Abraham (HKU)
Associate Professor
Journalism and Media Studies Centre
Mark Jit (LSHTM)
Professor
Department of Infectious Disease Epidemiology
Malik Peiris (HKU)
Chair Professor
Li Ka Shing Faculty of Medicine
Marc Lipsitch (Harvard)
Professor
Department of Epidemiology

In this second part, I will continue the story of cholera

and draw some conclusions

for the control of emerging infections.

The objectives of this part

are to define a natural experiment;

to define the mode of transmission of cholera

as it is now understood;

and to discuss the implications

of mode of communication for the impact of control measures.

At the end of August 1854,

following a series of smaller outbreaks in London,

a major epidemic of cholera began

near Golden Square in Soho,

a district in central London with a few thousand residents.

Within a few days, more than 100 people had died,

residents were fleeing the area,

and by the end of the outbreak

the death toll was more than 600.

Later, Snow would call it,

"The most terrible outbreak of cholera

which ever occurred in this kingdom."

He visited the death registry

and obtained the home addresses

of the cholera deaths in the area,

and then he went around the area door-to-door

to identify cases of cholera.

The result of his investigation

is this famous map of cholera cases.

Snow’s suspicions immediately fell on the Broad Street pump,

from where local residents drew their drinking water.

Snow noted that the cases were clustered

around the Broad Street pump,

with a few important exceptions.

Residents of the brewery were unaffected,

and Snow found that they drank

their own beer rather than water.

More than 500 residents of the workhouse

were largely unaffected,

and they had their own well.

As part of his investigation,

Snow determined the walking distance

to each nearby pump, and added this line to the map.

From any point inside the line,

it is quicker to walk to the Broad Street pump

than any other pump.

Most of the cases fall inside the line,

and of those outside, Snow determined

that half preferred to visit the Broad Street pump

even though it was slightly further

than another water source.

The evidence that really clinched it for John Snow

was when he heard about a case in Hampstead five miles away,

in a widow who used to live in Broad Street,

and so loved the water that she had the water

shipped to her on a weekly basis.

Having received a delivery

of contaminated water on Thursday,

she fell ill on Friday and died on the Saturday.

She and her maid were the only two people

to die of cholera in Hampstead.

On the evening of the 7th of September,

John Snow presented his findings to local officials,

and convinced them to remove the pump handle,

to prevent any new cases.

The officials agreed, and the epidemic subsided.

A subsequent investigation, not by John Snow,

established that the well had probably been contaminated

by the cesspit of the house nearest to the pump,

in which a baby had been ill with cholera

on the 28th of August.

John Snow is often credited with controlling the outbreak,

through removing the pump handle,

which is of course a non-pharmaceutical intervention.

However, as Snow noted from the epidemic curve shown here,

the epidemic was largely over by the 7th of September.

John Snow then proceeded to conduct another, larger study.

Snow noted that water was supplied to households

in London by a number of different water companies.

He obtained, from William Farr,

the addresses of the first 334 recorded cholera deaths

in a particular area of London

that was mainly served by two particular companies,

the Southwark and Victoria company,

and the Lambeth company,

and he visited each household to enquire

which company they obtained their water from.

He was able to obtain information for 330 of the 334 cases,

which is a remarkable effort.

Separately, he obtained information

on the number of houses in those areas of London

whose water was supplied by each of the two companies.

He then compared the incidence rates of cholera,

and identified a far higher rate among households

supplied by the Southwark and Victoria company.

Whereas the Lambeth company had recently moved

its waterworks upstream,

the Southwark and Victoria company drew their water

from the Thames in central London,

where contamination by fecal matter

could have been much greater.

This is an example of a natural experiment.

In Snow’s own words, "No experiment could have been devised

which would more thoroughly test the effect of water supply

on the progress of cholera."

What distinguishes a natural experiment

from a typical scientific experiment

is that the interventions are applied by nature,

rather than by the investigator.

However, the scientific community was not convinced

by Snow’s theory that cholera was spread

by the fecal-oral route,

and communicated through contaminated water.

Snow’s report on the water supply companies

and the Golden Square outbreak

was followed by scathing editorials in The Lancet,

continuing to support the miasma theory,

and branding Snow as an enemy of sanitary progress.

Other commentators also noted that

removal of the pump handle did not control the outbreak,

and questioned why some people who drank contaminated water

failed to fall ill if it was consumption

of contaminated water that caused the disease.

It was not until 1866, eight years after Snow had died,

that his theory was finally accepted.

By 1890, Dr John Simon,

the first UK Medical Officer of Health,

wrote that John Snow’s hypothesis of cholera was

“The most important truth yet acquired

for the prevention of epidemic disease."

Snow's work identifying cholera as a communicable disease

is now held up as a textbook example

of the arrangement of established facts

into chains of inference.

While he was not the first to blame contaminated water,

Snow is heralded for his elegant analysis of outbreaks,

including the one in Golden Square,

and the natural experiment

with different water supply companies,

which provided convincing evidence in support of his theory

that the disease was communicated in excretions

of cholera victims via contaminated water.

So what can we learn from this

that is relevant to our topic of

non-pharmaceutical interventions?

First, John Snow differed from his contemporaries

because he was searching for

the mode of communication or transmission

of cholera between people,

rather than the factors that placed people

at higher risk of infection.

By identifying the mode of communication of cholera,

that is the process by which the pathogen

spreads between hosts, Snow was able to identify

interventions that would be effective.

It was not essential to identify the agent,

which in the case of Vibrio cholerae

was publicized by Robert Koch 30 years later.

In the early stages of a contemporary

emerging infectious disease epidemic,

laboratory scientists will work day and night

to identify the pathogen.

As a concrete example, we can think of the outbreak

of Severe Acute Respiratory Syndrome in 2003.

It was a major breakthrough when the SARS coronavirus

was identified as the cause of SARS in 2003,

but it was more important for public health

when scientists realized that SARS

was transmitted by respiratory droplets,

mostly within hospitals rather than in the community,

and that isolation of cases

and respiratory protection of healthcare workers

was necessary and sufficient to control the outbreak.

In other words, the SARS epidemic

would not be controlled unless cases were identified,

isolated, and front-line staff treating those patients

wore protective equipment, and that doing

all of these things led to control

of the outbreak in a population.

Now, SARS transmission did occur outside hospitals,

but perhaps not at a high-enough rate

to sustain an epidemic.

Second, we learn that non-pharmaceutical interventions

can be effective in controlling

an infectious disease outbreak.

The Broad Street epidemic largely subsided

because 75% of the residents had left the area

within a week of the start of the outbreak,

and because exposure was so common

that those who were vulnerable

to infection were rapidly infected.

But if the contaminated pump had been identified sooner,

removal of the handle could have prevented

hundreds of cholera deaths.

In the separate analysis of water supply companies,

the decision by the Lambeth company

to move its waterworks upstream

turned out to save many lives among its clients,

and similar actions by other water companies

could have prevented more cholera deaths.

I will conclude this section by inviting you

to read more about the story of John Snow

and his pioneering work on cholera.

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