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John Snow and Cholera - www.chnri.org

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John Snow’s Investigations
of the Cholera Epidemics in
London, 1848-1854
Source of Information for the
John Snow Example
UCLA School of Public Health,
Department of Epidemiology
Website:
http://www.ph.ucla.edu/epi/snow.html
British cholera epidemics
• 1831-1832:
- first modern outbreak in Britain
- 23,000 deaths
- helped to launch the sanitary reform movement
• 1848-1849:
- 250,000 cases and 53,000 deaths
- prompted Snow (and others) to investigate
causes
The London Cholera Epidemic of
September 1848 – August 1849
• Based on his clinical experience and
review of epidemiologic characteristics
of cholera, Snow formulated a theory of
causation and transmission of the
disease.
The London Cholera Epidemic of
September 1848 – August 1849
• Snow’s hypotheses concerning cholera:
- a gastrointestinal disease, therefore
causal agent was likely ingested
- diarrhea as most prominent symptom,
therefore causal agent likely left the
body by this route
- if cholera excretions contaminated
rivers from which drinking water was
taken, then the disease could be widely
disseminated
The London Cholera Epidemic of
September 1848 – August 1849
Causal Hypothesis:
Sewage-contaminated drinking
water was a causal agent for the
cholera epidemic.
The London Cholera Epidemic of
1848-1849
• Snow tested this hypothesis by first
conducting an “ecologic” study of London
districts using:
- routine surveillance data on cholera cases
- population data at the district level
- available information on water companies
serving the districts
- available data on property values by district
The London Cholera Epidemic of
1848-1849
• Characteristics of an ecologic study:
- unit of analysis is not the individual or case
- unit of analysis is a geographical unit such as a
district or neighborhood or city or county
- useful to initially study a cause-effect relationship
when data are easily available and/or an individual
level study is not feasible
- does not directly link an individual case to an
exposure (unless everyone in the geographical
unit has a similar exposure)
The London Cholera Epidemic of
1848-1849
• Snow did not have exact information on
the sources of water for the districts.
• However, Snow knew that:
- the East and South districts were served by
water supplies obtained from known polluted
parts of the Thames River
- the rest of London received drinking water
relatively uncontaminated by sewage.
Districts
of
London
Population
in
1841
Deaths
from
Cholera
West
300,711
533
1.77
North
375,971
415
1.10
Central
373,605
920
2.48
East
392,444
1,597
4.06
South
502,548
4,001
7.95
1,948,369
7,466
3.83
Total
Deaths
to each 1,000
inhabitants
Deaths from Cholera in London; Registered from
September 23, 1848 to August 25, 1849
The London Cholera Epidemic of
1848-1849
• Limitations of ecologic study:
- Uncertainties about districts’ water supplies
-
some residences used water pumps
some districts served by more than one company
- Districts may vary by socioeconomic (SES)
factors
- Don’t know if cholera cases actually drank
contaminated water
The London Cholera Epidemic of
1848-1849
• Most of southern districts were served by
two water companies, both using sewagecontaminated water from Thames River.
- Southwark and Vauxhall Water Company
- Lambeth Water Company
In 1852, Lambeth Water Company
moved its intake to an uncontaminated
part of the Thames River
Cholera death rate in London
according to water company
supplying district of residence
(August 1853 – January 1854)
Water Company
Southwark & Vauxhall and
Kent
Deaths from cholera in
100,000
107
Sourthwark & Vauxhall
94
Lambeth and Southwark &
Vauxhall
61
Cholera death rate in London
according to water company
supplying sub-district of residence
(August 1853 – January 1854)
Water
company
Population Deaths from Deaths in
in 1851
cholera
100,000
Southwark & 167,654
Vauxhall
12 sub-districts
192
114
Both
301,149
companies
16 sub-districts
182
60
0
0
Lambeth
3 sub-districts
14,632
• Ecologic study of districts and subdistricts could not rule out:
- confounding by socio-economic (SES)
factors.
- Exposure misclassification (e.g., public
wells)
• Ecologic study could not link cases with
exposure. (Cases may not have drunk
contaminated water.)
The natural experiment:
16 sub-districts served by both water
companies, with pipes from both
companies going down every street,
serving residences in a virtually random
fashion.
The natural experiment:
individual-level study
The natural experiment:
“shoe-leather epidemiology”
• Home of each case was visited to determine
source of water to the residence:
- next-of-kin recall found to be inadequate
- confirmation by water bill receipt or by water
sample.
• Reports from water companies provided
denominator information (# houses supplied in
each district)
The natural experiment
• The preceding table included:
- districts served by both companies in an
intermingled fashion (i.e., the natural
experiment) and
- districts served by only one of the
companies.
- the relative risk = 315/ 37 = 8.5
The natural experiment
• Because Snow combined both districts with
and without intermingled supplies,
confounding bias by SES factors was still
possible:
- the districts served only by Lambeth Co. were
wealthier than those served only by Southwark &
Vauxhall.
• When Snow later focused only on the
districts with intermingled supplies, the
relative risk fell from 8.5 to 6.9.
The Broad Street Pump
Outbreak
INSTANCES OF THE COMMUNICATION OF CHOLERA THROUGH THE
MEDIUM OF POLLUTED WATER IN THE NEIGHBORHOOD OF BROAD
STREET, GOLDEN SQUARE
(August 31 – September 9)
The most terrible outbreak of cholera which ever occurred in this
kingdom, is probably that which took place in Broad Street, Golden
Square, and the adjoining streets, a few weeks ago. Within two hundred
and fifty yards of the spot where Cambridge Street joins Broad Street,
there were upwards of five hundred fatal attacks of cholera in ten days.
Source: Snow J. On the Mode of Communication of Cholera, p 38.
The Broad Street Pump
Outbreak
• Snow began the investigation with:
- a hypothesis from his previous work
-
sewage-contaminated drinking water is a
cause of cholera
- a working knowledge of the cluster area
(he resided nearby).
The Broad Street Pump Outbreak
• Piped water not contaminated by sewage.
• Water samples on 9/3 from pumps near Golden
Sq. found that all except the Broad St. pump
had visible impurities.
• However, local residents reported foul smells
from the Broad St. pump water the day before
the pump samples were taken.
• Days later, small flocculent particles were
detected in Broad St. sample.
The Broad Street Pump Outbreak:
difficulties of exposure assessment
• Variability in sampling results over time
-
sampling a day earlier may have found visible
contamination in Broad St pump water
• Uncertainties about relevant contaminant
- visible particles seen immediately?
- flocculent particles seen days later?
• High or unknown detection limits
• Lack of information on contaminant sources
(e.g.,local cesspool near pump)
The Broad Street Pump Outbreak:
case series evaluation
• 61 of 73 cases drank water from the Broad
St. pump; 6/73 did not; 6/73 unknown.
• 2 cases living far from the area used water
from the Broad St. pump.
• No outbreak among those in the outbreak
area who did not use Broad St. pump
- 5 deaths among 535 inmates
- 0 deaths among 70 brewery workers
Cholera outbreak in Golden Square,
Broad Street, London 1854
Lessons from Snow’s
Investigations
• Based on review of what was known about
cholera, Snow formed a hypothesis:
- sewage in drinking water causes cholera
• Snow tested hypothesis using available data
to conduct an ecologic study comparing
cholera rates in districts and subdistricts
Lessons from Snow’s
Investigations
• Snow recognized the limitations of the
ecologic study:
- could not directly link individual cases to exposure
to contaminated drinking water
- alternative explanation for findings: confounding
by SES factors
Lessons from Snow’s
Investigations
• Snow identified a study area and
study population that:
- provided an excellent exposure contrast
(high vs no exposure)
- minimized confounding by SES factors
-
districts with intermingled supplies of high and
low contamination serving homes in a virtual
random fashion - a natural experiment
Lessons from Snow’s
Investigations
• shoe-leather epidemiology approach
to case verification & exposure
assessment:
- visiting the homes of the cholera cases
- obtaining verification of source of water for
each case residence (receipt, water
sample)
Lessons from Snow’s
Investigations
• collected denominator data from water
companies in order to calculate disease
rates among exposed and unexposed.
Lessons from Snow’s
Investigations
• Explored alternative explanations:
- focused on districts with intermingled supplies to
minimize confounding by SES factors
- in the cluster investigation, Snow identified:
-
-
brewery workers and inmates unexposed to the
contaminated water but worked in cluster area in
order to rule out air pathway of exposure
high SES cholera cases living far from the cluster
area whose only link to the cluster was drinking from
Broad St pump
Lessons from Snow’s
Investigations
• Snow found an association between
exposure and disease and used this
information for prevention
• Snow utilized tables and maps to present a
strong case for a causal relationship between
sewage-contaminated drinking water and
cholera
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