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So!rce of Information for the
John Snow E"ample
UCLA School of Public Health,
Department of EpidemiologyWebsite
http!!"""#ph#ucla#edu!epi!sno"#html
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#ritish cholera epidemics
• $%&$'$%&(
- first modern outbreak in Britain
- 23,000 deaths
- helped to launch the sanitary reform movement
• $%)%'$%)*
- 250,000 cases and 53,000 deaths
- prompted Snow and others! to investi"ate causes
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$he London Cholera Epidemic of
Septem%er 1848 & '!g!st 184(
• +ased on his clinical eperience and
re-ie" of epidemiologic
characteristics of cholera, Sno"formulated a theory of causation and
transmission of the disease#
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$he London Cholera Epidemic of
Septem%er 1848 & '!g!st 184(
• Sno".s hypotheses concerning cholera
- a gastrointestinal disease, therefore causal
agent "as li/ely ingested
- diarrhea as most prominent symptom,therefore causal agent li/ely left the body
by this route
- if cholera ecretions contaminated ri-ers
from "hich drin/ing "ater "as ta/en, thenthe disease could be "idely disseminated
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$he London Cholera Epidemic of
Septem%er 1848 & '!g!st 184(
Causal Hypothesis
Se"age'contaminated drin/ing
"ater "as a causal agent for the
cholera epidemic#
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$he London Cholera Epidemic of
1848-184(
• Sno" tested this hypothesis by first
conducting an 0ecolo"ic# study of London
districts using
- routine sur-eillance data on cholera cases- population data at the district le-el
- a-ailable information on "ater companies
ser-ing the districts
- a-ailable data on property -alues by district
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$he London Cholera Epidemic of
1848-184(
• Characteristics of an ecologic study
- unit of analysis is not the individual or case
- unit of analysis is a "eo"raphical unit such as a
district or nei"hborhood or city or county- useful to initially study a cause-effect relationship
when data are easily available and$or an individual
level study is not feasible
- does not directly link an individual case to ane%posure unless everyone in the "eo"raphical
unit has a similar e%posure!
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$he London Cholera Epidemic of
1848-184(
• Sno" did not ha-e eact information on
the sources of "ater for the districts#
• Ho"e-er, Sno" /ne" that- the East and South districts "ere ser-ed by
"ater supplies obtained from /no"n polluted
parts of the 1hames 2i-er
- the rest of London recei-ed drin/ing "ater
relati-ely uncontaminated by se"age#
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Districtsof
London
Population in 1841
Deaths fromCholera
Deathsto each 1,000inhabitants
West 300,711 533 1.77
orth 375,!71 415 1.10
Central 373,"05 !#0 2.48
$ast 3!#,444 1,5!7 4%0"
&outh 50#,548 4,001 7%!5
'otal 1,!48,3"! 7,4"" 3.83
Deaths from Cholera in London3 2egistered from
September (&, $%)% to August (4, $%)*
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$he London Cholera Epidemic of
1848-184(
• Limitations of ecologic study
- &ncertainties about districts' water supplies- some residences used water pumps
- some districts served by more than one company
- (istricts may vary by socioeconomic S)S!
factors
- (on't know if cholera cases actually drankcontaminated water
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$he London Cholera Epidemic of
1848-184(
• *ost of southern districts were served by
two water companies, both usin" sewa"e-
contaminated water from +hames iver
- Southwark and .au%hall /ater ompany
- 1ambeth /ater ompany
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In 1852, Lambeth Water Company
moved its intake to an uncontaminated
part of the Thames River
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holera death rate in 1ondon
accordin" to water company
supplyin" district of residence
u"ust 453 6anuary 457!
Water Compan( Deaths from cholera in100,000
&outh)ar* + au-hall and.ent 107
&ourth)ar* + au-hall !4
Lambeth and &outh)ar* +au-hall
"1
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Ch l d th t i L d
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Cholera death rate in London
according to "ater company
supplying sub'district of residence
5August $%4& 6 7anuary $%4)8
Watercompan(
Populationin 1851
Deaths fromcholera
Deaths in100,000
&outh)ar* +au-hall
1# sub/districts
1"7,"54 1!# 114
oth
companies
1" sub/districts
301,14! 18# "0
Lambeth
3 sub/districts
14,"3# 0 0
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• Ecologic study of districts and sub'
districts could not rule out
- confounding by socio'economic 5SES8factors#
- Eposure misclassification 5e#g#, public
"ells8
• Ecologic study could not lin/ cases "ith
eposure# 5Cases may not ha-e drun/contaminated "ater#8
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$he nat!ral e"periment)1* s!%-districts served %+ %oth water
companies, with pipes from %othcompanies going down ever+ street,
serving residences in a virt!all+ random
fashion
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$he nat!ral e"periment)
individ!al-level st!d+
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$he nat!ral e"periment)
shoe'leather epidemiology.
• Home of each case "as -isited to determine
source of "ater to the residence- net'of'/in recall found to be inade9uate
- confirmation by "ater bill receipt or by "atersample#
• 2eports from "ater companies pro-ided
denominator information 5: houses supplied in
each district8
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$he nat!ral e"periment
• +ecause Sno" combined both districts "ithand "ithout intermingled supplies,
confoundin" bias by S)S factors "as still
possible
- the districts ser-ed only by Lambeth Co# "ere"ealthier than those ser-ed only by South"ar/ =
>auhall#
• When Sno" later focused only on the
districts "ith intermingled supplies, therelati-e ris/ fell from %#4 to ?#*#
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$he #road Street /!mp
0!t%rea
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@S1ACES B 1HE CBU@CA1@B B CHBLE2A 1H2BUH 1HE
ED@U B PBLLU1ED WA1E2 @ 1HE E@H+B2HBBD B +2BADS12EE1, BLDE SFUA2E
5August &$ 6 September *8
+he most terrible outbreak of cholera which ever occurred in thiskin"dom, is probably that which took place in Broad Street, 8olden
S9uare, and the ad:oinin" streets, a few weeks a"o /ithin two hundred
and fifty yards of the spot where ambrid"e Street :oins Broad Street,
there were upwards of five hundred fatal attacks of cholera in ten days
Source; Snow 6
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$he #road Street /!mp
0!t%rea
• Sno" began the in-estigation "ith
- a hypothesis from his pre-ious "or/- se"age'contaminated drin/ing "ater is a
cause of cholera
- a "or/ing /no"ledge of the cluster area5he resided nearby8#
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$he #road Street /!mp 0!t%rea
• =iped water not contaminated by sewa"e• /ater samples on >$3 from pumps near 8olden
S9 found that all ecept the +road St# pump
had visible impurities
• ?owever, local residents reported foul smellsfrom the Broad St pump water the day before
the pump samples were taken
• (ays later, small flocculent particles weredetected in Broad St sample
$he #road Street /!mp 0!t%rea)
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$he #road Street /!mp 0!t%rea)
diffic!lties of e"pos!re assessment
• >ariability in sampling results o-er time- samplin" a day earlier may have found visible
contamination in Broad St pump water
• Uncertainties about rele-ant contaminant
- visible particles seen immediately@
- flocculent particles seen days later@
• High or un/no"n detection limits
• Lac/ of information on contaminant sources
e",local cesspool near pump!
$he #road Street /!mp 0!t%rea)
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$he #road Street /!mp 0!t%rea)
case series eval!ation
• ?$ of
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Cholera o!t%rea in 2olden S3!are,
#road Street, London 184
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Lessons from Snow’s
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Lessons from Snow s
Investigations
• Based on review of what was known about
cholera, Snow formed a hypothesis;
- sewa"e in drinkin" water causes cholera
• Snow tested hypothesis usin" available data
to conduct an ecolo"ic study comparin"
cholera rates in districts and subdistricts
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Lessons from Snow’s
Investigations
• Sno" identified a study area and study
population that
- provided an e%cellent e%posure contrast hi"h
vs no e%posure!
- minimiAed confoundin" by S)S factors- districts with intermin"led supplies of hi"h and low
contamination servin" homes in a virtual random
fashion - a natural e%periment
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Lessons from Snow’s
Investigations
• shoe'leather epidemiology approach
to case -erification = eposure
assessment
- visitin" the homes of the cholera cases
- obtainin" verification of source of water for
each case residence receipt, watersample!
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Lessons from Snow’s
Investigations
• collected denominator data from water
companies in order to calculate diseaserates amon" e%posed and une%posed
f S
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Lessons from Snow’s
Investigations
• Eplored alternati-e eplanations;
- focused on districts with intermin"led supplies to
minimiAe confoundin" by S)S factors
- in the cluster investi"ation, Snow identified;- brewery workers and inmates une%posed to the
contaminated water but worked in cluster area in order to
rule out air pathway of e%posure
- hi"h S)S cholera cases livin" far from the cluster area
whose only link to the cluster was drinkin" from Broad St
pump
L f S ’
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Lessons from Snow’s
Investigations
• Snow found an association between
e%posure and disease and used this
information for prevention
• Snow utiliAed tables and maps to present a
stron" case for a causal relationship between
sewa"e-contaminated drinkin" water andcholera