John Snow and Cholera

8/18/2019 John Snow and Cholera

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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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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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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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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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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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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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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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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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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!



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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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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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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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Investigations

• collected denominator data from water

companies in order to calculate diseaserates amon" e%posed and une%posed

f S


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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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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

John Snow and Cholera

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