Transcript STAT 6395

Spring , 2008
STAT 6395
Special Topic in Statistics:
Epidemiology
Filardo and Ng, 2008
I. Epidemiology
The study of the distribution and determinants of healthrelated states or events in specified populations and the
translation of study results to control health problems
Distribution
• Persons affected
• Place
• Time
Epidemiology: The study of the distribution and determinants of health-related
states or events in specified populations and the translation of study results to
control of health problems
Determinants
All the physical, biological, social, cultural, and behavioral
factors that influence health
Epidemiology: The study of the distribution and determinants of health-related
states or events in specified populations and the translation of study results to
control of health problems
Health-related states or events
•
•
•
•
•
•
•
•
•
•
Diseases
Mortality (death)
Specific causes of death
Injuries
Disability
Health-related behaviors
Physiological measurements
Results of preventive regimens
Clinical outcomes
Provision and use of health services
Epidemiology: The study of the distribution and determinants of health-related
states or events in specified populations and the translation of study results to
control of health problems
Specified populations
• Residents of a defined geographic area
• Students who attend a specified school
• Persons who belong to a specified organization
• Workers at a specified workplace
Epidemiology: The study of the distribution and determinants of health-related
states or events in specified populations and the translation of study results to
control of health problems
Translation
Study Results
Scientific articles and presentations at meetings
Clinical guidelines
Prevention programs
Quality of care improvement programs
Patient safety programs
Epidemiology: The study of the distribution and determinants of health-related
states or events in specified populations and the translation of study results to
control of health problems
Control
Operations or programs aimed at reducing the
adverse impact of the disease on the community
1)
2)
3)
Prevention
Cure
Management
Epidemiology: The study of the distribution and determinants of health-related
states or events in specified populations and the translation of study results to
control of health problems
Primary prevention
An action taken to prevent the development
of a disease in a person who is well and does
not have the disease in question
Operations or programs aimed at reducing the adverse impact of the disease on
the community: 1) Prevention; 2) Cure; 3) Management
Secondary prevention
(Cure and management)
The identification and treatment of people
who have already developed a disease or
precursors of the disease, through screening,
at an early enough stage in the disease’s
natural history (early detection) such that
intervention will be more effective than if the
disease had been discovered later
Operations or programs aimed at reducing the adverse impact of the disease on
the community: 1) Prevention; 2) Cure; 3) Management
II. Objectives of Epidemiology
• To describe the extent of disease in the community
• To identify risk factors (factors that influence a person’s risk of acquiring a
disease) for disease and the etiology or cause of disease
• To study the natural history (course from onset to resolution) and
prognosis of disease
• To evaluate both existing and new preventive and therapeutic measures
(including health care delivery)
• To provide the foundation for developing public policy and regulatory
decisions relating to environmental problems
…bottom line, Epidemiology research
requires a multidisciplinary effort and
statisticians play a key role in:
• Hypothesis development
• Study execution
Hypothesis development
Epidemiologists study the specific distribution and
determinants of specific diseases
Development of hypotheses to test in an
epidemiologic study requires in-depth knowledge of
the disease and determinants under study
• Medical and biological sciences
• Social and behavioral sciences
Epidemiology Research  hypothesis development and study execution
Study execution
• Statistics ------------>Biostatistics
• Medical and biological sciences
• Social and behavioral sciences
Epidemiology Research  hypothesis development and study execution
Biostatistics
• Separate chance observations from
meaningful observations
• Sampling
• Sophisticated statistical analyses
Epidemiology Research  Biostatistics, medical and biological sciences, social
and behavioral sciences
Medical and biological sciences
• Microbiology, e.g., to identify infectious agents
• Clinical medicine and pathology, (e.g., to identify
cases of disease)
• Molecular biology, (e.g., to identify genotype of
individuals)
• Biochemistry, (e.g., to measure serum hormone
levels)
Epidemiology Research  Biostatistics, medical and biological sciences, social
and behavioral sciences
Social and behavioral science
• Design questionnaires for obtaining valid
information
• Design effective interventions for lifestyle
changes
Epidemiology Research  Biostatistics, medical and biological sciences, social
and behavioral sciences
Epidemiology Sub-disciplines
• Disease-specific subject matter
• Exposure-specific subject matter
• Health services research
Disease-specific subject matter
• Infectious disease epidemiology
• HIV/AIDS epidemiology
• Malaria epidemiology
• Chronic disease epidemiology
• Cancer epidemiology
• Cardiovascular epidemiology
• Perinatal epidemiology
• Neuroepidemiology
• Psychiatric epidemiology
Epidemiology disciplines  Disease-specific subject matter, Exposure-specific
subject matter, Health services research
Determinant (exposure)-specific subject
matter
•
•
•
•
•
•
Environmental epidemiology
Occupational epidemiology
Pyschosocial epidemiology
Genetic epidemiology
Nutritional epidemiology
Pharmacoepidemiology
Epidemiology disciplines  Disease-specific subject matter, Exposure-specific
subject matter, Health services research
Health services research
• Operations
research: the study of the
placement of health services in the community
and the optimum utilization of such services
• Program evaluation
• Clinical epidemiology or outcomes research:
the study of illness outcomes in persons seen
by providers of health care; evaluation of
medical treatments
Epidemiology disciplines  Disease-specific subject matter, Exposure-specific
subject matter, Health services research
Types of epidemiologic studies
• Experimental
• Observational (this course will focus on
this second type of studies)
Experimental Studies
• Studies in which conditions are under the control of
the investigator(s).
• The investigators assigns subjects to different
study groups. The effect of the treatment is
determined by comparing the outcome of interest
in these groups.
Type of studies  Experimental, Observational
Experimental studies (examples)
• Randomized clinical trial (unit of study is the
individual)
• Community trial (unit of study is the community)
Type of studies  Experimental, Observational
Observational studies
• Studies in which the investigators does not control
conditions, but rather observe nature taking its
course by gathering information, recording,
classifying, counting, and analyzing the collected
data.
• Exposure and disease outcomes would have
occurred whether or not the studies have been
performed because there were no ‘a priori’
intervention(s) on the part of the investigators.
Type of studies  Experimental, Observational
Observational studies (examples)
• Descriptive
• Analytic
Type of studies  Experimental, Observational
Descriptive studies
Studies aimed at describing the distribution of disease
or other health-related variables with respect to person
(age, gender, race, socioeconomic status), place
(census tract, county, state, country, urban/rural), and
time (season, year)
Type of studies  Experimental, Observational (descriptive)
Descriptive studies
• Often use routinely-collected data
• Can define high-risk groups
• Can be used for hypothesis generation, but
generally not hypothesis testing
Type of studies  Experimental, Observational (descriptive)
Analytic studies
• Test specific etiologic hypotheses
• To generate new etiologic hypotheses
• To suggest mechanisms of causation
• To generate preventive hypotheses
• To suggest or identify potential methods for
disease prevention
In these studies, the epidemiologist observes the relationship between an exposure
and a disease or other health outcome.
Type of studies  Experimental, Observational (analytic)
Definition: Exposure
A potential causal agent or characteristic, such as
infectious agent, behavior, dietary factor,
medication, medical treatment, genetic makeup,
environmental agent, or physiologic state (e.g.,
serum level of a hormone or nutrient; blood
pressure).
An exposure may be harmful or beneficial
Type of studies  Observational  Analytic
Types of analytic studies
• Cohort studies
• Case-control studies
Type of studies  Observational
Cohort studies
A study in which a group of
persons exposed to a factor
of interest and a group of
persons not exposed are
followed
and compared with respect
to the incidence rate of the
disease or other condition of
interest
Time
Type of studies  Observational  Cohort studies
Cohort studies (study schema)
Type of studies  Observational  Cohort studies
Case-Control studies
Studies in which a group of persons with a disease
(cases) and a comparison group of persons without
the disease (controls) are compared with respect to
the history of past exposures to factors of interest
Past
Type of studies  Observational  Case-Control
Present
Case-Control studies (study schema)
Type of studies  Observational  Case-Control
Time
Either descriptive or analytic studies
• Cross-sectional studies
• Ecologic studies
Type of studies  Observational
Cross-sectional studies
Studies of the distribution of exposures
and/or disease in a defined population
at one given point in time
Type of studies  Observational  Cross-sectional
Ecologic studies
Studies of the association between exposures and
disease in which the units of analysis are
populations or groups of people, rather than
individuals.
This involves the assessment of the correlation of
exposure rates and disease rates among different
groups or populations.
Causality, though???
Type of studies  Observational  Ecologic
Types of Epidemiologic studies
Example involving several
types of study designs
Ecologic study results
Case-Control study
results
Cohort study results
Randomized controlled trial
results
Which type of study is the ‘Gold Standard’
and/or more common/feasible?
Experimental
Observational
controlled setting
(difficult to reproduce in real life)
real life setting
Efficacy
Effectiveness
Focus of this course is on
observational Epidemiologic research
(research regarding the direct study of disease in human populations)
Some triumphs of observational
Epidemiology
• Smoking causes lung cancer
• Identification of cardiovascular disease risk factors
• Characterization of how HIV spreads through a
population
• Identification of occupational hazards (e.g., asbestos)
Three ‘Eras’ of Epidemiology
• Sanitary (1800-1875)
• Infectious disease (1875-1950)
• Chronic disease (1950-present)
Sanitary ‘Era’ (1800-1875)
• Miasma theory of disease – poisoning by foul emanations (miasma) from the
soil, water, and air.
• Created national vital statistics systems: much valuable descriptive
epidemiology
• Demonstrated clustering of disease in slums and among the poor
• Solutions – sewage systems, drainage, clean water supplies, garbage
collection, decent housing
• Incorrect miasma theory, but solutions were a major contribution to public health
Lesson:
prevention doesn’t necessarily require understanding of cause
Sanitary, Infectious disease, Chronic disease
Infectious disease ‘Era’ (1875-1950)
• Germ theory: single microscopic agents relate one-toone to specific diseases
• Epidemiology took a back seat to laboratory science,
although in the1920s-30s, the germ theory was
broadened to accommodate the interactive roles of
host (immune and nutritional status), environment, and
agent in infectious disease
• Other epidemiologic contributions:
– Occupational exposures as causes of cancer
– Specific vitamin deficiencies as causes of disease
Sanitary, Infectious disease, Chronic disease
Chronic Disease (Modern) ‘Era’
• By end of World War II, infectious diseases were under
control in the developed countries; coronary heart
disease and lung cancer were epidemic
• Multifactor causation of chronic disease
• Focus on identification and control of risk factors at the
individual level
• Black box approach: emphasis on risk factor
identification with only a secondary concern about
mechanism or pathogenesis
Most of the methodology we will cover in this course
was developed during this ‘Modern Era’.
Sanitary, Infectious disease, Chronic disease
New ‘Era’(???)
• Emerging infectious diseases (e.g., HIV)
• Continued burden of infectious diseases in majority of world
• Traditional chronic disease epidemiology has hit a wall in its ability to
discover important new risk factors
• Advances in molecular biology and genetics allow the study of
pathogenesis and causality at the molecular and genetic levels using
epidemiologic approaches
• The need to be concerned with causal pathways at multiple levels,
including the societal level, as opposed to an exclusive focus on risk
factors at the individual level, has become apparent to many
Sanitary, Infectious disease, Chronic disease, ???
Levels of causality
• Societal or population
• Individual
• Biochemical
• Cellular
• Molecular
What causes lung cancer?
 Individual/societal:
Cigarette
smoking
(nicotine
addiction)
 Biochemical: Specific chemicals in cigarette smoke
that cause the mutations
 Cellular: Specific phenotypic changes in the cells
that result in loss of growth control
 Molecular: Mutations in DNA
What causes AIDS?
 Societal:
Poverty
Prostitution
 Individual/societal: a. Multiple sexual partners;
b. Intravenous drug use
X Biochemical:
 Cellular: Infection -the HIV viruses progressively
destroy lymphocytes (a types of white blood cells)
 Molecular: Mutations in DNA -the viral DNA is
incorporated into the DNA of the infected lymphocyte
On the mode of communication of cholera
John Snow, M.D.
London, 1855
John Snow
1857
Why study Snow?
• Appreciate those who came before us and paved
the way
• Brilliant piece of work: lucid and thorough
• Snow’s work on cholera illustrates a key
epidemiologic principle:
…the most important information to have about any
communicable disease is its mode of communication
John Snow (1813-1858)
• Physician
• Pioneer
in
both
anesthesiology
epidemiology
and
• Experiments in administration of anesthesia
himself may have contributed to his early
death
Snow administered
chloroform to
Queen Victoria
for the birth of
Prince Leopold and
Princess Beatrice
Cholera: Acute Gastrointestinal Disease
• Incubation period: 12-72 hours
• Sudden onset of severe vomiting
• Followed shortly by voluminous, watery, non-bloody
diarrhea, described as rice water stool (white and
opalescent)
• Abdominal cramps
• Severe cases: severe dehydration, circulatory
collapse, renal failure (death may occur within a few
hours of first symptoms)
• Case fatality rate may range from 1-50%, depending
on strain of Vibrio cholerae and treatment
In 1817,
four years after John Snow’s birth,
cholera emerged from the Indian
subcontinent, where it had existed for
centuries, to spread across the world.
Cholera Epidemics in Great Britain
• 1831-32
– 56,000 deaths
• 1848-49
– 125,000 deaths
• 1853-54
– Cholera returns to England
Cholera: Apparently contradictory facts in
1854
• Local spread with evidence of direct communication
from person to person
• Failure to spread to many in close contact with the
sick
• Cases occur without traceable relation to prior cases
• Highest rates in low-lying areas and in filthy
environments
- Exceptions too numerous to be disregarded
• Unpredictability of its spread around the world and
its geographic distribution
Cholera: Miasma Theory
Report of the London General Board of Health on
cholera epidemic of 1848-49:
“…it appears as if some organic matter, which
constitutes the essence of the epidemic, when
brought in contact with other organic matter
proceeding
from
living
bodies,
or
from
decomposition, has the power of so changing the
condition of the latter as to impress it with poisonous
qualities of a peculiar kind similar to its own.”
Cholera: Elaborations of Miasma theory
• Localizing influences
• Predisposition
• Spontaneous generation of “cholera poison”
• “Poison” spread by diffusion through
the
atmosphere vs. poison attached itself to solid bodies
• “Poison” communicated by an effluvium (contagion)
given off by the sick
William
Farr
Farr’s elaboration of Miasma theory
• Soil at low elevations, especially near the banks of the
Thames River, contained much organic matter that
produced deadly miasmata.
• Miasmata diffused through the atmosphere in a cloud or
mist
• Concentration of miasmata would be greater at lower
elevations than in higher elevations, accounting for the
geographic distribution in the London epidemic of 1849.
Farr’s theory did have some consistency with the facts.
John Snow’s Germ Theory
• Cholera caused by a germ cell, not yet identified
• 2 main modes of transmission of germ cell, which was
found in the evacuations of cholera victims:
1. Drinking water contaminated with sewage
2. Contaminated food, bedding, or clothing
Snow was firmly convinced of his theory by start of
1853-54 epidemic.
Snow’s ecologic observations prior to the
1853-54 epidemic
Cholera “has never appeared except where there
has been ample opportunity for it to be conveyed by
human connections.”
• Epidemics of cholera followed major routes of
commerce and warfare.
• Cholera always appeared first at seaports, when
extending to a new island or continent.
Snow’s observations from ‘case’ histories
• Cholera can be communicated from the sick to the
healthy.
• Persons attending those with cholera do not necessarily
become afflicted.
• Close contact with a cholera patient is not necessary to
become afflicted.
Snow’s conclusion:
“…cholera is communicated from person to person,
but not through the air”
When cholera returned to London in
August 1853, Snow had a definite
hypothesis:
cholera was spread by
contaminated water.
Water supply of south districts of London
Lambeth Water Company
• Until 1852, drew water from the Thames River in
London, contaminated with London’s sewage
• In 1852, moved intake 22 miles up river and far from the
contaminated water
Southwark and Vauxhall Water Company
• Continued to draw water from the contaminated Thames
Water
Company
1849
Epidemic
1853-54
Epidemic
Lambeth
Contaminated
(London)
Pure
(Up River)
Southwark &
Vauxhall
Contaminated
(London)
Contaminated
(London)
Cholera Deaths in South Districts
of London
Deaths/100,000
Water Company 1849
S&V, Lambeth
1290
Epidemic
1853
61
S&V
1420
94
S&V, Kent
2050
107
Cholera deaths (per 100,000) in south
districts of London
District
Water supply
1849
1853
Bermondsey
S&V
1610
150
St. Saviour
S&V, Lam
1530
146
St. George
S&V, Lam
1640
143
St. Olave
S&V
1810
134
Rotherhithe
S&V, Kent
2050
112
Newington
S&V, Lam
1440
57
Wandsworth
S&V, others
1000
51
Camberwell
S&V, Lam
970
40
Lambeth
Lam, S&V
1200
34
Notes: 1. Lambeth was supplied mostly by Lambeth Water Co.
2. Rotherhithe supplied partly by Kent in 1853
Cholera Deaths in south districts of
London, 1853, sub-district analysis
Water
Supply
S&V
Lambeth
Both
Cholera
Deaths
192
Deaths/
100,000
116
14,632
0
0
301,149
182
60
Population
167,654
How many cholera deaths would we expect in
sub-districts supplied by Lambeth if they had the
same death rate as those supplied by S&V?
Cholera Deaths in south districts of
London, 1853, sub-district analysis
Water
Supply
Population
S&V
167,654
Lambeth
14,632
Both
301,149
Cholera
Deaths
192
0
182
Deaths/
100,000
116
0
60
Expected deaths for Lambeth = (116/100,000)*14,632 = 16
Additional Observations on Sub-districts:
• Among the sub-districts supplied by both companies,
those supplied mainly by Lambeth had a low death rate,
while those supplied mainly by S&V had a high death
rate.
• Two sub-districts supplied only by S&V also contained a
number of pump-wells. These sub-districts had a low
death rate.
By the return of cholera in July 1854, Snow recognized the full
significance of the intermixing of the water supplies of the 2
companies
“…in the sub-districts … supplied by both Companies,
the mixing of the supply is of the most intimate kind… A
few houses are supplied by one Company and a few by
the other, according to the decision of the owner or
occupier at that time when the Water Companies were
in active competition …”
By the return of cholera in July 1854, Snow recognized
the full significance of the intermixing of the water
supplies of the 2 companies
“…in many cases a single house has a supply different
from that on either side. Each company supplies both
rich and poor, both large houses and small; there is no
difference either in the condition or occupation of the
persons receiving the water of the different Companies.”
An Experiment on the Grandest Scale
“…no fewer than three hundred thousand people of both
sexes, of every age and occupation, and of every rank
and station, from gentlefolks down to the very poor,
were divided into two groups without their choice …”
An Experiment on the Grandest Scale
“…one group being supplied with water containing the
sewage of London,
…the other group having water quite free from such
impurity”
What type of study John Snow conducted?
What type of study John Snow conducted?
Cohort Study
• A study in which a group of persons exposed to a factor
of interest and a group of persons not exposed are
followed and compared with respect to the incidence
rate of the disease or other condition of interest.
• Exposed group: persons using S&V water supply
• Comparison group: persons using Lambeth water
supply
Compared cholera mortality rates in the two groups
Calculation of mortality rates required
numerators and denominators
Mortality rate in exposed group
=
(Cholera deaths among persons supplied with S&V water) / (Number of persons supplied with S&V water)
Mortality rate in comparison group
=
(Cholera deaths among persons supplied with Lambeth water) / (Number of persons supplied with Lambeth water)
To determine numerators and denominators,
Snow needed a way to classify each death and every
person in the population by water supply
The Numerators
• For each cholera death in the relevant districts, Snow
obtained information on the water supply
• “The inquiry was necessarily attended with a good
deal of trouble.”
• Chemical test problematic because S&V water had 40
times more NaCl than Lambeth
The Denominators
“ …a return had been made to Parliament of the entire
number of houses supplied with water by each of the Water
Companies, but ... the number of houses which they supplied
in particular districts was not stated…”
• Therefore Snow had to include all the south districts of
London in his study, not just the districts where the
water supply was intermingled.
A daunting undertaking,
so Snow obtained an assistant (Mr. Whiting)
Cholera deaths in south London districts during first 4 weeks of
1854 Epidemic, by water supply
Water
Supply
S&V
Lambeth
Cholera
Deaths
286
Houses
40,046
14
26,107
Deaths/
10,000
Houses
71.4
5.4
Relative risk = 71.4/5.4 ≈ 13.2
Thames
22
Pump-wells
4
Ditches
4
Unknown
4
London
277
287,345
9.6
(-S&V)
Note: Houses, not persons, used in denominator
More data …
“ …as the epidemic advanced, the disproportion between the
number of cases in houses supplied by the Southwark and
Vauxhall Company and those supplied by the Lambeth
Company, became not quite so great, although it continued
very striking…”
• Cholera likely was imported from Baltic Fleet to
Thames River, which was initially the primary source
of the epidemic
• Later, cholera was also spread by other means,
diluting the water company effect
Cholera deaths in south London districts during first 7 weeks of
1854 Epidemic, by water supply
Water
Supply
S&V
Lambeth
Cholera
Deaths
Houses
Deaths/
10,000
Houses
1263
40,046
315
98
26,107
37
Relative risk = 315/37 ≈ 8.5
London
1422
(-S&V, Lambeth)
256,423
59
Cholera deaths in south London districts during last 7 weeks of
1854 Epidemic, by water supply
Water
Supply
S&V
Lambeth
Cholera
Deaths
Houses
Deaths/
10,000
Houses
2353
40,046
573
302
26,107
115
Relative risk = 573/115 ≈ 5
Cholera deaths in south London districts during the 1854
epidemic, by water supply
Water
Supply
S&V
Lambeth
Cholera
Deaths
4,093
461
Deaths/
Population 10,000
266,516
153
173,748
26
Relative risk = 153/26 ≈ 5.9
London
10,367
2,362,236
43
Note:
populations supplied by water companies estimated by
Registrar General.
Was the Use of Houses in the Denominators
Valid?
Water
Supply
Persons/
Household
Population
Houses
S&V
266,516
40,046
6.7
Lambeth
173,748
26,107
6.7
2,362,236
322,576
7.3
London
Cholera Deaths in South Districts of London,
Sub-district Analysis, 1849 vs. 1854
Water
Supply
Cholera Deaths
1849
1854
S&V
2261
2458
Both
3905
2547
Lambeth
1644
89
Here we are back to ecologic analysis
Cholera Outbreak in the Golden Square Area of London, 8/31 - 9/9, 1854.
Within 250 yards of the intersection of Cambridge and Broad Streets, there
were more than 500 fatal cases
Snow immediately suspected contamination of the water of the
much-used street pump on Broad Street near Cambridge Street.
Snow mapped the places of residence of
cholera decedents from August 31 September
2
in
the
broader
neighborhood…
…and found that 73 of 83 deaths had
taken place within a short distance of the
pump.
Snow investigated the water source of the 73
decedents who lived near Broad Street pump
• 61 -- drank water from the pump
• 6 -- did not
• 6 -- could get no information
10 Deaths in houses located nearer to
another street pump
• 5 always used the Broad Street pump,
as they preferred its water
• 3 (children) went to school near the
Broad Street pump
Handle of the pump was removed on
September 8
• Legend has it that the removal of the pump handle
caused the end of the epidemic
• Snow himself wondered whether removing the pump
handle had a beneficial effect
– Epidemic was already subsiding
– Much of the population in the neighborhood had fled
“…it is impossible to decide whether the water from the
pump still contained the cholera poison in an active state.”
Snow’s investigation of Golden Square
Outbreak: workhouse near Broad St.
• Surrounded by houses in which deaths from cholera occurred
• Only 5 deaths among 535 inmates
• Workhouse had a pump-well on the premises
• Also received water from the Grand Junction Water Works
• Did not use Broad Street pump
• Would have expected more than 100 deaths based on mortality in
surrounding streets
Snow’s investigation of Golden Square
Outbreak: Brewery on Broad St.
• Located near the pump
• More than 70 workers
• None died of cholera
• Workers drank malt liquor, not water
• Deep well located in brewery
• Workers never obtained water from Broad Street Pump
Snow’s recommendations for prevention of
cholera during an epidemic
• Observe strictest cleanliness around the sick.
• Wash linens of patients as soon as they are removed.
• Boil water for drinking and preparing food (unless known to
come from clean source).
• Wash or heat to 212°F all food.
• Healthy should not live in same room as sick.
• Pit-men should work 4 hour shifts, and not eat in mines.
• Educate the people about communicability of cholera.
Snow’s recommendations for long-term
prevention of cholera
• Effect good and perfect drainage.
• Provide water supply free from contamination with
contents of sewers, cesspools, house-drains, and refuse
of people who navigate the rivers.
• Provide model lodging-houses for the vagrant class and
sufficient house room for the poor in general.
• Teach habits of personal and domestic cleanliness
among the people.
• Screen persons arriving from infected places.
Medical establishment slow to catch on
• 1855 report of Scientific Committee for Scientific
Enquiries in Relation to the Cholera Epidemic of 1854:
“…on the whole of evidence, it seems impossible to
doubt that the influences, which determine in mass the
geographical distribution of cholera in London, belong
less to the water than to the air.”
• 1856 Report on the last two cholera epidemics of London
as affected by the consumption of impure water: “…under
the specific influence which determines an epidemic
period, fecalized drinking-water and fecalized air equally
may breed and convey the poison.”
Sanitary ‘movement’ eventually succeeded in
spite of its incorrect miasma theory
• Extensive improvements in several of London’s water
supplies, including Southwark and Vauxhall, had already
been ordered before the 1853-54 epidemic.
• In next London epidemic (1866), William Farr himself
used epidemiology to show that the source of the
epidemic was impure water from the East London Water
Company.
What We Know Now?
• Cholera caused by a bacterium (Vibrio cholerae;
discovered in 1883 by Robert Koch)
• Small intestine is primary site of infection
• Diarrhea caused by cholera toxin produced by Vibrio
cholerae
• Treatment: intravenous or oral fluid and electrolytes,
depending on severity of illness
• Environmental reservoir for Vibrio cholerae in the sea,
where it lives on zooplankton and shellfish
Vibrio Cholerae
2005