Download John Snow

Epidemiology Kept Simple
Chapter 1
Epidemiology Past & Present
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Epidemiology Defined
• Greek roots
epi = upon (as in “epidermis”)
demos = the people (as in “democracy”)
ology = “to speak of”, “to study”
• Modern definitions of epidemiology refer to
–Distributions of
–Health determinants
–Disease and other health-related outcomes
–Populations
–Application in control of health problems
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Public Health Defined
• Definition of public health
– organized effort
– Intended to reduce of morbidity & mortality and
improve health
• Core disciplines public health (CEPH / ASPH)
– Epidemiology
– Biostatistics
– Health administration
– Behavioral sciences
– Environmental health sciences
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Epi compared to public health
• Epidemiology 
“study of”
• Public health 
“organized effort”
Epi = the
“methodological
backbone” of public
health
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Basic Terms
•
•
•
•
Morbidity = related to disease or disability
Mortality = related to death
Endemic = normal occurrence of a condition
Epidemic = much greater than normal
occurrence of a condition
• Pandemic = an epidemic on multiple continents
• Incidence = rate or risk of developing a condition
• Prevalence = proportion of population with a
condition
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Uses of Epi (Morris 1957)
1.
2.
3.
4.
5.
6.
7.
Historical study
Community diagnosis
Working of health services
Individual chances
Completing the clinical picture
Identify new syndromes
Search for causes (paramount for prevention)
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Demographic Transition
↓ mortality
↓ fertility
↑ aging  change in
shape of pop’n pyramid
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Epidemiologic Transition
shift from acute contagious diseases
to
chronic lifestyle diseases
Leading Causes of Death
1900
1990
1 Pneumonia / influenza Heart disease
2 Tuberculosis
3 Diarrhea
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Neoplasms
Cerebrovascular
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Reasons for Epi.Transition
During the first half of 20th century:
• Improved standard of living
• Vaccination
• Improved nutrition
• Sanitation and vector control
During the second half of 20th century
• Birth control
• Improvements in lifestyle
• Medical technology (e.g., antibiotics, treatment of hypertension,
improved anesthetic safety)
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Causes of Death, U.S., 1950–1990
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Mortality Trends of Selected Cancers
U.S., 1940 - 1995
• Respiratory and
prostate increased
• Colorectal &
stomach declined
• Breast cancer
about the same
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Life Expectancy Trends
Dramatic increases
all groups
Rank
White women
Black women
White males
Black men
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Selected Historical Figures &
Events
• Hippocrates (400BCE)
• Age of scientific enlightenment (17th & 18th
centuries)
• John Graunt (1620 – 1674)
• Pierre Charles Louis (1787 – 1872)
• John Snow (1813 – 1858)
• Germ Theory (mid 19th century)
• Modern epidemiology (post WWII)
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Enlightenment
The birth of modern medicine
and public health must be
studied in the context of the
Western Enlightenment (pp.
11–12).
Barzun, J. (2001). From
Dawn to Decadence: 500
Years of Western Cultural
Life: New York: HarperCollins.
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Demographic Approach
John Graunt
(1620–74)
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17th Century Life Table
Age
% surviving
6
64
16
40
26
25
36
16
46
10
56
6
60
3
76
1
80
0
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Lesson from Graunt
(Rothman, 1996)
• Was brief
• Made reasoning clear
• Subjected theories to multiple and varied
tests
• Invited criticism
• Willing to change ideas when confronted
with contradictory evidence
• Avoided simplistic interpretations of data
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Germ Theory (Highlights)
• Until the 19th century, germ theory played
second fiddle to vague theories of pollution (e.g.,
miasma theory)
• Examples of early contagionists
– Fracastoro (16th century Italian)
– Henle & Koch (German physiologists)
– John Snow (epidemiologist’s hero)
– Pasteur (1865 experimental proof in
silkworms)
– Daniel Salmon (vector borne transmission)
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John Snow, Our Hero
Snow’s cholera theory:
• Epidemics follow routes of
commerce
• Agent is free-living & multiplies
within the host
• Transmission is water-borne,
spread via fecal contamination,
ingested orally
• Patho-physiology: diarrhea 
fluid loss  smudging of blood
 asphyxiation  death
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John Snow
(1813–1858)
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Snow’s Methods
• Ecological comparisons:
compared cholera rates by region
• Cohort comparisons: compared
cholera rates in exposed and nonexposed households
• Case-control comparisons:
compared exposure to
contaminated water in cases and
non-cases
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Ecological
Comparisons
• Southwark Water
Company  high
neighborhoods rates
• Mixed service 
intermediate rates
• Lambeth Water Co.
neighborhoods 
no cases
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Cohort Comparisons
Water
Source
Southwark
Lambeth
Both
Cases
1263
98
1422
Homes
40,046
26,107
256,423
Rate per
10,000
315
37
59
Example of calculation:
Southwark rate = 1263 / 40,046 = .0315 = 315 / 10,000
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Snow’s
map
quasi
casecontrol
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Snow’s Quasi Case-Control Analysis
• Cases more likely to be exposed to Broad St.
pump water than non-cases (“controls”)
• Among cases:
– 61 used Broad St. water, 6 did not, and 6 were
uncertain
• Among controls: Broad St. water use was rare
– e.g., at the Brewery (no cases) “the men
…were allowed a certain quantity of malt
liquor, and … do not drink water at all”
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Post-WWII (Modern)
Epidemiology
• Epidemiologic transition  shift in focus
from acute infectious diseases to chronic
life style diseases
• Exemplar studies
– The British Doctors Study
– The Framingham Heart Study
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