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Chapter 7
Epidemiology in Community Health
Care
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Objectives:
Upon mastery of this chapter, you should be able to:
Explore the historical roots of epidemiology.
Explain the host, agent, and environment model.
Describe theories of causality in health and illness.
Explain a “web of causation” matrix that assists you
with recognition of multicausal factors in disease or
injury occurrences.
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Objectives:
Define immunity and compare passive immunity,
active immunity, cross-immunity, and herd
immunity.
Explain how epidemiologists determine populations
at risk.
Identify the four stages of a disease or health
condition.
List the major sources of epidemiologic
information.
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Introduction
In this unit, the focus is on epidemiology,
communicable disease control, and
environmental health and safety. These
are cornerstone areas of public health
practice and are just as important at the
local level as at the regional, national,
and international levels.
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Definition of health
The World Health Organization
(WHO) defines the health as “ a state
of complete well-being, physical,
social, and mental, and not the absence
of diseases …. 1988
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Definition of Epidemiology
Epidemiology :is the study of
distribution and determine of healthrelated states or events in specific
populations, and the application of this
study to control of health problems
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What is Epidemiology?
Science of Public Health that studies:
Distribution of disease
Determinants of health/disease
Specific populations
Look for patterns of disease
Time, place, personal characteristics
Interventions
Prevention is key
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What does Epidemiology study?
Health-related
Infectious disease
Chronic disease
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Historical Roots of
Epidemiology
Ancient Times:
Hippocrates, Greek physician 460 to 375 BC
Middle Ages:
1348: Plague (Black Death)
18th century:
Florence Nightingale (1820–1910)
19th century: modern epidemiology
sanitary statistics
infectious-disease epidemiology
chronic-disease epidemiology
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Epidemiologic Information
1) Case definition
2) Person
3) Place
4) Time
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1) Case Definition
Standard set of criteria
Clinical and lab
Allows for comparison
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Case Definition example :
Smallpox
Clinical Description
• An illness with acute onset of fever >101 °F followed by a rash
characterized by vesicles or firm pustules in the same stage of
development without other apparent cause.
Laboratory Criteria for Confirmation
• Isolation of smallpox (variola) virus from a
clinical specimen, or
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Case Definition Gradient
Low
Specificity
Suspected
Probable
High
Confirmed Specificity
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2) Person
Age
Sex
Race/Ethnicity
Socio-Economic Status
Behaviors
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3) Place
Geographic Distribution
Natural
Clustering vs. uniform
Home
Work
School
Hospital room
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Regional HIV and AIDS statistics and features, 2006
Adults & children
living with HIV
Sub-Saharan Africa
Middle East & North Africa
South and South-East Asia
East Asia
Latin America
Caribbean
Eastern Europe & Central
Asia
Western & Central Europe
North America
Oceania
TOTAL
24.7 million
[21.8 – 27.7 million]
Adults & children
newly infected with
HIV
2.8 million
[2.4 – 3.2 million]
460 000
68 000
[270 000 – 760 000]
[41 000 – 220 000]
7.8 million
860 000
[5.2 – 12.0 million]
[550 000 – 2.3 million]
Adult (15‒49)
prevalence [%]
5.9%
[5.2% –
6.7%]
0.2%
Adult & child
deaths due to
AIDS
2.1 million
[1.8 – 2.4 million]
36 000
[0.1% –
0.3%]
[20 000 – 60 000]
[0.4% –
1.0%]
[390 000 – 850 000]
0.6%
590 000
750 000
100 000
0.1%
43 000
[460 000 – 1.2 million]
[56 000 – 300 000]
[<0.2%]
[26 000 – 64 000]
1.7 million
140 000
[1.3 – 2.5 million]
[100 000 – 410 000]
250 000
27 000
[190 000 – 320 000]
[20 000 – 41 000]
1.7 million
270 000
[1.2 – 2.6 million]
[170 000 – 820 000]
740 000
22 000
[580 000 – 970 000]
[18 000 – 33 000]
1.4 million
43 000
[880 000 – 2.2 million]
[34 000 – 65 000]
81 000
7100
[50 000 – 170 000]
[ 3400 – 54 000]
39.5 million
4.3 million
[34.1 – 47.1 million]
[3.6 – 6.6 million]
0.5%
65 000
[0.4% –
1.2%]
[51 000 – 84 000]
[0.9% –
1.7%]
[14 000 – 25 000]
[0.6% –
1.4%]
[58 000 – 120 000]
[0.2% –
0.4%]
[ <15 000]
[0.6% –
1.1%]
[11 000 – 26 000]
[0.2% –
0.9%]
[2300 – 6600]
1.2%
0.9%
0.3%
0.8%
0.4%
1.0%
[0.9% 1.2%]
19 000
84 000
12 000
18 000
4000
2.9 million
[2.5 – 3.5 million]
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Geographic Distribution
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4) Time
Onset of symptoms
Incubation Period
Infectious Period
Seasonality
Baseline vs. epidemic
Interval
• Long-term trends
• Shorter for environmental exposure
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Mortality versus Morbidity rate
• Mortality rate : is a measure of the number
of deaths (in general, or due to a specific
cause) in some population, scaled to the size
of that population, per unit time.
* Morbidity rate : refers to the number of
individuals who have contracted a disease
during a given time period
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Examples :
*Mortality rate :
Ex. Number of deaths from TB is 39 per 10
100000 population in KSA.
*Morbidity rate :
Ex.: the number of HIV cases is 432 per
100000 population in South Africa.
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Epidemiologic Triangular:
Host, Agent, & Environment
1)Host
susceptible human or animal who harbors and
nourishes a disease-causing agent
2)Agent
a factor that causes or contributes to a health
problem or condition
3)Environment
all the external factors surrounding the host that
might influence vulnerability or resistance
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Epidemiologic Triangular
Agent
Host
Environment
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Epidemiologic Triangular
Agent
Host
Environment
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Theories of Causality in
Health and Illness
Relationship between a cause and its effect
1) Chain of Causation:
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Chain of Infection
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Web of Causation:
Infant Mortality
2) Multiple Causation:
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Transmission
a) Direct
Contact
Droplet
b) Indirect
Airborne
Vehicle
Vector
• Mechanical vs. biologic
Portal of Entry
Portal of Exit
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Definition & Types of
Immunity
A host’s ability to resist a particular infectious
disease–causing agent.
Acquired immunity: is the resistance acquired by a
host as a result of previous natural exposure to
infectious agent
Ex. : having a measles once protects against future
infection
* Acquired immunity may be induced by active or
passive immunity
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Types of immunization :
1- Active immunity:
refers to immunization of an individual
by administration of an antigen
(infectious agent or vaccine ) and usually
characterized by the presence of an
antibody produced by the individual host
*long-term immunization
Ex.: vaccination of the children against
measles .
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Types of immunization :
2- Passive immunity:
refers to immunization through the transfer
of specific antibody from an immunized
individual to no immunized individual .
* short-term immunization.
Ex .: used of immunoglobulin to hepatitis
patient .
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Populations at Risk
The risk of developing a health problem is
directly influenced by their biology,
environment, lifestyle, and system of health
care.
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Four Stages of a Disease or
Health Condition
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Major Sources of
Epidemiologic Information
Existing Data
Vital Statistics
Census Data
Reportable Diseases
Disease Registries
Environmental Monitoring
Informal Observational Studies
Scientific Studies
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Incidence rate
Incidence rate:
Number of new disease cases per population at risk
• High incidence implies high disease occurrence
• Low incidence implies low disease occurrence
Measured over a given time interval
Usage:
1-Determine probability of developing a specific disease
2-Used to detect etiologic factors
refers to all new cases of a disease or health
condition appearing during a given time.
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Incidence rate formula :
Incidence rate =
Number of persons developing a disease
Total number
x100%
Ex.: the number of people age 30-39 at risk at
10,000, since all the time new people are
entering the population at risk ( 100 cases )
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Summary
Case definition, person, time, place
Know disease/agent
Triangular epidemiology
Passive and active immunization
Incidence rate calculation
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