Post on 02-Mar-2020
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Introduction To Epidemiology
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Lectures 1&2
EVALUATION
First Midterm 20%
Second Midterm 25% 50%
Class participation/assignments 5 %
Final Exam 50%
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Endemic: a disease or pathogen present or usually prevalent
in a given population or geographic region at all times
Hyperendemic: equally endemic in all age groups of a
population
Holoendemic: endemic in most of the children in a population,
with the adults in the same population being less often
affected
Epidemic: a disease occuring suddenly in numbers far
exceeding those attributable to endemic disease; occuring
suddenly in numbers clearly in access of normal expectancy
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TERMINOLOGY
Pandemic: a widespread epidemic distributed or occuring
widely throughout a region, country, continent, or globally
Epizootic: of, or related to a rapidly spreading and widely
diffused disease affecting large numbers of animals in a given
region
Incidence: rate of occurrence of an event; number of new cases
of disease occuring over a specified period of time; may be
expressed per a known population size
Prevalence: number of cases of disease occurring within a
population at any one given point in time
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TERMS ASSOCIATED WITH DISEASE CAUSATION, ETC.
Host
Agent
Environment
Fomites
Vector
Carrier – active
Incubatory
Convalescent
Healthy
Intermittent
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What is epidemiology? What is an epidemiologist?
Greek (English)
epi (among)
demos (people)
logy (study)
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“Epidemiology . . .
is a Greek word that means to put
people to sleep with charts and
graphs.”
- Dr. Mark Johnson
In testimony before the
House Judiciary Committee
on the spread of HIV
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DEFINITIONS OF EPIDEMIOLOGY
Oxford English Dictionary
THE BRANCH OF MEDICAL SCIENCE WHICH TREATS OF
EPIDEMICS
Kuller LH: Am J Epid 1991
EPIDEMIOLOGY IS THE STUDY OF "EPIDEMICS" AND THEIR
PREVENTION
Anderson G: Modern Epidemiology
THE STUDY OF THE OCCURRENCE OF ILLNESS
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DEFINITIONS OF EPIDEMIOLOGY
Lilienfeld A: in Foundations of Epidemiology
THE STUDY OF THE DISTRIBUTION OF A DISEASE OR A
PHYSIOLOGICAL CONDITION IN HUMAN POPULATIONS AND
OF THE FACTORS THAT INFLUENCE THIS DISTRIBUTION.
Last JM: A Dictionary of Epidemiology
The study of the distribution and determinants of
health and disease related states in populations, and the
application of this study to control health problems.
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What is Epidemiology? EPIDEMIOLOGY
is the study of the nature, cause, control and determinants of the frequency
and distribution of disease, disability, and death in human populations.
Epidemiology:
is the study of factors influencing the occurrence, transmission,
distribution, prevention and control of disease in a defined population
“epidemiology’s full value is achieved only when its
contributions are placed in the context of public health action,
resulting in a healthier populace.” (Koplan et al. 1999)
“the product of [epidemiology] is research and information
and not public health action and implementation”.
(Atwood et al. 1997)
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MEASURING DISEASE FREQUENCY
HAS SEVERAL COMPONENTS
Classifying and categorizing disease
Deciding what constitutes a case of disease in a study
Finding a source for ascertaining the cases
Defining the population at risk of disease
Defining the period of time of risk of disease
Obtaining permission to study people
Making measurements of disease frequency
Relating cases to population and time at risk
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EPIDEMIOLOGIST
AN EPIDEMIOLOGIST IS A PUBLIC HEALTH SCIENTIST, WHO IS
RESPONSIBLE FOR CARRYING OUT ALL USEFUL AND EFFECTIVE
ACTIVITIES NEEDED FOR SUCCESSFUL EPIDEMIOLOGY
PRACTICE
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WHAT IS THE UNIQUE SKILL OF EPIDEMIOLOGISTS?
MEASURING DISEASE
FREQUENCY IN
POPULATIONS
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Epidemiologists are required to have some knowledge of the disciplines of public health, clinical medicine, pathophysiology, statistics, and the social sciences.
• Public health, because of the emphasis on disease prevention.
• Clinical medicine, because of the emphasis on disease classification and diagnosis.
• Pathophysiology, because of the need to understand basic biological mechanisms in disease.
• Statistics, because of the need to quantify disease frequency and its relationships to antecedents.
• Social sciences, because of the need to understand the social context in which disease occurs and presents.
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Purpose of Epidemiology
The ultimate purpose of Epidemiology is prevention of
diseases and promotion of health
1. Identify causes and risk factors for disease.
2. Determine the extent حجم / رقعة of disease in the
community.
3. Study natural history and prognosisتشخيص of disease.
4. Evaluate preventive and therapeutic measures
5. Provide foundation for public policy سياسة عامة
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Uses of Epidemiology
To study the cause (or etiology) of disease(s), or conditions, disorders,
disabilities ……… etc.
determine the primary agent responsible or ascertain تأكد من
causative factors
determine the characteristics of the agent or causative factors
define the mode of transmission
determine contributing factors
identify and determine geographic patterns 30
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Uses of Epidemiology
To determine, describe, and report on the natural course
of disease, disability, injury, and death.
To aidلمساعدة in the planning and development of health
services and programs
To provide administrative and planning data
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Example: Use of Epidemiology
In the United States, the National Center for Health
Statistics is a data source for information on health and
disease (Centers for Disease Control
&prevention)
http://www.cdc.gov/nchs
Globally, the World Health Organization is a data
source for information on health and disease
http://www.who.int 32
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LEADING CAUSES OF DEATH, USA, 2000
(number of deaths)
1. Heart Disease
2. Neoplasms (a new and abnormal growth of tissue in a part of the
body, especially as a characteristic of cancer.
3. Cerebrovascular Disease
4. Chronic Pulmonary Disease
5. Accidents/Injuries
6. Diabetes mellitus
7. Influenza and pneumonia
8. Alzheimer’s Disease
9. Nephritis, nephrotic syndrome
10. Septicemia
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PAKISTAN: LEADING CAUSES OF PREMATURE DEATH, 1990
1. Diarrhea
2.
3.
Lower Respiratory Infections - child
4. Rheumatic heart disease
5. Chronic liver disease
6. Congenital malformations
7. Birth Diseases
8. Ischemic heart disease
9. Child Septicemia
10. Injuries
Tuberculosis
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LEADING CAUSES OF DEATH WORLDWIDE, 2000
(Based on number of global deaths)
1. Ischemic Heart Disease
2. Cerebrovascular Disease
3. Lower Respiratory Infections
4. HIV/AIDS
5. COPD (Chronic obstructive pulmonary disease)
6. Perinatal Conditions
7. Diarrhoeal Diseases
8. Tuberculosis
9. Road Traffic Injuries
10. Lung Cancers
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Basic features of Epidemiology
1. Studies are conducted on human population
2. It examines patterns of events in people
3. Can establish cause-effect relationship without the
knowledge of biological mechanism
4. It covers a wide range of conditions
5. It is an advancing science
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Forms of Epidemiology
• Clinical Epidemiology
• Descriptive Epidemiology
• Predictive Epidemiology
• Etiologic Epidemiology
• Genetic Epidemiology
• Occupational Epidemiology
• Social Epidemiology
• Spatial Epidemiology
etc…
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What?
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EPIDEMIOLOGIC APPROACHES (TYPES)
DESCRIPTIVE Health and disease in the community
What? How many? Who? When? Where?
What are the
health problems
of the
community?
What are the
attributes of
these illnesses?
How many people
are affected?
Over what
period of time?
Where do the
affected people
live, work or
spend leisure
time?
ANALYTIC Etiology, prognosis and program evaluation
Why? How?
What are the
causal agents?
What factors
affect outcome?
By what mechanism
do they operate?
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What are the
attributes of
affected persons?
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TWO BROAD TYPES OF EPIDEMIOLOGY
Examining the distribution of a disease
in a population, and observing the basic
features of its distribution in terms of
time, place, and person.
Typical study design:
community health survey (approximate
synonyms - cross-sectional study,
descriptive study)
Testing a specific hypothesis about
the relationship of a disease to a
putative cause, by conducting an
epidemiologic study that relates the
exposure of interest to the disease of
interest.
Typical study designs: cohort, case-
control
DESCRIPTIVE EPIDEMIOLOGY ANALYTIC EPIDEMIOLOGY
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THE BASIC TRIAD OF DESCRIPTIVE EPIDEMIOLOGY
THE THREE ESSENTIAL CHARACTERISTICS OF DISEASE WE LOOK FOR
IN DESCRIPTIVE EPIDEMIOLOGY:
Think of this as the standard dimensions used to track the occurrence of
a disease.
In Descriptive Epidemiology:
Who? - person
Where? - place
When? - time
TIME PLACE
PERSON
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TIME WHEN does the disease occur?
• Temporal
• Range from hours to decades
Changing or stable?
Time since an event
Calendar Time
Age (time since birth)
Characteristics Relating to Time
Secular change (long-term)
Point epidemics (short-term)
Cyclic trends
Seasonal variation
Graphic format often used
• y-axis (vertical) - frequency
• x-axis (horizontal) - time
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SECULAR CHANGE (LONG-TERM)
Incidence Rates of Cancers in Women Incidence Rates of Cancers in Men 44
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POINT EPIDEMICS (SHORT-TERM)
• Short-term changes occur over limited time frames
Hours
Days
Weeks
Months
• Used for short-term exposures or diseases with
short incubation and/or illness durations
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CYCLIC TRENDS
• Cyclic trends may be either long-term or short term events.
• Some are “seasonal” while others are cyclic due to other
factors:
School year
Military deployment
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Cyclic Trends
Migratory Birds?
Seasonal Variation
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PLACE WHERE are the rates higher? or lower?
Geographically restricted or widespread (pandemic)?
Geographic location of reservoir
Relation to water or food supply.
Multiple clusters or one?
presence or agents or vectors
climate
geology
population density
economic development
nutritional practices
medical practices
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Dr. John Snow and Cholera
CHARACTERISTICS RELATING TO PLACE
International
Variation within countries
• Urban-rural
• Local
Building Maps
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PERSON
WHO is getting the disease?
Many variables are involved and studied, but factors such as
sex, age & race often have a major effect.
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CHARACTERISTICS OF PERSON
Age
Sex
Ethnic group
Socio-economic status
Gender
Concurrent disease مرض متزامن
Diet, exercise, smoking
Nativity (place of birth)
Religion
Marital status
Behavior
Occupation 50
Age
Sex
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EXAMPLE
You have been asked to investigate an event in
which 2,220 people were exposed and 1,520 of
them died.
Your role as an epidemiologist is to ask
questions about person, place and time.
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HOW DO WE ASK QUESTIONS?
SURVEYS
- OF SURVIVORS
- OF NEXT-OF-KIN ذوى القربى
- OF OTHER RELATED PERSONS
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WITH QUESTIONS YOU LEARN
THAT ...
Person: Men, women and children were all exposed and
at risk. The majority of people who died were
wealthy ثري and young men between 18-50 years
(when compared to survivors).
Place: All those exposed were within 1 block of one
another, the climate was cold.
Time: Mid April, people died within hours of the
precipitating exposure.
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DESCRIPTIVE EPIDEMIOLOGY IS A NECESSARY ANTECEDENT OF ANALYTIC
EPIDEMIOLOGY
To undertake an analytic epidemiologic study you
must first:
Know where to look
Know what to control for
Be able to formulate hypotheses compatible with فرضيات
laboratory evidence
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A COMMON ERROR IN EPIDEMIOLOGY IS MOVING TO ANALYTIC EPIDEMIOLOGY
WITHOUT HAVING A SOLID BASE IN THE DESCRIPTIVE EPIDEMIOLOGY OF THE
CONDITION.
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Three essential characteristics that are examined
to study the cause(s) for disease in analytic
epidemiology are...
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Host Environment
Agent
The Basic Triad Of Analytic Epidemiology
AGENTS
Nutrients
Poisons
Allergens
Radiation
Physical trauma
Microbes
Psychological experiences
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• Necessary for disease to occur
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HOST FACTORS
Genetic endowment
Immunologic state
Age
Personal behavior
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ENVIRONMENT
External conditions
Physical or biologic or social
Crowding
Atmosphere
Modes of communication – phenomena in the
environment that bring host and agent together,
such as:
• Vector
• Vehicle
• Reservoir
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• Contribute to the disease process
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EPIDEMICS ARISE WHEN HOST,
AGENT, AND ENVIRONMENTAL
FACTORS ARE NOT IN BALANCE
• Due to new agent
• Due to change in existing agent (infectivity, pathogenicity,
virulence)
• Due to change in number of susceptibles in the population
• Due to environmental changes that affect transmission of
the agent or growth of the agent
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DIFFERENCES BETWEEN LABORATORY SCIENCES AND FIELD SCIENCES
In the Laboratory:
• Mostly experimental
• Variables controlled by the investigator
• All variables known
• Replication easy
• Results valid
• Meaning of results for humans uncertain.
• Little need for statistical manipulation of data.
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DIFFERENCES BETWEEN LABORATORY SCIENCES AND FIELD SCIENCES
In the Field:
• Mostly observational
• Variables controlled by nature
• Some variables unknown
• Replication difficult; exact replication impossible
• Results often uncertain
• Meaning of results for humans clear
• Statistical control often very important
Men Women Children Total
1st class 67% 3% 0 38%
2nd class 92% 14% 0 59%
3rd class 84% 54% 66% 62%
Total 82% 26% 48% 62%
EVERY HEALTH OUTCOME HAS SOME INTERESTING AND USEFUL EPIDEMIOLOGIC
CHARACTERISTIC
DEATH RATES BY SOCIAL CLASS FROM A CERTAIN CAUSE AMONG 1,316 PEOPLE
WHAT CAUSE OF DEATH IS THIS?
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THE PREVIOUS SLIDE SHOWS DEATH
RATES BY CLASS OF TICKET ON THE
TITANIC, A LARGE OCEAN LINER THAT
SANK AFTER COLLIDING WITH AN
ICEBERG IN 1912