Download Epidemiology

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Chapter 1
Foundations of Epidemiology
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Objective:
Define epidemiology
 Epidemiology is the study of the distribution and
determinants of health-related states or events in
human populations, and the application of this study to
prevent and control health problems.
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Defining epidemiology
 The word epidemiology is based on the
Greek words
 epi—prefix meaning on, upon, or befall;
 demos—root meaning the people; and
 logos—suffix meaning the study
 In other words, epidemiology is the study of
what befalls the population
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Key terms in the definition
 Study - Epidemiology involves sound
methods of scientific investigation.
 Methods rely on careful observation
and the use of valid comparison
groups to determine whether the
observed health events differ from
what might be expected
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Key terms in the definition
 Distribution
Study of frequency and pattern of
health events in the population
 Frequency – number, and number in
relation to the population
 Pattern – the health-related state or
event by person, place, and time
characteristics

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Key terms in the definition
 Determinants

Search for causes and other
factors of health-related states or
events
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Key terms in the definition
 Health-related states or events
Disease states
 cholera, influenza, pneumonia, mental
illness)
 Conditions associated with health
physical activity, nutrition, environmental
poisoning, seat belt use, and provision
and use of health services
 Events
injury, drug abuse, and suicide

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Key terms in the definition
 Application of this study to prevent and
control health problems
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Objective:
Define descriptive epidemiology
 Answering the who, what, when, and
where questions is prerequisite to
effective education, screening,
prevention, and control programs
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The importance of descriptive
epidemiology
 Descriptive epidemiology involves
characterization of the distribution of
health-related states or events by

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Person - who
Place - where
Time – when
Clinical criteria - what
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Objective:
Define analytic epidemiology
 Analytic epidemiology involves
identifying and quantifying associations,
testing hypotheses, and identifying
causes of health-related states or events
 Explains why and how health-related
states or events occur
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Objective: Identify some activities
performed in epidemiology
 Identifying risk factors for disease, injury,
and death
 Describing the natural history of disease
 Identifying individuals and populations at
greatest risk for disease
 Identifying where the public health
problem is greatest
 Monitoring diseases and other healthrelated events over time
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Objective: Identify some activities
performed in epidemiology
 Evaluating the efficacy and effectiveness of
prevention and treatment programs
 Providing information useful in health planning
and decision making for establishing health
programs with appropriate priorities
 Assisting in carrying out public health programs
 Being a resource person
 Communicating public health information
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Objective: Explain the role of
epidemiology in public health practice
and individual decision making
 Epidemiological findings contribute to

preventing and controlling disease, injury,
disability, and death
 How?
By providing information leading to
informed public health policy and planning,
as well as individual health decision
making
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Epidemiologic information for influencing
public health policy and planning and
individual decisions
 Public health surveillance
 Causes of disease
 Completing the clinical picture
 Program evaluation


Efficacy
Effectiveness
See Table 1.1 of text
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Questions that need epidemiology
 Screening/Diagnosis
What is the sensitivity/specificity of a
test?
 Is prostate specific antigen a good test for
prostate cancer?
 Causes
 Why did this patient suffer a stroke?
 Is obesity the cause of metabolic
syndrome?

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Questions that need
epidemiology
 Treatment
Is this the best treatment for Parkinson's
disease?
 Is my surgery as good as that of everyone else?
 Prognosis
 What are the chances of a recurrent heart
attack?
 How long will this knee joint prosthesis last?

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Questions that need epidemiology
 Health promotion and protection
Do current school meals harm children's future health?
 Will the Irish smoking ban in public places work better than the
English policy?
 Health and disease surveillance
 Why are there 10 fold international differences in suicide rates?
 When will the next influenza pandemic occur?
 Health inequalities
 Why should life expectancy be nearly five years lower in
unskilled manual workers?
 Do health services reduce or increase health inequalities?

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Objective: Define epidemic, endemic,
and pandemic
 Epidemic – Health-related state or event
in a defined population above the
expected over a given period of time
 Endemic – Persistent, usual, expected
health-related state or event in a defined
population over a given period of time
 Pandemic – Epidemic affecting a large
number of people, many countries,
continents, or regions
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Objective: Describe common
source, propagated, and mixed
epidemics
 Common source

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Point
Intermittent
Continuous
 Propagated

Spread from person-to-person
 Mixed epidemics

A mixture of common source and mixed
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Common source
 Tend to result in more cases occurring
more rapidly and sooner than host-tohost epidemics
 Identifying and removing exposure to the
common source typically causes the
epidemic to rapidly decrease
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Common source
 Examples
anthrax, traced to milk or meat
from infected animals
 botulism, traced to soil-contaminated
food
 and cholera traced to fecal contamination
of food and water

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Propagated
 Arise from infections being transmitted from
one infected person to another
 Transmission can be through direct or indirect
routes
 Host-to-host epidemics rise and fall more
slowly than common source epidemics
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Propagated
 Examples
tuberculosis
 whooping cough
 Influenza
 measles

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Mixed epidemics
 Occurs when a common source
epidemic is followed by person-to-person
contact and the disease is spread as a
propagated outbreak
 Example – Shigellosis occurred among a
group of 3000 women attending a music
festival. Over the next few weeks,
subsequent generations of shigella
cases spread by person-to-person
transmission from festival attendees.
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Disease transmission
 Disease transmission usually occurs by
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direct, person-to-person contact (e.g.,
STDs)
fomite-borne (e.g., Hepatitis A spread by a
contaminated eating utensil)
vehicle-borne (e.g., HIV/AIDS spread
through needle sharing drug users)
vector-borne transmission (e.g., Malaria
spread through mosquitoes)
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Accurate assessment requires
a standard case definition
 A standard set of criteria, or case
definition, assures that cases are
consistently diagnosed, regardless of
where or when they were identified and
who diagnosed the case
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Objective: Define the concepts
and principles of case as used in
epidemiology
 A case is a person who has been
diagnosed as having a disease, disorder,
injury, or condition
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Primary case, index case
 The first disease case in the population
is the primary case.
 The first disease case brought to the
attention of the epidemiologist is the
index case.
 The index case is not always the primary
case.
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Secondary case
 Those persons who become infected
and ill after a disease has been
introduced into a population and who
become infected from contact with the
primary case
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Different levels of diagnosis
 Suspect

An individual who has all of the signs and
symptoms of a disease or condition, yet
not diagnosed
 Confirmed

All criteria met
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Different levels of diagnosis
 As more information (such as laboratory
results) becomes available to the
physician, he or she generally upgrades
the diagnosis. When all criteria are met
and they meet the case definition, the
case is classified as a confirmed case.
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Objective: Describe the epidemiology
triangle for infectious disease
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Triangle is based on the
communicable disease model
 Shows the interaction and interdependence of
agent, host, environment, and time as used in
the investigation of diseases and epidemics.
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Agent is the cause of the disease
Host is an organism, usually a human or an
animal, that harbors a disease
Environment includes those surroundings and
conditions external to the human or animal that
cause or allow disease transmission
Time accounts for incubation periods, life
expectancy of the host or the pathogen, and
duration of the course of the illness or condition.
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Stopping an epidemic
 An epidemic can be stopped when one
of the elements of the triangle is
interfered with, altered, changed, or
removed from existence, so that the
disease no longer continues along its
mode of transmission and routes of
infection
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Some disease transmission
concepts
 Fomites
Objects such as clothing, towels,
and utensils that may harbor a
disease agent and are capable of
transmitting it; usually used in the
plural
 Example – transmission of
cutaneous anthrax from drums to
an individual

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Vector
 An invertebrate animal (e.g., tick, mite,
mosquito, bloodsucking fly) capable of
transmitting an infectious agent among
vertebrates
 Can spread an infectious agent from an
infected animal or human to other
susceptible animals or humans through its
waste products, bite, body fluids, or
indirectly through food contamination
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Plague
 Yersinia pestis (the bacteria that cause plague) is
found in animals throughout certain parts of the
world, most commonly in rats, but occasionally in
other wild animals, such as prairie dogs. Plague
transmission from these infected animals
generally occurs in one of three ways:
 Bites from infected rodent fleas (85%)
 Direct contact with infected tissue or bodily fluids

For example, people can become directly infected with
plague by handling infected rodents, rabbits, or wild
carnivores that prey on these animals when plague
bacteria enter through the person's skin.
 Inhaling infected droplets.
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Reservoir
 The habitat (living or
nonliving) on which an
infectious agent lives,
grows, and multiplies
and is dependent on for
its survival in nature
 Humans often serve as
both reservoir and host
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Zoonosis
 When an animal transmits a disease to a human
 Examples – Rabies, Rocky Mountain
spotted fever, shigellosis
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Carrier
 A carrier contains, spreads, or harbors
an infectious organism
 Example – Typhoid Mary
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Modes of disease transmission
 Direct transmission – direct physical
contact such as touching with
contaminated hands, skin-to-skin
contact, kissing, or sexual intercourse
 Indirect transmission – occurs when
pathogens or agents are transferred or
carried by some intermediate item,
organism, means, or process to a
susceptible host, resulting in disease
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The chain of infection
 There is a close association between the
triangle of epidemiology and the chain of
infection
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Other modes of causation
 Infectious diseases are no longer the
leading cause of death in industrialized
nations so a more advanced model of
the triangle of epidemiology has been
proposed that better reflects the
behavior, lifestyle, and chronic disease
issues found in modern times
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Objective: Describe the advanced
epidemiology triangle for chronic diseases
and behavioral disorders
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Objective: Define the three levels of
prevention used in public health and
epidemiology
 Primary prevention (occurs prior to
exposure)
 Immunization
 Sanitation
 Education
 Media campaigns
 Warning labels
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Active primary prevention
 Requires behavior change on part of
subject
 Wearing protective devises
 Health promotion
 Lifestyle changes
 Community health education
 Ensuring healthy conditions at
home, school and workplace
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Passive primary prevention
 Does not require behavior change
Vitamin fortified foods
 Fluoridation of public water supplies

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Secondary prevention
 Occurs to reduce the progress of disease
 The disease already exists in the person

Cancer screening – cancer already
present. The goal is to detect the cancer
before clinical symptoms arise in order to
improve prognosis and prevent conditions
from progressing and from spreading
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Tertiary prevention
 To reduce the limitation of disability from
disease
 The disease has already occurred
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Physical therapy for stoke victims
Halfway houses for recovering alcoholics
Shelter homes for the developmentally
disabled
Fitness programs for heart attack patients
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Conclusion
 Epidemiology involves application of
scientific methods for describing the
frequency and pattern of health-related
states or events
 Epidemiology identifies causes of healthrelated states or events and modes of
transmission
 Epidemiology guides public health
planning and decision making
 Epidemiology assists individuals in
making informed health behavior
changes
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