Epidemiologic Triads &Epidemiologic Triads & Natural History of Disease Natural History of Disease
Dr. Salwa A. Tayel & Dr. Mohammad Afzal Mahmood
KSU Department of Family & Community MedicineSeptember, 2013
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By the end of this lecture students will be able to:
• Explain epidemiologic triads as a model of study of disease
causation
• Describe natural history and spectrum of infectious diseases
and their implications for public health.
OBJECTIVES OF THE LECTURE
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Concepts
• Descriptive epidemiological triad• Analytical epidemiological triad• Natural history of disease• Spectrum of disease• Public health implications
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Purpose of studying causal models
• Studying how different factors can lead to ill health
generates knowledge for disease prevention &
control
• The classic epidemiological triangle or triad help understand the relation between a disease, disease causing agent and environment
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Epidemiological Triads
Descriptive Epidemiology Triad:
• Person• Place • Time
Analytical Epidemiology Triad:
• Agent• Host• Environment
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Descriptive Epidemiology
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 laboratory evidence
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Person
• Age • Gender • Marital status• Ethnicity/Race • Behavior / life-style factors• Socio-economic status
– Education– Occupation– Income
• Biological factors, passive/active immunity, concomitant illness
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Place
• Geographically restricted or widespread (pandemic)?
• Relation to water or food supply (clusters: multiple / one)
• Residence (rural, urban, remote)• Climate (temperature, humidity)
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Time
• Changing or stable?
• Seasonal variation.
• Clustered (epidemic) or evenly distributed (endemic)?
• Point source or propagated.
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Time Trends
• Point source e.g. food-borne outbreaks), in terms of hours / days
• Seasonal - cyclicity (e.g. common cold, influenza), in terms of months
• Propogative (e.g. water borne epidemics), in terms of weeks / months
• Secular (e.g. morbidity / mortality of non-communicable diseases), in terms of years
• Cluster in time / place
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The Basic Triad Of Analytic Epidemiology
THE THREE PHENOMENA ASSESSED IN ANALYTIC EPIDEMIOLOGY ARE:
HOST
ENVIRONMENTAGENT
The Analytical Epidemiologic Triad The Analytical Epidemiologic Triad
This model comprises a susceptible host (the person at risk for the disease), a disease agent (the proximate cause), and an environmental context for the interaction between host and agent.
Thus, development of disease is a combination of events:• A harmful agent • A susceptible host • An appropriate environment
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AgentsAgents
• Biological (micro-organisms)
• Physical (temperature, radiation, trauma, others)
• Chemical (acids, alkalis, poisons, tobacco,
medications / drugs, others)
• Environmental (nutrients in diet, allergens, others)
• Nutritional (under- or over-nutrition)
• Psychological experiences
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Host Factors
• Host factors are intrinsic factors that influence an individual’s exposure, susceptibility, or response to a causative agent. These include:
• Genetic endowment • Immunologic state• Personal behavior (life-style factors): diet,
tobacco use, exercise, etc• Personal characteristics (described before, under
“person”), including: age, gender, socio-economic status, etc.
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Environmental factors are extrinsic factors which affect the agent
and the opportunity for exposure. These include:
– Physical factors: e.g. geology, climate (temperature,
humidity, rain, etc)
– Biological factors: e.g. insects that transmit an agent
– Socioeconomic factors: e.g. crowding, sanitation, and the
availability of health services
Phenomena which bring the host and agent together: vector,
vehicle, reservoir, etc
Environment
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•Agent factors include infectious microorganisms, e.g. virus,
bacterium, parasite, or other agents.
•They may be necessary but not always sufficient alone to
cause disease.
•Host factors are intrinsic factors that influence an individual’s
exposure, susceptibility, or response to a causative agent
•Environmental factors are extrinsic factors which affect the
agent and the opportunity for exposure.
Summary of Analytical Triad
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Example
The number of people who become diseased with tuberculosis will depend on:
• characteristics of the agent,• environmental factors,• And host factors
Explain some of these factors
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Agent:Amount, infectivity,
pathogenicity, virulence,….
Environment:Physical, biological, social
Host:Intrinsic factors, genetic, physiologic factors,
psychological factors, immunity
Health
or
Illness
?
The Analytical Epidemiology TriadThe Analytical Epidemiology Triad
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Natural History of Disease
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Natural history of disease
• Natural history of disease refers to the progress of a
disease process in an individual over time, in the
absence of intervention.
• The natural history of a disease describes the course
of the disease in an individual starting from the moment
of exposure to the causal agents till one of the possible
outcomes occurs.
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• Induction : time to disease initiation
• Incubation:– time to symptoms (infectious disease)
• Latency: time to detection (for non-infectious disease) or to infectiousness
Natural history Phenomena
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22222222
Natural history of disease
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Natural history of disease
Susceptible Host
Subclinical Disease
Clinical Disease
Outcome:
Stage of Recovery, Complications, Disability, or Death
Point of Exposure
Screening
Onset of symptoms
Diagnosis sought
Natural History of DiseaseDetectable subclinical disease
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The problem
• The problem is that we might know about disease onset when symptoms occur but most likely we will only know about the disease when a person seeks care for the symptoms.
• In some situations an investigator will only become aware of a case after a diagnosis is made.
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Importance of studying Importance of studying Natural history of diseaseNatural history of disease
• The understanding of this progression from disease onset to cure or death is important for epidemiologists.
• Natural history is as important as causal
understanding for the prevention and control of
disease.
• The earlier you can become aware of the attack
the more likely you will be able to intervene and
save lives.
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Reference books
• Principles of Epidemiology in Public Health Practice. Third Edition. An Introduction to Applied Epidemiology and Biostatistics. Centers for Disease Control and Prevention (CDC)
• Gordis L. Epidemiology. 2009
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