![Page 1: Epidemiologic Triads & Natural History of Disease Dr. Salwa A. Tayel & Dr. Mohammad Afzal Mahmood KSU Department of Family & Community Medicine September,](https://reader030.vdocuments.us/reader030/viewer/2022032600/56649db05503460f94a9e224/html5/thumbnails/1.jpg)
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
1
![Page 2: Epidemiologic Triads & Natural History of Disease Dr. Salwa A. Tayel & Dr. Mohammad Afzal Mahmood KSU Department of Family & Community Medicine September,](https://reader030.vdocuments.us/reader030/viewer/2022032600/56649db05503460f94a9e224/html5/thumbnails/2.jpg)
2
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
April 19, 2023 2Triads & Natural History
![Page 3: Epidemiologic Triads & Natural History of Disease Dr. Salwa A. Tayel & Dr. Mohammad Afzal Mahmood KSU Department of Family & Community Medicine September,](https://reader030.vdocuments.us/reader030/viewer/2022032600/56649db05503460f94a9e224/html5/thumbnails/3.jpg)
Concepts
• Descriptive epidemiological triad• Analytical epidemiological triad• Natural history of disease• Spectrum of disease• Public health implications
April 19, 2023 Triads & Natural History 3
![Page 4: Epidemiologic Triads & Natural History of Disease Dr. Salwa A. Tayel & Dr. Mohammad Afzal Mahmood KSU Department of Family & Community Medicine September,](https://reader030.vdocuments.us/reader030/viewer/2022032600/56649db05503460f94a9e224/html5/thumbnails/4.jpg)
44
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
April 19, 2023 Triads & Natural History
![Page 5: Epidemiologic Triads & Natural History of Disease Dr. Salwa A. Tayel & Dr. Mohammad Afzal Mahmood KSU Department of Family & Community Medicine September,](https://reader030.vdocuments.us/reader030/viewer/2022032600/56649db05503460f94a9e224/html5/thumbnails/5.jpg)
Epidemiological Triads
Descriptive Epidemiology Triad:
• Person• Place • Time
Analytical Epidemiology Triad:
• Agent• Host• Environment
April 19, 2023 Triads & Natural History 5
![Page 6: Epidemiologic Triads & Natural History of Disease Dr. Salwa A. Tayel & Dr. Mohammad Afzal Mahmood KSU Department of Family & Community Medicine September,](https://reader030.vdocuments.us/reader030/viewer/2022032600/56649db05503460f94a9e224/html5/thumbnails/6.jpg)
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
April 19, 2023 Triads & Natural History 6
![Page 7: Epidemiologic Triads & Natural History of Disease Dr. Salwa A. Tayel & Dr. Mohammad Afzal Mahmood KSU Department of Family & Community Medicine September,](https://reader030.vdocuments.us/reader030/viewer/2022032600/56649db05503460f94a9e224/html5/thumbnails/7.jpg)
Person
• Age • Gender • Marital status• Ethnicity/Race • Behavior / life-style factors• Socio-economic status
– Education– Occupation– Income
• Biological factors, passive/active immunity, concomitant illness
April 19, 2023 Triads & Natural History 7
![Page 8: Epidemiologic Triads & Natural History of Disease Dr. Salwa A. Tayel & Dr. Mohammad Afzal Mahmood KSU Department of Family & Community Medicine September,](https://reader030.vdocuments.us/reader030/viewer/2022032600/56649db05503460f94a9e224/html5/thumbnails/8.jpg)
Place
• Geographically restricted or widespread (pandemic)?
• Relation to water or food supply (clusters: multiple / one)
• Residence (rural, urban, remote)• Climate (temperature, humidity)
April 19, 2023 Triads & Natural History 8
![Page 9: Epidemiologic Triads & Natural History of Disease Dr. Salwa A. Tayel & Dr. Mohammad Afzal Mahmood KSU Department of Family & Community Medicine September,](https://reader030.vdocuments.us/reader030/viewer/2022032600/56649db05503460f94a9e224/html5/thumbnails/9.jpg)
Time
• Changing or stable?
• Seasonal variation.
• Clustered (epidemic) or evenly distributed (endemic)?
• Point source or propagated.
April 19, 2023 Triads & Natural History 9
![Page 10: Epidemiologic Triads & Natural History of Disease Dr. Salwa A. Tayel & Dr. Mohammad Afzal Mahmood KSU Department of Family & Community Medicine September,](https://reader030.vdocuments.us/reader030/viewer/2022032600/56649db05503460f94a9e224/html5/thumbnails/10.jpg)
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
April 19, 2023 Triads & Natural History 10
![Page 11: Epidemiologic Triads & Natural History of Disease Dr. Salwa A. Tayel & Dr. Mohammad Afzal Mahmood KSU Department of Family & Community Medicine September,](https://reader030.vdocuments.us/reader030/viewer/2022032600/56649db05503460f94a9e224/html5/thumbnails/11.jpg)
The Basic Triad Of Analytic Epidemiology
THE THREE PHENOMENA ASSESSED IN ANALYTIC EPIDEMIOLOGY ARE:
HOST
ENVIRONMENTAGENT
![Page 12: Epidemiologic Triads & Natural History of Disease Dr. Salwa A. Tayel & Dr. Mohammad Afzal Mahmood KSU Department of Family & Community Medicine September,](https://reader030.vdocuments.us/reader030/viewer/2022032600/56649db05503460f94a9e224/html5/thumbnails/12.jpg)
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
April 19, 2023 12Triads & Natural History
![Page 13: Epidemiologic Triads & Natural History of Disease Dr. Salwa A. Tayel & Dr. Mohammad Afzal Mahmood KSU Department of Family & Community Medicine September,](https://reader030.vdocuments.us/reader030/viewer/2022032600/56649db05503460f94a9e224/html5/thumbnails/13.jpg)
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
April 19, 2023 Triads & Natural History 13
![Page 14: Epidemiologic Triads & Natural History of Disease Dr. Salwa A. Tayel & Dr. Mohammad Afzal Mahmood KSU Department of Family & Community Medicine September,](https://reader030.vdocuments.us/reader030/viewer/2022032600/56649db05503460f94a9e224/html5/thumbnails/14.jpg)
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.
April 19, 2023 Triads & Natural History 14
![Page 15: Epidemiologic Triads & Natural History of Disease Dr. Salwa A. Tayel & Dr. Mohammad Afzal Mahmood KSU Department of Family & Community Medicine September,](https://reader030.vdocuments.us/reader030/viewer/2022032600/56649db05503460f94a9e224/html5/thumbnails/15.jpg)
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
April 19, 2023 15Triads & Natural History
![Page 16: Epidemiologic Triads & Natural History of Disease Dr. Salwa A. Tayel & Dr. Mohammad Afzal Mahmood KSU Department of Family & Community Medicine September,](https://reader030.vdocuments.us/reader030/viewer/2022032600/56649db05503460f94a9e224/html5/thumbnails/16.jpg)
1616
•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
April 19, 2023 16Triads & Natural History
![Page 17: Epidemiologic Triads & Natural History of Disease Dr. Salwa A. Tayel & Dr. Mohammad Afzal Mahmood KSU Department of Family & Community Medicine September,](https://reader030.vdocuments.us/reader030/viewer/2022032600/56649db05503460f94a9e224/html5/thumbnails/17.jpg)
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
April 19, 2023 17Triads & Natural History
![Page 18: Epidemiologic Triads & Natural History of Disease Dr. Salwa A. Tayel & Dr. Mohammad Afzal Mahmood KSU Department of Family & Community Medicine September,](https://reader030.vdocuments.us/reader030/viewer/2022032600/56649db05503460f94a9e224/html5/thumbnails/18.jpg)
1818
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
April 19, 2023 18Triads & Natural History
![Page 19: Epidemiologic Triads & Natural History of Disease Dr. Salwa A. Tayel & Dr. Mohammad Afzal Mahmood KSU Department of Family & Community Medicine September,](https://reader030.vdocuments.us/reader030/viewer/2022032600/56649db05503460f94a9e224/html5/thumbnails/19.jpg)
Natural History of Disease
![Page 20: Epidemiologic Triads & Natural History of Disease Dr. Salwa A. Tayel & Dr. Mohammad Afzal Mahmood KSU Department of Family & Community Medicine September,](https://reader030.vdocuments.us/reader030/viewer/2022032600/56649db05503460f94a9e224/html5/thumbnails/20.jpg)
20202020
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.
April 19, 2023 20Triads & Natural History
![Page 21: Epidemiologic Triads & Natural History of Disease Dr. Salwa A. Tayel & Dr. Mohammad Afzal Mahmood KSU Department of Family & Community Medicine September,](https://reader030.vdocuments.us/reader030/viewer/2022032600/56649db05503460f94a9e224/html5/thumbnails/21.jpg)
April 19, 2023 Triads & Natural History
• 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
21
![Page 22: Epidemiologic Triads & Natural History of Disease Dr. Salwa A. Tayel & Dr. Mohammad Afzal Mahmood KSU Department of Family & Community Medicine September,](https://reader030.vdocuments.us/reader030/viewer/2022032600/56649db05503460f94a9e224/html5/thumbnails/22.jpg)
22222222
Natural history of disease
April 19, 2023 22Triads & Natural History
![Page 23: Epidemiologic Triads & Natural History of Disease Dr. Salwa A. Tayel & Dr. Mohammad Afzal Mahmood KSU Department of Family & Community Medicine September,](https://reader030.vdocuments.us/reader030/viewer/2022032600/56649db05503460f94a9e224/html5/thumbnails/23.jpg)
2323
Natural history of disease
![Page 24: Epidemiologic Triads & Natural History of Disease Dr. Salwa A. Tayel & Dr. Mohammad Afzal Mahmood KSU Department of Family & Community Medicine September,](https://reader030.vdocuments.us/reader030/viewer/2022032600/56649db05503460f94a9e224/html5/thumbnails/24.jpg)
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
April 19, 2023 Triads & Natural History 24
![Page 25: Epidemiologic Triads & Natural History of Disease Dr. Salwa A. Tayel & Dr. Mohammad Afzal Mahmood KSU Department of Family & Community Medicine September,](https://reader030.vdocuments.us/reader030/viewer/2022032600/56649db05503460f94a9e224/html5/thumbnails/25.jpg)
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.
April 19, 2023 25Triads & Natural History
![Page 26: Epidemiologic Triads & Natural History of Disease Dr. Salwa A. Tayel & Dr. Mohammad Afzal Mahmood KSU Department of Family & Community Medicine September,](https://reader030.vdocuments.us/reader030/viewer/2022032600/56649db05503460f94a9e224/html5/thumbnails/26.jpg)
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.
April 19, 2023 26Triads & Natural History
![Page 27: Epidemiologic Triads & Natural History of Disease Dr. Salwa A. Tayel & Dr. Mohammad Afzal Mahmood KSU Department of Family & Community Medicine September,](https://reader030.vdocuments.us/reader030/viewer/2022032600/56649db05503460f94a9e224/html5/thumbnails/27.jpg)
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
27Triads & Natural History