Epidemiologic Triads & Natural History of Disease Dr. Salwa A. Tayel & Dr. Mohammad Afzal Mahmood KSU Department of Family & Community Medicine September, 2013

OBJECTIVES OF THE LECTURE 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. 19 April 2017 Triads & Natural History 2

Triads & Natural History Concepts Descriptive epidemiological triad Analytical epidemiological triad Natural history of disease Spectrum of disease Public health implications 19 April 2017 Triads & Natural History

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 4 19 April 2017 Triads & Natural History

Epidemiological Triads Descriptive Epidemiology Triad: Person Place Time Analytical Epidemiology Triad: Agent Host Environment 19 April 2017 Triads & Natural History

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 19 April 2017 Triads & Natural History

Triads & Natural History Person Age Gender Marital status Ethnicity/Race Behavior / life-style factors Socio-economic status Education Occupation Income Biological factors, passive/active immunity, concomitant illness 19 April 2017 Triads & Natural History

Triads & Natural History Place Geographically restricted or widespread (pandemic)? Relation to water or food supply (clusters: multiple / one) Residence (rural, urban, remote) Climate (temperature, humidity) 19 April 2017 Triads & Natural History

Triads & Natural History Time Changing or stable? Seasonal variation. Clustered (epidemic) or evenly distributed (endemic)? Point source or propagated. 19 April 2017 Triads & Natural History

Triads & Natural History 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 19 April 2017 Triads & Natural History

The Basic Triad Of Analytic Epidemiology THE THREE PHENOMENA ASSESSED IN ANALYTIC EPIDEMIOLOGY ARE: HOST Define host, agent? AGENT ENVIRONMENT

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 19 April 2017 Triads & Natural History

Triads & Natural History Agents 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 19 April 2017 Triads & Natural History 13

Triads & Natural History 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. 19 April 2017 Triads & Natural History

Triads & Natural History Environment 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 19 April 2017 Triads & Natural History

Triads & Natural History Summary of Analytical Triad 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. 16 19 April 2017 Triads & Natural History

Triads & Natural History 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 19 April 2017 Triads & Natural History

The Analytical Epidemiology Triad Host: Intrinsic factors, genetic, physiologic factors, psychological factors, immunity Health or Illness ? Agent: Amount, infectivity, pathogenicity, virulence,…. Environment: Physical, biological, social 18 19 April 2017 Triads & Natural History 18

Natural History of Disease

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. 20 20 19 April 2017 Triads & Natural History

Natural history Phenomena Induction : time to disease initiation Incubation:– time to symptoms (infectious disease) Latency: time to detection (for non- infectious disease) or to infectiousness Several important landmarks in natural history are the initiation of the disease itself (however we are defining it), the onset of symptoms, the point where a disease can be and/or is usually detected, and for communicable diseases, the point when the disease becomes transmissible. Epidemiology is still working on standardizing its terminology. The terms presented here are used by a major epidemiology textbook (Rothman and Greenland, Modern Epidemiology) to represent these four stages: Rothman applies the term induction period to the period of time between exposure to a causal agent and the initiation of the disease. Since the etiology of many diseases involves a combination of factors, Rothman considers each causal agent as having its own induction period. The beginning of actual disease, even if it can be defined, is often unobservable. Nevertheless, the concept of the induction period is a useful one. 19 April 2017 Triads & Natural History

Natural history of disease 22 22 19 April 2017 Triads & Natural History

Natural history of disease 19/2/2007 Natural history of disease 23 Dr. Salwa Tayel 23

Natural History of Disease Detectable subclinical disease Onset of symptoms Susceptible Host Point of Exposure Subclinical Disease Clinical Disease Outcome: Stage of Recovery, Complications, Disability, or Death Screening Diagnosis sought 19 April 2017 Triads & Natural History

Triads & Natural History 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. 19 April 2017 Triads & Natural History

Importance of studying Natural 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. 19 April 2017 Triads & Natural History

Triads & Natural History 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 Triads & Natural History