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Epidemiologic Triads & Natural History of Disease

Epidemiologic Triads & Natural History of Disease

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Epidemiologic Triads & Natural History of Disease

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

  2. 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. Triads & Natural History 2

  3. Concepts • Descriptive epidemiological triad • Analytical epidemiological triad • Natural history of disease • Spectrum of disease • Public health implications Triads & Natural History

  4. 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 Triads & Natural History

  5. Epidemiological Triads Descriptive Epidemiology Triad: • Person • Place • Time Analytical Epidemiology Triad: • Agent • Host • Environment Triads & Natural History

  6. 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 Triads & Natural History

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

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

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

  10. 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 Triads & Natural History


  12. 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 Triads & Natural History

  13. 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 Triads & Natural History

  14. 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. Triads & Natural History

  15. 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) Biologicalfactors: 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 Triads & Natural History

  16. 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 Triads & Natural History

  17. 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 Triads & Natural History

  18. 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 Triads & Natural History

  19. Natural History of Disease

  20. Natural history of disease Naturalhistoryof 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 Triads & Natural History

  21. 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 Triads & Natural History

  22. Natural history of disease 22 22 Triads & Natural History

  23. Natural history of disease 23

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

  25. 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. Triads & Natural History

  26. 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. Triads & Natural History

  27. 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