Descriptive Epidemiology Ahmed Mandil Prof of Epidemiology KSU College of Medicine

Headlines Definition of Health Definition of Epidemiology Uses and Applications of Epidemiology Descriptive versus Analytical Epidemiology Epidemiological triads Related disciplines

Definition of Health A state of complete physical, mental and social well-being, and not mere absence of infirmity (WHO, 1945)

Definition of Epidemiology Epidemiology may be defined as: the study of distribution and determinants of health-related states or events in specified populations; and the application of this study to control of health problems (Last, 2000)  

Uses and Applications of Epidemiology - Establishing etiological hypothesis (causation) - Studying natural history of disease - Describing health status of populations - Evaluating interventions  

Establishing etiological hypothesis (causation) This includes the study of the different factors (genetic / environmental / interaction between both), which may be responsible for ill health. This includes investigation of causes of communicable diseases, as well as risk factors for non-communicable diseases and injuries. All this can lead to identification of the most effective preventive methods, which have the overall goal of improving the health of populations.

Studying natural history of disease This includes the study of the course and outcome (i.e. natural history) of diseases (in individuals and groups). This includes following up the affected people from the time of exposure, through subclinical stages; appearance of signs and symptoms (clinical disease); and final outcome (recovery, chronicity, disability, death)

Describing health status of populations Epidemiology is also often used to describe the health status of populations groups. Knowledge of disease burden (morbidity, mortality) in populations is essential for health authorities, which seek to use limited resources to the best possible effect by identifying priority health programmes for prevention and care.

Evaluating Interventions In addition to investigating etiological hypothesis, studying natural history of diseases, describing health status of population groups, epidemiologists have become involved in evaluating interventions. This inlcudes evaluating the effectiveness and efficiency of health promotion and educations programmes; preventive measures as well as public health services including management schemes.

What Is The Unique Skill Of Epidemiologists? MEASURING DISEASE FREQUENCY IN POPULATIONS

Measuring Disease Frequency Has Several Components Classifying and categorizing disease Deciding what constitutes a case of disease in a study Finding a source for ascertaining the cases Defining the population at risk of disease Defining the period of time of risk of disease Obtaining permission to study people Making measurements of disease frequency Relating cases to population and time at risk

Two Broad Types of Epidemiology DESCRIPTIVE EPIDEMIOLOGY ANALYTIC EPIDEMIOLOGY Examining the distribution of a disease in a population, and observing the basic features of its distribution in terms of time, place, and person. Typical study design: community health survey (approximate synonyms - cross-sectional study, descriptive study) Testing a specific hypothesis about the relationship of a disease to a putative cause, by conducting an epidemiologic study that relates the exposure of interest to the disease of interest. Typical study designs: cohort, case-control

Epidemiological Triads Descriptive Epidemiology Triad: Person Place Time Analytical Epidemiology Triad: Agent Host Environment

The Basic Triad Of Descriptive Epidemiology THE THREE ESSENTIAL CHARACTERISTICS OF DISEASE WE LOOK FOR IN DESCRIPTIVE EPIDEMIOLOGY: TIME PLACE PERSON

Time Changing or stable? Seasonal variation. Clustered (epidemic) or evenly distributed (endemic)? Point source or propagated.

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

Place Geographically restricted or widespread (pandemic)? Relation to water or food supply (clusters: multiple / one) Residence (rural, urban, sub-urban) Weather (temperature, humidity) Natural / political

Person Age Socio-economic status Gender Ethnicity/Race Behavior

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

A COMMON ERROR IN EPIDEMIOLOGY IS MOVING TO ANALYTIC EPIDEMIOLOGY WITHOUT HAVING A SOLID BASE IN THE DESCRIPTIVE EPIDEMIOLOGY OF THE CONDITION.

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

Agents Biological (micro-organisms) Physical (temperature, radiation, trauma, others) Chemical (acids, alkalis, poisons, tobacco, others) Environmental (nutrients in diet, allergens, others) Psychological experiences 22

Host Factors Genetic endowment Immunologic state Age Personal behavior

Environment Crowding Atmosphere Modes of communication – phenomena in the environment that bring host and agent together, such as: Vector Vehicle Reservoir

Epidemiologists are required to have some knowledge of the disciplines of: • Public health, because of the emphasis on disease prevention. • Clinical medicine, because of the emphasis on disease classification and diagnosis.  • Pathophysiology, because of the need to understand basic biological mechanisms in disease. • Statistics, because of the need to quantify disease frequency and its relationships to antecedents. • Social sciences, because of the need to understand the social context in which disease occurs and presents.

Further Reading Porta M. A dictionary of epidemiology. 5th Edition. Oxford, New York: Oxford University Press, 2008. Gordis L. Epidemiology. 4th edition. Philadelphia, Pennsylvania: Elsevier Science, 2008  Beaglehole R, Bonita R, Kjellstrom T. Basic epidemiolgy. 2nd edition. Geneva: World Health Organization, 2006 Paneth N. Introduction to epidemiology. University of Pittsburgh.