Epidemiology, Concepts and Applications Dr Faris Al Lami MBChB MSc PhD FFPH
Objectives Define Epidemiology Describe the main uses of Epidemiology Describe the main types of Epidemiology Describe Person, Place and Time Model Describe Agent, Host and Environment Model
Definition The study of the distribution and determinants of health-related states or events in specified populations, and the application of this study to control of health problems Last s Dictionary
Epidemiology EPI DEMO LOGOS Upon,on,befall People,population,man the Study of The study of anything that happens to people That which befalls man
Epidemiology Describes health events cause and risk factors of disease clinical pattern of disease Identify syndromes Identify control and/or preventive measures
So, Epidemiology Is the basic science of public health Provides insight regarding the nature, causes, and extent of health and disease Provides information needed to plan and target resources appropriately
History of Epidemiology HIPPOCRATES (400 BC): On Airs, Waters, and Places Hypothesized that disease might be associated with the physical environment, including seasonal variation in illness. JOHN GRAUNT (1662): Nature and Political Observations Made Upon the Bills of Mortality First to employ quantitative methods in describing population vital statistics. JOHN SNOW (1850): Describe and test the hypothesis that cholera was transmitted by contaminated water.
History of Epidemiology (cont.) DOLL & HILL (1950): Describe and test the association between smoking and lung cancer. FRANCES at al. (1950): Field trial of the Poliomyelitis vaccine in school children. DAWBER et al. (1955): Study risk factors for cardiovascular disease in the Framingham Heart Study.
Uses of Epidemiology (1) 1. Community diagnosis; i.e., what are the major health problems occurring in a community 2. Establishing the history of a disease in a population; e.g., identifying the periodicity of an infectious disease 3. Describing the natural history of disease in the individual; e.g., infection-asymptomatic phaseclinical disease-death) 4. Describing the clinical picture of disease; i.e., who gets the disease, who dies from the disease, and what the outcome of the disease is.
Uses of Epidemiology (2) 5. Estimating risk; e.g., what factors increase the risk of heart disease, automobile accidents, and violence 6. Identifying syndromes and precursors; e.g., the relationship of high blood pressure to stroke, kidney disease, and heart disease 7. Evaluating prevention/intervention programs; e.g., vaccine and clinical trials 8. Investigating epidemics/diseases of unknown etiology
Kinds of Epidemiology Descriptive Analytic Study of the occurrence and distribution of disease Determine risk factors of disease. Experimental Manipulation of the cause is should be followed by an alteration in the effect not due to chance
Basic Triad of Descriptive Epidemiology Person: Who is getting sick? Place: Where is the sickness occurring? Time: When is the sickness occurring? PPT = person, place, time PERSON PLACE TIME
Person Age Sex Occupation Immunization status Underlying disease Medication Nutritional status Socioeconomic factors Crowding Hobbies Pets Travel Personal Habits Stress Family unit School Genetics Religion
Place (where?) Geographically restricted or widespread Climate effects (temperature, humidity, combined effects..) Urban / sub-urban-squatter / rural Relation to environmental exposure (water, food supply, etc) Multiple clusters or one?
Time (when?) Changing or stable? Clustered (epidemic) or evenly distributed (endemic)? Time-trends: Point source, propagated, seasonal, secular, combinations
Secular Trend, The long-time trend of disease occurrence Tetanus by year, USA, 1955-2000 During 2000, a total of 35 cases of tetanus were reported. 900 800 700 600 500 400 300 200 100 0 1955 1960 1965 1970 1975 1980 1985 1990 1995 2000 Year
Seasonal A cyclic variation in disease frequency by time of year & season. Seasonal fluctuations in, Environmental factors Occupational activities Recreational activities
Whooping Cough - Four-monthly admissions, 1954-1973
Epidemic An increase in occurrence of the disease above the expected in a defined geographic area within a defined time period
Endemic, Epidemic and Pandemic Endemic - The habitual presence (or usual occurrence) of a disease within a given geographic area Epidemic - The occurrence of an infectious disease clearly in excess of normal expectancy, and generated from a common or propagated source Pandemic - A worldwide epidemic affecting an exceptionally high proportion of the global population Number of Cases of Disease Time
Natural history of disease Exposure Pathologic changes Onset of symptoms Usual time of diagnosis Stage of susceptibility Stage of subclinical disease Stage of clinical disease Stage of recovery, disability or death PRIMARY PREVENTION SECONDARY PREVENTION TERTIARY PREVENTION
The natural history of disease STAGE 1: DESCRIPTION: EXAMPLE: Susceptibility Risk factors which assist the development of disease exist, but disease has not developed Smoking
The natural history of disease (cont d) STAGE 2: DESCRIPTION: EXAMPLE: Presymptomatic disease Changes have occurred to lead toward illness but disease is not yet clinically detectable Alveoli deteriorate
The natural history of disease (cont d) STAGE 3: Clinical Disease DESCRIPTION: Detectable signs and/or symptoms of disease exist EXAMPLE: Emphysema detected by pulmonary function test
The natural history of disease (cont d) STAGE 4: DESCRIPTION: EXAMPLE: Disability Disease has progressed to the point of causing a residual effect Person has difficulty breathing
The Epidemiology Triangle Time
Agents Biological (micro-organisms) Physical (temperature, radiation, trauma, others) Chemical (acids, alkalis, poisons, tobacco, others) Environmental (nutrients in diet, allergens, others) Psychological experiences
Host Factors Genetic endowment Immunologic status Personal characteristics Personal behavior Definitive versus intermediate (in vector-borne diseases)
Environment Living conditions (housing, crowding, water supply, refuse, sewage, etc) Atmosphere / climate Modes of communication: phenomena in the environment that bring host and agent together, such as: vector, vehicle, reservoir, etc)