General Epidemiology Lecture Notes PDF

GENERAL EPIDEMIOLOGY (Communicable & Non- communicable diseases) Dr\ Lotfi Fahmi Issa Objectives By the end of this lecture, the student should be able to: ❖ Define epidemiology and list its uses ❖ Describe the key features of epidemiology ❖ Describe the basi...

GENERAL EPIDEMIOLOGY (Communicable & Non- communicable diseases) Dr\ Lotfi Fahmi Issa Objectives By the end of this lecture, the student should be able to: ❖ Define epidemiology and list its uses ❖ Describe the key features of epidemiology ❖ Describe the basic measures used in epidemiology ❖ Define the role of epidemiology in public health practice. ❖ Know natural history of disease.& epidemiologic triad. Epidemiology ◼ Greek roots ◼ epi = upon ◼ demos = the people ◼ ology = study of OR science Epidemiology (cont.) Def. of Epidemiology: Epidemiology is defined as: - The study of the occurrence, distribution and determinants of disease or any health related conditions in defined population and the application of this study to control health problems in order to promote and protect health of the Community. Epidemiology (cont.) Comparison of epidemiology & medicine: ◼ Main unit of concern ◼ Epi → population ◼ Medicine → individual Epidemiology (cont.) Origin of epidemiology: - Study of epidemics of communicable diseases. - Then epidemiology extend to endemic communicable and non communicable infectious diseases. - Modern epidemiology: applied to chronic diseases, injuries, occupational and environmental health and behaviors related to health. What is Epidemiology? Epidemiology is the Study of Distribution Determinants Disease (health related event) Division of Epidemiology ◼ Descriptive Epidemiology - Occurrence - Distribution (Person, time and place) ◼ Analytic Epidemiology - Determinants (Risk factors & Causes)  Epidemiology Descriptive Analytical Studies the distribution of Provide Why and How of such health-related states or events events Provides What, Who, Tests the etiological hypothesis When, and Where of health-related events. Who is affected, when does Identifies the underlying causes affection occur, where does or risk factors of diseases. the event occur Disease Occurrence Pattern of disease occurrence: Endemic: The constant presence and/or usual prevalence of a disease or infectious agent in a population within a geographic area. Persistent occurrence of disease at the same level (Malaria) Epidemic: Sudden unusual occurrence of disease in specific population and specific period of time (Corona Virus) Outbreak: Localized epidemic (school) Pandemic: Epidemic in more than one country (Corona Virus) Epidemo-endemic: Epidemicity of an endemic disease. Sporadic: a disease that occurs infrequently and irregularly. Scattered cases over wide area. Tetanus, Rabies…. Disease Occurrence (cont.) Another pattern of occurrence: 1. Emergent diseases: Diseases occur for the first time, its natural history is less or not known clearly (need effort, investigation and studies) (Corona Virus, HIV……. 2. Re - emergent diseases: Diseases has disappeared for a period of time and then coming back, may be in more severe form (malaria, tuberculosis, cholera…….) Disease Distribution Epidemiology is concerned with the frequency and pattern of health events in a population: ❖ Frequency: The number of health events such as the number of cases of meningitis or diabetes in a population & the relationship of that number to the size of the population. The resulting rate allows epidemiologists to compare disease occurrence across different populations. ❖ Pattern: The occurrence of health-related events by time, place, and person. Disease Distribution (cont.) I- Distribution in time (when) A- Secular (long-term) trends: i.e., changes in periodicity and natural history of the disease over many years (Antigenic dirft and shift of influenza virus) - Graphing the annual cases or rate of a disease over a period of years -------- shows long-term or secular trends in the occurrence of the disease-------- Assess direction of disease occurrence (increasing, decreasing, or essentially flat) - Evaluate programs or make policy decisions (what caused an increase or decrease in the occurrence of a disease, and use past trends as a predictor of future incidence of disease.  Disease Distribution (cont.) B) Periodic change Periodic changes are the changes that repeat themselves after a fixed interval of time. Two years cyclical periodicity. (measles before MMR vaccination with accumulation of susceptible children) C) Seasonal pattern (seasonal variation of influenza) Disease Distribution (cont.) II. Distribution in place: - Mapping of disease occurrence (map spot, epidemic map) - Urban and rural difference To know if the risk of disease is present in the person living there or in the environment or both. III. Distribution with regard to person): - Age, sex, etc - Social and economic factors - Psychological factors - Physiological factors - Genetic - Lifestyle - Occupational factors (Occupation) - Other factors Determinants (Risk Factors) of Disease Determinants: - are the causes and risk factors that influence the occurrence of disease and other health-related events. - illness happens only when the right accumulation of risk factors or determinants exists in an individuals. Risk factor is the factor when present increase the probability of occurrence of an event (disease). These factors may be : - known risk factors - Suspected risk factors Data Sources of Epidemiology 1. Census 2. Population estimates 3. Vital records: A) Birth registration B) Death registration 4. Notification of infectious diseases 5. Disease registers 6. Hospital records 7. Special subgroups records 8. Morbidity surveys Tools in Epidemiology Data from 1. Epidemiologic studies 2. Other sources of data 3. Measures of disease frequency and association 4. Proportions, ratio, and rate. Community diagnosis Preventive and control measures Proportion, Ratio & Rate A / A+ B ❑ Proportion: Are special kinds of ratios where the denominator is the total while the Proportion of male to total in a class (class 20 students; 5 diseased and 15 non diseased P (diseased) = 5/20 =25% ◼ Ratio A/ B When one number expressed in relation to another by dividing the 1st to the 2nd. Diseased to non diseased ratio = 1 : 4 Proportion, Ratio & Rate Rate: The numerator: is the frequency of an event during a time period, The denominator: is the number of persons exposed to that event in the time period. A / (event of time) E.g: birth rate. are a special form of a ratio which represents the probability of a certain event. Measures of Disease Frequency Prevalence ◼ The prevalence is defined as the number of persons with a disease at a specified point in time. ◼ P = Number of having the disease at a specific time / Number of individuals in the population at that point of time X 1000 Incidence ◼ Incidence is defined simply as the number of new events (e.g., new cases of a disease) in a defined population within a specified period of time. ◼ I = Number of new cases at a specific time / Number of individuals in the population at that point of time X 1000  Commonly Asked Question to Quantify Disease Frequency Question Measure of disease frequency 1. How many people in a given Point prevalence population have the disease at this point in time? 2. How many people in a given Interval prevalence population ever had the disease during a given interval of time? 3. How many people in a given Incidence population newly developed the disease during a given period of time? Uses of Epidemiology 1 Search for causes: etiologic and risk factors in the pathogenesis of the disease. 2 Study of the natural history of diseases 3 Community diagnosis 4 Working of health services (develop and evaluate health programs) 5 Complete clinical picture Uses of Epidemiology (cont.) 6 Identify new syndromes (group of manifestations occurring together) 7 Clinical researches 8 Describe the distribution, and magnitude of health and disease problems in human populations. 9 Provide data essential to the planning, implementation and evaluation of services for prevention and control of the disease. 10 Assessment of the health of the population and community. Epidemiology in Public Health Practice Epidemiology is essential for carrying out five functions in public health: 1 Public health surveillance: ongoing systematic collection, analysis, interpretation and dissemination of health data for effective investigation, prevention and control of diseases in the community. 2 Disease investigations: for a case or cluster of cases to identify additional unreported cases (to control spread), to identify the source of infection or learn more about the disease itself. 3 Case definition: set of criteria for deciding whether a person has a particular disease or other related conditions for ensuring that every case is diagnosed in the same way regardless of when or where it occurred or who identified it. Epidemiology in Public Health Practice (cont.) 4- Analytic studies: for testing hypothesis to find that if there is a strong association between exposure and disease or not (if association, implement control measures). 5- Program evaluation: for achieving acceptable results. Example: evaluation of immunization program Natural History of the Disease Single causation theory Multiple causation theory - Pre-pathogenesis period - Pathogenesis period. Incubation period. Clinical stage. Ending stage - Complete recovery - Residual disability - Death - Carrier state  Natural History of the Disease Onset of Usual time of Exposure symptoms diagnosis Pathologic changes Stage of Stage of sub Stage of Stage of susceptibility clinical clinical recovery, disease disease disability or death PRIMARY PREVENTION SECONDARY PREVENTION TERTIARY PREVENTION  Disease Model “Epidemiologic Triad” Model Epidemiologic Triad (cont.) Disease is the result of forces within a dynamic system consisting of: Agent Host Environment  Epidemiologic Triad (cont.) any susceptible Host organism invaded by an agent Agent Environment the element that all other factors that inhibit must be present in order or promote disease for the diseases to occur. occurrence. Causative Agents Biological Chemical Physical Agents Agents Agents Viruses Pesticides Heat Rickettsiae Food additives Light Bacteria Pharmacologics Radiation Fungi Industrial chemicals Noise Protozoa Air pollutants Vibration Cigarette smoke Speeding object Host Factors 1 Resistance and immunity - Non specific resistance: skin, mm, secretion, etc. - Specific immunity:. Natural immunity - passive (Abs from mother) - active (clinical and sub clinical infections). Artificial immunity - passive (Immune globulin and gamma globulin) - active (vaccines and toxoid). Herd immunity 2 Sociocultural and habitual factors (lifestyle factors) 3 Genetic factors 4 Sociodemographic factors: age, sex, education, income, marital status, etc Herd Immunity ◼ Definition: - Herd immunity is defined as the magnitude of the immunity against any disease in the community. - Resistance to the spread of an infectious disease within a population that is based on pre-existing immunity of a high proportion of individuals as a result of previous infection or vaccination. - Determining factors: - Previous experience (previous disease). - Immunization program. - Level of susceptible persons due to 1 Immigration 2 Failure of immunization program Herd Immunity(cont.) ◼ Measuring Herd immunity: Indirect measures ❑ Morbidity and mortality indices. Direct measures Measuring immunoglobulin, etc. Environmental Factors i. Physical environment - Geographical - Geological - Climatic ii. Biological living or biotic components such as Human beings, animals, plants, trees,……. ii. Socicultural environment - Population density - Educational status - Economic level - Medical care facilities - Means of transportation Referrences - Maxy- Rosenau-Last: Public Health and preventive Medicine 15th edition 2017. - Park’s textbook of preventive and social medicine 20th edition 2020. - Raivir Bhalwar Textbook of Public Health and Community Medicine 1st edition 2009.