Epidemiology lecture notes

Transcription

1 Epidemiology lecture notes By Avhinesh Kumar February

2 Objectives At the end of the lecture, the student is able to: o o o o o define epidemiology and its uses. describe the purposes and goals of epidemiology. identify the historical achievements in epidemiology. define specified terms (e.g. morbidity, mortality, incidence, prevalence rates, and population at risk). o define statistical terms (e.g. range, mean, median, average, mode, significance, standard deviation). examine the primary, secondary and tertiary stages of the infectious process. 2

3 oepi : among odemos: People ologos: Study 3

4 Epidemiology Based on two fundamental assumptions: First, human disease does not occur at random Second, human disease has causal and preventive factors 4

5 Definitions Epidemiology It is the study of frequency, distribution, and determinants of diseases and other health-related conditions in a human population and the application of this study to the prevention of disease and promotion of health 5

6 Components of the definition 1.Study: Systematic collection, analysis and interpretation of data Epidemiology involves collection, analysis and interpretation of health related data Epidemiology is a science 6

7 Components 2. Frequency: the number of times an event occurs Epidemiology studies the number of times a disease occurs It answers the question How many? Epidemiology is a quantitative science 7

8 Components 3. Distribution: Distribution of an event by person, place and time Epidemiology studies distribution of diseases It answers the question who, where and when? Epidemiology describes health events 8

9 Components 4. Determinants: Factors the presence/absence of which affect the occurrence and level of an event Epidemiology studies what determines health events It answers the question how and why? Epidemiology analyzes health events 9

10 Components 5. Diseases & other health related events Epidemiology is not only the study of diseases The focus of Epidemiology are not only patients It studies all health related conditions Epidemiology is a broader science 10

11 Components 6. Human population Epidemiology diagnoses and treats communities/populations Clinical medicine diagnoses and treats patients Epidemiology is a basic science of public health 11

12 Components 7. Application Epidemiological studies have direct and practical applications for prevention of diseases & promotion of health Epidemiology is a science and practice Epidemiology is an applied science 12

13 History of Epidemiology Seven land marks in the history of Epidemiology 1. Hippocrates (460BC): Environment & human behaviors affects health 2. John Graunt (1662): Quantified births, deaths and diseases 3. Lind (1747): Scurvy could be treated with fresh fruit 13

14 History 4. William Farr (1839): Established application of vital statistics for the evaluation of health problems 5. John Snow (1854): tested a hypothesis on the origin of epidemic of cholera 6. Alexander Louis (1872): Systematized application of numerical thinking (quantitative reasoning) 14

15 History 7. Bradford Hill (1937): Suggested criteria for establishing causation Epidemiological thought emerged in 460 BC Epidemiology flourished as a discipline in 1940s 15

16 Purpose/use of Epidemiology The ultimate purpose of Epidemiology is prevention of diseases and promotion of health How? 1. To investigate nature / extent of health-related phenomena in the community / identify priorities. To study natural history and prognosis of healthrelated problems 16

17 Purpose/use of Epidemiology To identify causes and risk factors To recommend / assist in application of / evaluate best interventions (preventive and therapeutic measures) To provide foundation for public policy 17

18 Clinician Epidemiologist Patient s diagnostician Investigations Diagnosis Therapy Cure Community s diagnostician Investigations Predict trend Control Prevention 18

19 Types of Epidemiology Two major categories of Epidemiology 1.Descriptive Epidemiology Defines frequency and distribution of diseases and other health related events Answers the four major questions: how many, who, where, and when? 19

20 Types 2. Analytic Epidemiology Analyses determinants of health problems Answers two other major questions: how? and why? Generally, Epidemiology answers six major questions: how many, who, where, when, how and why? 20

21 Basic Epidemiological assumptions 1.Human diseases doesn t occur at random or by chance 2. Human diseases have causal and preventive factors 21

22 I keep six honest serving man; they taught me all I know. 22

23 Descriptive Epidemiology Who? When? Where? Descriptive epidemiology covers time, place, and person epidemiologist becomes very familiar with the data. learns the extent and pattern of the public health problem being investigated which months, which neighborhoods, and which groups of people have the most and least cases. 23

24 Descriptive Epidemiology creates a detailed description of the health of a population that can be easily communicated with tables, graphs, and maps. identify areas or groups within the population that have high rates of disease. This information in turn provides important clues to the causes of the disease, and these clues can be turned into testable hypotheses 24

25 Time occurrence of disease changes over time. changes occur regularly unpredictable. Influenza winter Dengue rainy seasons hepatitis B and salmonellosis can occur at any time. 25

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27 Epi Curve for E.Coli outbreak n= Number of cases Date of onset 27

28 Time Secular (long-term) trends 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 Seasonality Disease occurrence can be graphed by week or month over the course of a year or more to show its seasonal 28

29 Time Day of week and time of day some conditions, displaying data by day of the week or time of day may be informative. Analysis at these shorter time periods is particularly appropriate for conditions related to occupational or environmental exposures that tend to occur at regularly scheduled intervals 29

30 Farm Tractor Deaths by Day of Week 30

31 Road Deaths by Day of Week 31

32 Seasonal Pattern of Rubella, Influenza and Rotavirus 32

33 Place Describing the occurrence of disease by place provides insight into the geographic extent of the problem and its geographic variation. birthplace, site of employment, school district, hospital unit, recent travel destinations 33

34 Place Two main types of maps used: Choropleth maps use different shadings/colors to indicate the count / rate of cases in an area Spot maps show location of individual cases 34

35 Data Characterized by Place 35

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38 Person Person attributes include age, sex, ethnicity/race, and socioeconomic status 38

39 Data Characterized by Person 39

40 Data Characterized by Person 40

41 Infant Mortality Rates for 2002, by Race and Ethnicity of Mother 41

42 Analytic Epidemiology Used to help identify the cause of disease Typically involves designing a study to test hypotheses developed using descriptive epidemiology 42

43 Exposure and Outcome A study considers two main factors: exposure and outcome Exposure refers to factors that might influence one s risk of disease Outcome refers to case definitions 43

44 Case Definition A set of standard diagnostic criteria that must be fulfilled in order to identify a person as a case of a particular disease Ensures that all persons who are counted as cases actually have the same disease 44

45 Case Definition Typically includes clinical criteria (lab results, symptoms, signs) and sometimes restrictions on time, place, and person 45

46 Developing Hypotheses A hypothesis is an educated guess about an association that is testable in a scientific investigation Descriptive data provide information to develop hypotheses Hypotheses tend to be broad initially and are then refined to have a narrower focus 46

47 Example Hypothesis: People who ate at the church picnic were more likely to become ill Exposure is eating at the church picnic Outcome is illness this would need to be defined, for example, ill persons are those who have diarrhea and fever Hypothesis: People who ate the egg salad at the church picnic were more likely to have laboratory-confirmed Salmonella Exposure is eating egg salad at the church picnic Outcome is laboratory confirmation of Salmonella 47

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49 Epidemic Disease Occurrence Endemic - amount of a particular disease that is usually present in a community (baseline levels) endemic refers to the constant presence and/or usual prevalence of a disease or infectious agent in a population within a geographic area. Sporadic - refers to a disease that occurs infrequently and irregularly 49

50 Epidemic Disease Occurrence Epidemic - refers to an increase, often sudden, in the number of cases of a disease above what is normally expected in that population in that area. Outbreak carries the same definition of epidemic, but is often used for a more limited geographic area 50

51 Epidemic Disease Occurrence Pandemic - refers to an epidemic that has spread over several countries or continents, usually affecting a large number of people. 51

52 Natural history of disease The progression of disease process in an individual overtime in the absence of intervention Four stages in the natural history of a disease 1.Stage of susceptibility Presence of factors No disease 52

53 Natural history 2. Stage of sub-clinical disease Presence of pathogenic changes (biological onset) No disease manifestations 3. Stage of clinical disease Presence of sign and symptoms (clinical onset) 4. Stage of recovery, disability,or death 53

54 Levels of disease prevention Three major levels of disease prevention Primary prevention Targeted at healthy people Objectives are Promotion of health Prevention of exposure and Prevention of disease 54

55 Levels of disease Secondary prevention Targeted at sick individuals Objective is to stop or slow the progression of disease and to prevent or limit permanent damage through early detection & treatment 55

56 Levels of disease Tertiary prevention Targeted at people with chronic diseases & disabilities that can t be cured Objective is to prevent further disability or death and to limit impacts of disability through rehabilitation 56