Descriptive epidemiology

Presentation on Descriptive epidemiology (General)

1. Descriptive Epidemiology - Reshma Ann Mathew 1

2. Classification of Epidemiologic methods 2

3. Observational studies allow nature to take its own course; the investigator measures but does not intervene . Descriptive analysis is limited to a description of the occurrence of a disease in a population. 3

4. In descriptive epidemiology, it is concerned with observing the distribution of disease in human population and identifying the characteristics with which the disease seems to be associated 4

5. Procedures in Descriptive Studies 5

6. Step 1: Defining the Population Descriptive studies are investigations of population. A defined population should not only be in terms of total no ., but also in terms of age, sex, occupation, etc. 6

7. The defined population- i ) could be a whole geographic region or a representative sample ii) could be a specially selected group - based on age, sex, occupation, etc iii) should be large enough so that it is meaningful iv) should be stable without migration into or out v) should not be different from other communities in the region. 7

8. Step 2: Defining the Disease The epidemiologist defines the disease which can be measured and identified in the defined population with a degree of accuracy . This is different from the clinician’s definition of a disease 8

9. Step 3: Describing the disease Time Place Person Year, Season Month, Week Day, Hour of onset Duration Climatic zones Country, Region Urban/Rural Local community Towns Cities Institutions Age Sex Marital state Occupation, Social status, Education Birth order Family size Height Weight Blood pressure, Blood cholesterol, Personal habits 9

10. 1) Time distribution 10

11. Epidemic curve A graph of time distribution of epidemic cases is called epidemic curve. 11

12. a) Short term fluctuations An epidemic is the best known short term fluctuation. It is defined as “ The occurrence in a community or region of cases of an illness or other health related events clearly in excess of normal expectancy” 12

13. Types- 1) Common source epidemics- i )Single/Point exposure ii)Continuous/Multiple exposure 2) Propagated epidemics- i )person to person ii)arthropod vector iii)animal reservoir 3) Slow(modern) epidemics 13

14. i ) Common source epidemics 1) Single exposure- It can occur due to an infectious agent or as a result of contamination of the environment and develops within one incubation period. Eg : Bhopal gas tragedy, Minamata disease The epidemic curve rises and falls rapidly , usually has one peak It tends to be explosive (i.e. clustering of cases within a short time) 14

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16. 2) Continuous exposure- It is when the exposure from the same source is prolonged and the epidemic continues over more than one incubation period. The epidemic reaches a sharp peak , but tails off gradually over a longer period of time. Eg : A well of contaminated water or nationally distributed vaccine (polio vaccine) or food ; water borne cholera . 16

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18. II) Propagated epidemics Types - person to person, arthropod, animal The epidemic shows gradual rise and tails off over a much longer period of time It is more likely to occur where there is i ) regular supply of new susceptible individuals- Births, Immigrants ii) lowering herd immunity 18

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20. 20 Course of a typical propagated epidemic

21. b) Periodic Fluctuations 1) Seasonal trend- Seasonal variation is characteristic of many communicable diseases. Eg : Measles, upper respiratory tract infections(seasonal rise during winter), Malaria, etc. Non-infectious diseases and conditions may sometimes exhibit seasonal variation. Eg : Sunstroke, hay fever. 21

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23. 2) Cyclic trend- Some diseases occur in cycles spread over short periods of time (days, weeks, months or years) . Eg : Influenza pandemics are known to occur at intervals of 7-10yrs due to antigenic variations. Non-infectious conditions may also occur in this trend. Eg : Automobile accidents in the US are more frequent on weekends. 23

24. c) Long term trends It refers to changes in the occurrence of disease over a long period of time. Eg : Coronary disease, diabetes showing consistent upward trend and a decline in TB , polio in developed countries during the past 50 yrs. 24

25. 2) Place distribution 25

26. a) International variation Descriptive studies have shown that the pattern of a disease is not the same everywhere Eg : Cancer of the stomach is very common in Japan , but unusual in the US. 26

27. b) National variation There are variations in disease occurrence within countries . Eg : The distribution of endemic goitre, fluorosis , malaria, nutritional deficiencies show variations in their distribution in India. 27

28. c) Rural-Urban Variations Due to differences in population density , levels of sanitation , deficiencies of medical care , education and environment factors, there exists a rural-urban variation Chronic bronchitis , cardiovascular diseases, accidents are more frequent in urban than rural areas. Skin and zoonotic diseases and soil transmitted helminths may be more frequent in rural than urban areas. 28

29. d) Local distributions These variations can be studied with the help of “spot or shaded” maps . If the map showed clustering , it may suggest a common source of infection . Eg : Study of Cholera epidemic by John Snow in 1854 29

30. Study by John Snow, 1854 Spot map of deaths from cholera in Golden Square area, London, 1854 This pump was later suspected and proved to be a source of infection 30

31. Migration studies The use of migrant studies is a way of distinguishing genetic and environmental factors . Carried out in 2 ways- Study of genetically similar groups but living under different environmental conditions. Eg : Twins Study of genetically different groups living in a similar environment . Eg : Men of Japanese origin living in USA have higher rate of coronary heart disease than the Japanese in Japan 31

32. 3) Person distribution The disease can be characterised by defining a person who develops a disease based on age , sex, occupation, marital status, social factors, habits and other host factors . 32

33. a) Age Certain diseases are more frequent in certain age groups than others. Eg : Measles in childhood, cancer in middle age and atherosclerosis in old age. Many chronic and degenerative diseases show a progressive increase in prevalence with advancing age . 33

34. Bimodality There may be two separate peaks instead of one in the age incidence curve of a disease. This is known as bimodality as seen in Hodgkin’s lymphoma , breast cancer . It indicates that there are two different sets of causative factors even though the clinical and pathological manifestations of the disease is the same in all ages. 34

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36. b) Sex Variations occur due to- Biologic difference like sex linked genetic inheritance Cultural and behavioural differences between the sexes in social settings. Eg : 4:1 male to female ratio in lung cancer due to cigarette smoking. 36

37. c) Ethnicity Differences in racial and ethnic origin. Eg : Tuberculosis, sickle cell anemia 37

38. d) Marital status In a study, the mortality rates were lower for married people than unmarried It is because according to demographers and sociologists, marriages are selective with respect to health of the individual. Healthier the individual, the more likely to get married . 38

39. e) Occupation Occupation may alter the habit pattern of employees (Sleep, alcohol, smoking, etc) Workers in a particular occupation are exposed to certain types of risk. Eg : Workers in coal mines are likely to suffer from silicosis 39

40. f) Social class Health and diseases are NOT equally distributed in social classes. Certain diseases show higher prevalence in upper class (Diabetes, Coronary heart disease, hypertension) 40

41. g) Behaviour Behavioural factors such as smoking, sedentary life, over-eating, drug abuse lead to certain diseases (Coronary heart disease, Cancer, etc) Factors like mass movement ( Eg : Pilgrimages) may also lead to transmission of infectious diseases. 41

42. h) Stress The effects of stress are seen based on the patient’s response (Susceptibility to disease, Exacerbation of symptoms, etc) I) Migration Due to migration of people, there is also transmission of the disease from one place to another. 42

43. Step 4: Measurement of disease Types- Cross sectional studies - Prevalence can be obtained. It is based on a single examination of a cross section of population at one point in time. More useful for chronic diseases 43

44. 2) Longitudinal studies- Incidence can be obtained. The observations are repeated in the same population over a prolonged period of time by means of follow up examination. Longitudinal is more useful , but it is time consuming. 44

45. Step 5: Comparing with known indices Comparisons are made with known indices to arrive at clues to the disease’s etiology 45

46. Step 6: Formulation of a hypothesis A hypothesis is a supposition , arrived at from observation or reflection An epidemiologic hypothesis should specify- The population - characteristics of the people to whom the hypothesis applies Specific cause Expected outcome -the disease 46

47. Dose response relationship -The amount of the cause needed to lead to the stated incidence of the effect . Time response relationship -Time period between exposure to the cause and observation of the effect. 47

48. Uses of Descriptive epidemiology It provides data regarding the magnitude of the disease load and types of disease problems in terms of morbidity and mortality rates and ratios. It provides clue to disease etiology and help in the formulation of an etiological hypothesis . 48

49. It provides background data for planning, organizing and evaluating preventive service . Contribute to research by describing variations in disease occurrence by time, place and person. 49

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