Dr Santosh K Yatnatti

1. Descriptive epidemiology Dr Santosh K Yatnatti

2. Plan of presentation • Introduction • Aims of epidemiology • Epidemiological methods • Descriptive epidemiology • Steps in descriptive epidemiology • Describing the disease under time, place and person • Measurement of disease • Uses

3. Introduction • EPIDEMIOLOGY: • The study of the distribution and determinants of health-related states or events in specified populations and the application of this study to the control of health problems. • - John M Last

4. Aims of epidemiology 1. To describe the distribution & magnitude of the health and disease problems 2. To identify etiological factors 3. To provide data essential to the planning, implementation & evaluation of services for the prevention, control & treatment of diseases & to the setting up of priorities among those services.

5. Epidemiological study design • Observational Studies • Descriptive studies • Analytical studies • Ecological studies • Cross-sectional studies • Case control studies • Cohort Studies • Experimental or Interventional Studies • Randomized controlled trials • Field trials • Community trials

6. Descriptive epidemiology • Distribution of disease occurs in patterns in a community and that the patterns may lead to the generation of hypothesis about causative factors. This aspect of epidemiology is known as descriptive epidemiology. • First phase of an epidemiological investigation • These studies are concerned with observing distribution of disease or health-related characteristics in human population • Identifying the characteristics with which the disease in question seems to be associated

7. When is the disease occurring? - Time distribution Where is it occurring? – Place distribution Who is getting the disease? – Person distribution Descriptive epidemiology (contd..)

8. Steps in Descriptive studies • Defining the population to be studied • Defining the disease under study • Describing the disease by • Time • Place • Person • Measurement of disease • Comparing with known indices • Formulation of an etiological hypothesis

9. 1. Defining the population • Size and composition • Whole population or representative sample or specially selected group • Health facility should be close enough to provide relatively easy access for patients requiring medical services • Provides the denominator

10. 2. Defining the disease • Definition – Precise & valid • To identify those who have the disease from those who do not • Operational definition- it is defined as the disease or condition can be identified and measured in the defined population with degree of accuracy. • Operational definition spells out clearly the criteria by which the disease can be measured.

11. 2. Defining the disease (contd..) • Such criteria in the case of tonsillitis would include the presence of enlarged, red tonsils with white exudate which on throat swab culture grow predominantly S. pyogenes Once established the case definition must be adhered to throughout the study.

12. 3. Describing the disease • Primary objective: Describe the occurrence & distribution – Time, place & person • Identifying those characteristics associated with presence or absence of disease in individual • Involves systematic collection & analysis of data

13. The pattern of disease may be described by the time of its occurrence that is hour of onset, day of the week, month, year It raises the question whether the disease is seasonal in occurrence Periodic increase or decrease Gives important clues about source or etiology of the disease Helps in taking preventive measures Time trends Short term fluctuations Periodic fluctuations Long term or secular trends Time distribution

14. Short term fluctuations • Epidemic It is defined as “the occurrence in a community or a region of cases of an illness or health-related events clearly in excess of normal expectancy.” The community or region and the time period in which cases occur are specified precisely Types of epidemics • Common source • Propagated • Slow/ modern

15. Types of epidemics • 1. Common source epidemics • a. Single exposure or “ Point source” epidemics • b. Continuous or Multiple exposure epidemic • 2. Propagated epidemics - Person to person Eg; epidemics of hepatitis - Arthropod vector Eg; plague - Animal reservoir Eg; Rabies • 3. Slow or modern epidemics

16. A. Common source epidemics • Single exposure or “ Point source” epidemics • Exposure to the disease agent is brief & simultaneous • All the cases occur within single incubation period • The epidemic curve rises & falls rapidly with no secondary waves • Explosive/ Clustering of cases in a narrow time • Eg: Bhopal gas tragedy, Minamata disease, food poisoning.

17. Exposure

18. Continuous or Multiple exposure epidemic • Prolonged (Continuous/ repeated/) exposure to the same source • Eg; commercial sex worker may be a common source in a gonorrhoea, but since she will infect her clients over a period of time there may be no explosive rise in the number of cases • Epidemics – more extended & irregular

19. B. Propagated epidemic • Person to person transmission of an infectious agent • Epidemic usually shows gradual rise & tails off over a much longer period of time • Speed of spread depends on • Herd immunity • Secondary attack rate • Opportunities for contact

20. Periodic fluctuations Seasonal trend: • Seasonal variation of disease occurrence may be related to environmental conditions which directly or indirectly favour disease transmission • Eg: Measles – Early Spring season URTI– Winter season GE – Summer season

21. Periodic fluctuations (contd..) Cyclic trend: • Diseases occur in cycles – Days, weeks, months or years Eg; Automobile accidents – Saturdays Measles- 2-3 years Rubella – 6-9 years Influenza- 7-10 years

22. Long term or secular trends Progressive increase or decrease in the occurrence of a disease over a long period of time Ex: CHD, Lung cancer, Diabetes Typhoid fever, Diphtheria

23. Place distribution • Geographic patterns provide an important source of clues about the causes of the diseases • Variations: • International • National • Rural – Urban • Local distribution

24. Place distribution (contd..) International variations • Variation in the pattern of disease in different parts of the world • Eg: Cancer, CHD • Helps to identify factors which are crucial in the cause & prevention of disease

25. Place distribution (contd..) National variation: • Variation in occurrence of disease within the country • Eg: Endemic goitre, lathyrism, flurosis, malaria It helps • To demarcate the affected areas • For providing appropriate health care services

26. Place distribution (contd..) Rural – urban variation: • Chronic bronchitis, lung cancer, accidents, cardiovascular diseases, mental illness and drug dependence are more frequent in urban than in rural areas. On the other hand, skin, zoonotic diseases and soil transmitted helminthes may be more frequent in rural areas than in urban areas. death rates especially infant and maternal mortality rates, are higher for rural than urban areas. • Variations may be due to population density, social class, medical care deficiencies, education & environmental factors • It helps • To identify high risk groups • To provide guide lines for prevention & control

27. Place distribution (contd..) Local distribution: • Inner & outer city variations in disease frequency Spot or shaded maps: • Areas of high & low frequency • Boundaries • Pattern of disease distribution • If the map shows “clustering” of cases, it may suggest a common source of infection or common risk factor shared by all cases.

29. Place distribution (contd..) • John snow of England in his investigation of cholera epidemic in 1854 in the Golden Square district of London was able to focus attention on the common water pump in Broad street as the source of infection. Based on his descriptive findings, Snow was able to hypothesize that cholera was a water-borne disease, long before the birth of bacteriology

30. Migration studies • Migration of human population from one country to another provides unique opportunity to evaluate the role of the possible genetic & environmental factors in the occurrence of disease in a population. Supposing there are marked geographic differences in the occurrence of a disease in a two areas, area “A” & area “B”. Let us assume that the environments in these two places are very different. The question arises whether the environmental differences in the two areas account for the variations in the occurrence of the disease in question. • Samples of a population in area “A” should be sent to area “B”, and vice versa to study change in incidence of disease.

31. Migration studies (contd..) • Migrant studies can be carried out in two ways. • (a) Comparison of disease and death rates for migrants with those of their kin who have stayed at home. This permits study of genetically similar groups but living under different environmental conditions or exposures. If the disease and death rates in migrants are similar to the country of adoption over a period of time, the likely explanation would be change in the environment. • (b) Comparison of migrants with local population of the host country provides information on genetically different groups living in a similar environment. If the migration rates of disease and death are similar to the country of origin, the likely explanation would be the genetic factors.

32. Migration studies (contd..) • Eg. Men of Japanese ancestry living in USA experience a higher rates of CHD than do the Japanese in Japan. • Another example, Japan has higher rate of stomach cancer and a lower rate for colon cancer than the United States has. However, third generation descendants of Japanese immigrants to USA have rates of stomach and colon cancer like those of the total US population.

33. Person distribution • Age • Sex • Ethnicity • Marital status • Occupation • Social class • Behaviour • Stress • Migration

34. Person distribution (contd..) • Age Certain diseases are more frequent in certain age groups Eg; Measles in childhood Cancer in middle age Atherosclerosis in old age • Bimodality Sometimes there may be two separate peaks in the age incidence curve of a disease Eg; Hodgkin’s disease, leukaemia, and female breast cancer.

35. Person distribution contd.. • Sex • Is another host characteristic which is often studied in relation to disease, using such indices as sex-ratio, sex-specific morbidity and mortality rates. • Eg; Diabetes,hyperthyrodism,obesity, are more common in women, and diseases such as lung cancer,CHD, are less frequent in women • Ethnicity • Eg; tuberculosis, essential hypertension,cancer,sickle cell anaemia • Marital status • It was found that mortality rates were always lower for married males and females than for the unmarried,of the same age and sex

36. Person distribution contd.. • Occupation • Workers in coal mines are more likely to suffer from silicosis, sedentary occupation face the risk of heart disease. • Social class • Epidemiological studies have shown that health and disease are not equally distributed in social classes. certain diseases have shown a higher prevalence in upper classes than in lower classes. Eg; coronary heart disease, hypertension, diabetes

37. Person distribution contd.. • Behaviour • Human behaviour is increasingly looked upon as a risk factor in coronary heart disease, cancer, obesity. • The behaviour factors which have attracted the greatest attention are cigarette smoking, sedentary life, over-eating and drug abuse. • Stress • Stress has been shown to affect a variety of variables related to patients response, eg; susceptibility to disease, exacerbation of symptoms.

38. 4.Measurement of diseases • To obtain estimates of magnitude of health & disease problem in a population • Mortality • Morbidity • Incidence • Longitudinal studies • Prevalence • Cross sectional studies

39. Longitudinal studies There is an increasing emphasis on the value of longitudinal studies in which observations are repeated in same population over prolonged period of time by means of follow-up examinations. • Longitudinal studies are useful 1 To study the natural history of disease and its future outcome 2 For identifying the risk factors of disease 3 For finding out incidence rate

40. Cross-sectional studies • Is the simplest form of observational study. It is based on single examination of a cross section of population at one point of time. Cross- sectional studies are more useful for chronic than short-lived diseases.

41. 5.Comparing with known indices • Comparison • Different population • Different sub groups • Helps to identify groups who are at increased risk • Gives clues about disease etiology

42. 6.Formulation of hypothesis A hypothesis is a supposition, arrived at from observations or reflections An epidemiological hypothesis should specify the following • The population –the Characteristics of the persons to whom the hypothesis applies. • Specific cause being considered • Expected outcome-the disease • Dose- response relationship- the amount of the cause needed to lead to a stated incidence of the effect • Time- response relationship- the time period that will elapse between exposure to the cause and observation of the effect

43. 6.Formulation of hypothesis contd., Eg: The smoking of 30-40 cigarettes per day causes lung cancer in 10 percent of smokers after 20 years of exposure

44. Uses of Descriptive studies • Provide data regarding magnitude of the disease • Provides clues to disease etiology • Provides background data for planning, organizing and evaluating preventive and curative services • Contribute to research by describes variation in disease occurrence by time, place and person

45. References • K. Park, Park’s Textbook of Preventive and Social Medicine 19th edition 2007. • Brian Macmahon, Epidemiology principles and methods, 1st edition 1970. • R Beaglehole, R Bonita, T Kjellstrom Text Book of Basic Epidemiology, 1st edition 2006. • John M. Last, Dictionary of Epidemiology, 4th edition 2001.