Epidemiology

1. Epidemiology

2. Classically speaking • Epi = upon (among) • Demos = people • Ology = science • Epidemiology = the science which deals with what falls upon people…..

3. A Modern Definition • “The study of the distribution and determinants of health-related states in specified populations, and the application of this study to control health problems." (Last J) Search for knowledge Apply in health service

4. Objectives of Epidemiology 1. To describe the distribution and magnitude of health and disease problems in the population. 2. To identify the etiological factors – risk factors in the population. 3. To provide the data essential to planning, implementation and evaluation of services for prevention, control and treatment of disease and to setting up of priorities for these services.

5. The ultimate aim of epidemiology is • to eliminate or reduce health problem or its consequences and • to promote health and well-being of society as a whole.

6. Purposes of Epidemiology To investigate nature / extent of health-related phenomena in the community / identify priorities To study natural history and prognosis of health-related problems 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

7. Component: Disease Frequency- Rate and Ratio e.g Rate- incidence rate, prevalence rate etc Ratio- sex ratio, doctor-population ratio Distribution of Disease- Disease in community find causative factor Generate hypothesis Descriptive epidimiology

8. Determinants of Disease- To test hypothesis Analytic epidemiology Help in develop sound scientific program

9. Incidence • Number of new cases of a disease which come into being during a specified period of time. (Number of new cases of specific disease during a given period)/(population at risk during that period) x 1000 Importance: If incidence increasing, it may indicate failure or ineffectiveness of control measure of a disease and need for better/new health control measure.

10. Prevalence • Number of current case (old and new) of a specified disease at a point of time • It help to estimate the burden of disease • Identify potentially high-risk populations. They are essentially helpful to plan rehabilitation facilities, manpower needs, etc. (Number of current case of a specified disease at a point of time)/(estimated population at the same point of time) x 100 • Point prevalence AND Period prevalence

11. Relationship between incidence and prevalence • Prevalence =Incidence x Duration

12. Approach of an epidemiologist • Asking questions • making comparisons ● Asking questions may provide clues to cause or aetiology of disease e.g. What is the event, what is its magnitude, where did it happen, when did it happen, who were affected, why did it happen?

13. Making comparisons will help draw inferences to support asking questions. • This comparison may be: ● Between those with the disease and those without the disease; ● Those with risk factor and those not exposed to risk factor;

14. Terms to know • Endemic: constant presence of a disease in a given population • epidemic: outbreak or occurrence of one specific disease from a single source, in a group population, community, or geographical area, in excess of the usual level of expectancy • pandemic: epidemic that is widespread across a country, or large population, possible worldwide

15. Epidemiology versus clinical medicine Epidemiology Clinical medicine Unit of study is case Concerned with only sick Patient comes to doctor Seeks diagnosis, derives prognosis, prescribes specific treatment • Unit of study is a defined population or population at risk • Concerned with sick as well as healthy • Investigator goes to the community • identify source of infection, mode of spread, an Etiological factor, future trend or recommend control measures

16. Three essential characteristics that are examined to study the cause(s) for disease in analytic epidemiology are... • Host • Agent • Environment

18. Host Factors • Behaviors • Genetic predisposition • Immunologic factors • Influence the chance for disease or its severity

19. Agents • Biological • Physical • Chemical • Necessary for disease to occur

20. Environment • External conditions • Contribute to the disease process

21. Epidemics arise when host, agent, and environmental factors are not in balance • Due to new agent • Due to change in existing agent (infectivity, pathogenicity, virulence) • Due to change in number of susceptibles in the population • Due to environmental changes that affect transmission of the agent or growth of the agent

22. EPIDEMIOLOGICAL METHODS . The methods he employs can be classified as: 1. Observational studies a. Descriptive studies b. Analytical studies – Case control studies – Cohort studies 2. Experimental/interventional studies – Randomized control studies – Field trials – Community trials

23. Descriptive observations pertain to the “who, what, where and when of health-related state occurrence”. However, analytical observations deal more with the ‘how’ of a health-related event occur. Randomized control trial (often used for new medicine or drug testing), field trial (conducted on those at a high risk of conducting a disease), and community trial (research on social originating diseases)

24. Descriptive Studies Steps in conducting a descriptive study. Descriptive studies form the first step in any process of investigation. These studies are concerned with observing the distribution of disease in populations. 1. Defining the population. 2. Defining disease under study. 3. Describing the disease. 4. Measurement of disease 5. Compare 6. Formulate hypothesis-

25. Defining the population. Defined population may be the whole population or a representative sample. • It can also be specially selected group such as age and sex groups, occupational groups, hospital patients, school children, small community, etc.

26. 2. Defining disease under study. 3. Describing the disease. Disease is examined by the epidemiologist by asking three questions: ● When is the disease occurring—time distribution? ● Where is it occurring—place distribution? ● Who is getting the disease—person distribution?

27. A. Time Distribution • Short-term fluctuations. Common source epidemics - single exposure/point source—bhopal tragedy Propagated-infectious :Hep A • Periodic fluctuations; Seasonal –measles (early spring) cyclic- ,, in pre-vaccinated era (peak 2-3 yr) • Long-term or secular trends; diabetes, CVD

28. B. Place Distribution • International variations: Cancer of stomach very common in Japan less common in US. oral cancer- India Breast cancer- Low-japan, high-western • National variations, e.g. Distribution of fluorosis,

29. Rural-urban differences, e.g. CVD, Mental illness more common in urban areas. Skin diseases, worm infestations more common in rural areas. • Local distributions, e.g. Spot maps- John Snow in London to incriminate water supply as cause of cholera transmission in London.

30. cholera cases in proximity to water pump, 1854

32. C. Person Distribution Age:e.g. Measles is common in children, Cancer in middle age Degenerative diseases in old age. Sex: Women- Lung cancer-less Hyperthyroidism- more c. Social class- Diabetes, Hypertenson– upper class

33. 4. Measurement of disease- Mortality/ Morbidity 5. Compare- Between different population, subgroups 6. Formulate hypothesis. On basis of all data epidemiologist form hypothesis.

34. Cross-sectional studies • Cross-sectional study is also called prevalence study. • Cross-sectional study is the simplest form of observational study. • It is based on single examination of cross-section of population at one point of time. • If the sampling methodology is accurate, results can be projected to the entire population. • They are more useful for chronic illnesses, e.g. hypertension. • Cross-sectional studies save on time and resources, but provide very little information about natural history of disease and incidence of illness.

35. Case- control studies • It start from effect and then proceed to cause • Both exposure and outcome have occurred before start of the study • The study proceeds backwards from effect to cause

36. Select subjects based on their disease status. A group of individuals that are disease positive (the "case" group) is compared with a group of disease negative individuals (the "control" group). • The control group should ideally come from the same population that gave rise to the cases.

37. Basic steps in a case-control study • 1. Selection of cases and controls • 2. Matching • 3. Measurement of exposure • 4. Analysis and interpretation.

38. A 2×2 table is constructed, displaying exposed cases (A), exposed controls (B), unexposed cases (C) and unexposed controls (D). • To measure association is the odds ratio (OR), which is the ratio of the odds of exposure in the cases (A/C) to the odds of exposure in the controls (B/D), i.e. OR = (AD/BC).

39. The first step is to find out 1. Exposure rates among cases =a/(ac) = 33/35 =94.2% 2. Exposure rate among the controls =b/(bd) = 55/82 =67%

40. If the exposure rate among the cases is more than the controls. • We must see if the exposure rate among the cases is significantly more than the controls. This is done by using the chi-square test • It is significant if p is less than 0.05.

41. Odds ratio • It is a measure of strength of association between the risk factor and outcome. • The derivation of the odds ratio is based on three assumptions: ● The disease being investigated is relatively rare ● The cases must be representative of those with the disease ● The controls must be representative of those without the disease.

42. Odds ratio a.d/b.c 33X27/55X2 = 8.1 • People who smoke less than 5 cigarettes per day showed a risk of having lung cancer 8.1 times higher as compared to non-smokers.

43. OR is > 1- "those with the disease are more likely to have been exposed," OR close to 1 then the exposure and disease are not likely associated. OR <1-exposure is a protective factor in the causation of the disease.

44. Case control studies are usually faster and more cost effective • Sensitive to bias (selection bias). • The main challenge is to identify the appropriate control group; • The distribution of exposure among the control group should be representative of the distribution in the population that gave rise to the cases.

45. Cohort Study • It look at cause and proceed to effect • study before the disease is manifest and proceed to study over a period of time for the disease to occur. • Cohort means a group of people sharing a common experience. • Cohort studies are often prospective studies, they can be retrospective also, or a combination of both prospective and retrospective components can be brought in.

46. Steps in a cohort study: • Selection of study subjects • Obtaining data on exposure • Selection of comparison groups • Follow-up • Analysis.

47. The incidence rates of CHD among smokers i.e. a/(a+b) =84/3000 =28 per 1000 The incidence rates of CHD among non-smokers i.e. =c/(c+d) =87/5000 =17.4 per 1000

48. Then, we must determine if the incidence rate among the smokers is significantly more than among the non-smokers by using the chi-square test.

49. Relative risk (RR) • It is ratio of incidence of the disease among the exposed and incidence among the non-exposed. RR (incidence of disease among exposed)/ (Incidence of disease among non-exposed) =a/(ab)/c/(cd) =28/17.4 =1.6 • If RR is more than 1, then there is a positive association • If RR is equal to 1, then there is no association • Smokers develop CHD 1.6 times more than nonsmokers.

50. Attributable risk (AR) • This is defined as amount or proportion of disease incidence that can be attributed to a specific exposure. • It indicates to what extent the disease under study can be attributed to the exposure: (incidence of disease among exposed) - (incidence of disease among non exposed)/ (Incidence of disease among exposed) =28-17.4/28 = 10.6/28 = 0.379 =37.9% • 37. 9% of CHD among the smokers was due to smoking.