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Basic epidemiology for disease surveillance

Basic epidemiology for disease surveillance. IDSP training module for state and district surveillance officers Module 7. Elements included in the module. Basic epidemiology relevant to surveillance Ratios, proportions and rates Incidence, prevalence and case fatality Data presentation

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Basic epidemiology for disease surveillance

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  1. Basic epidemiology for disease surveillance IDSP training module for state and district surveillance officers Module 7

  2. Elements included in the module • Basic epidemiology relevant to surveillance • Ratios, proportions and rates • Incidence, prevalence and case fatality • Data presentation • Tables • Graphs • Maps

  3. Definition of epidemiology Epidemiology is the study of the distribution and determinants of health-related events or states in population groups and the application of this study to the control of health problems (Last JM ed. Dictionary of Epidemiology, Oxford University Press, 1995)

  4. The clinician Deals with patients Takes a history Conducts a physical Makes a diagnosis Proposes a treatment Follows up the patient The epidemiologist Deals with populations Frames the question Investigates Draws conclusions Gives recommendations Evaluates programmes Comparing the job of a clinician and the job of an epidemiologist

  5. The basic principles of descriptive epidemiology • Time • When did the event happen? • Place • Where did the event happen? • Person • Who was affected?

  6. Time Cases of acute hepatitis by date of onset, Baripada, January-March 2004 Investigation 45 started Cases 40 Deaths 35 30 25 Strike Number of cases and deaths 20 15 10 5 0 3/1/04 1/1/04 1/3/04 1/5/04 1/7/04 1/9/04 2/2/04 2/4/04 2/6/04 2/8/04 3/3/04 3/5/04 3/7/04 1/11/04 1/13/04 1/15/04 1/17/04 1/19/04 1/21/04 1/23/04 1/25/04 1/27/04 1/29/04 1/31/04 2/10/04 2/12/04 2/14/04 2/16/04 2/18/04 2/20/04 2/22/04 2/24/04 2/26/04 2/28/04

  7. Underground water supply Pump from river bed Place Attack rate of acute hepatitis by zone of residence, Baripada, Orissa, India, 2004 Attack rate 0 - 0.9 / 1000 1 - 9.9 / 1000 10 -19.9 / 1000 20+ / 1000 Chipat river

  8. Person Attack rate of acute hepatitis by age and sex, Baripada, Orissa, India, 2004

  9. Role of the host, the agent and the environment in the occurrence of disease Biologic, Chemical, Physical (injury, trauma) Social Psychological AGENT VECTOR Sanitation Weather Pollution Socio-Cultural Political Genotype Nutrition Immunity Behaviour ENVIRONMENT HOST

  10. Uses of epidemiology • Examine causation • Study natural history • Description of the health status of population • Determine the relative importance of causes of illness, disability and death • Evaluation of interventions • Identify risk factors

  11. 1. Examine causation Genetic factors Good health Ill health Environmental factors (Biological, chemical, physical, psychological factors) Life style related factors

  12. 2. Study natural history Death Sub-clinical disease Clinical disease Good health Recovery

  13. Prevalence of anemia among adolescent girls, Mandla, MP, India 2005 3. Description of the health status of population

  14. 4. Determine the relative importance of causes of illness, disability and death * Disability-adjusted life years

  15. 5. Evaluation of interventions Treatment, Medical care Good Health Ill Health Health promotion Preventive measures Public health services

  16. 6. Identify those sections of the population which have the greatest risk from specific causes of ill health Factors associated with anemia among pregnant women, Orissa, 2004

  17. Epidemiological approaches • Descriptive epidemiology: • What is the problem? • Who is involved? • Where does the problem occurs? • When does the problem occurs? • Analytical epidemiology: • Attempts to analyze the causes or determinants of disease • Intervention or experimental epidemiology: • Clinical or community trials to answer questions about effectiveness of control measures

  18. 1. Count the number of new AIDS cases in two cities No. of new of AIDS cases City A 58 City B 35 Count, divide and compare: The basis of epidemiology

  19. Count, divide and compare: The basis of epidemiology 2. Divide the number of cases by the population City A: 58/25,000/ 1 year City B: 35/7,000/ 2 years

  20. Count, divide and compare: The basis of epidemiology 3. Compare indicators City A: 232/100,000/ year City B: 250/100,000/ year

  21. A ratio places in relation two quantities that may be unrelated • The quotient of two numbers • Numerator NOT necessarily INCLUDED in the denominator • Allows to compare quantities of different nature = 5 / 2 = 2.5/1

  22. Examples of ratio • Number of beds per doctor • 85 beds for 1 doctor • Number of participants per facilitator • Sex ratio: • Male / Female

  23. A proportion measures a subset of a total quantity • The quotient of two numbers • Numerator NECESSARILY INCLUDED in the denominator • Quantities have to be of the same nature • Proportion always ranges between 0 and 1 • Percentage = proportion x 100 2 / 4 = 0.5=50%

  24. Example of proportion • Tuberculosis cases in a district: • 400 male cases • 200 female cases • Question • What is the proportion of male cases among all cases? • What is the proportion of female cases among all cases?

  25. Observed in 2004 2 ----- = 0.02 / year 100 A rate measures the speed of occurrence of health events • The quotient of two numbers • Defined duration of observation • Numerator • Number of EVENTS observed for a given time • Denominator (includes time) • Population at risk in which the events occur

  26. Example of rate • Mortality rate of tetanus in country X in 1995 • Tetanus deaths: 17 • Population in 1995: 58 million • Mortality rate = 0.029/100,000/year • Rate may be expressed in any power of 10 • 100, 1,000, 10,00, 100,000

  27. Measures of disease frequency • Prevalence • Number of cases of a disease in a defined population at specified point of time • Incidence • Number of new cases, episodes or events occurring over a defined period of time

  28. Prevalence Number of people with the disease or condition at a specified time P = X Factor Total population at risk

  29. Incidence rate Number of people who get the disease or condition in a specified time I = X Factor Total population at risk

  30. Case fatality ratio • Divide • Number of deaths • Number of cases • Example: Measles outbreak • 3 deaths • 145 cases • Case fatality ratio: 2.1%

  31. Presenting health information • Tables • Graphs • Histograms • Line diagrams • Bar chart • Pie chart • Scatter plot • Map

  32. Tables • Data presented in columns and rows by one or more classification variable • Title- Concise, self explanatory explaining clearly all information being presented • Rows and columns should be clearly labeled • Categories should be clearly shown

  33. Example of one way table: Data tabulated by one variable Age distribution of a sample of 100 villagers

  34. Example of two way table: Data tabulated by two variable Age and sex distribution of a sample of 100 villagers

  35. Graphs • Charts based on length • Bar charts (horizontal, vertical, grouped, stacked) • Charts based on proportion • Pie chart • Geographic co-ordinate charts (maps) • Spot map • Area map

  36. 45 40 35 30 25 20 15 10 5 0 July July July July July May May May May May April April April April April June June June June June March March March March March August August August August August January October January October January October January October January October February February February February February November December November December November December November December November December September September September September September Line graph for time series Malaria in Kurseong block, Darjeeling District, West Bengal, India, 2000-2004 Incidence of malaria Incidence of Pf malaria Incidence of malaria per 10,000 2000 2001 2002 2003 2004 Months

  37. Histogram to display a frequency distribution • Graphic representation of the frequency distribution of a continuous variable • Rectangles drawn in such a way that their bases lie on a linear scale representing different intervals • Areas are proportional to the frequencies of the values within each of the intervals • No spaces between columns • No scale breaks • Equal class intervals • Epidemic curve is an example of histogram with time on the x axis

  38. Histogram Urinary iodine excretion status, 24 N Parganas, West Bengal, India, 2004 80 60 40 Percentage 20 0 0-19.9 20-49.9 50-99.9 100-300 > 300 Urinary Iodine Excretion levels (µg/L)

  39. Epidemic curve Acute hepatitis by week of onset in 3 villages, Bhimtal block, Uttaranchal, India, July 2005 90 80 70 60 50 Number of cases 40 30 20 10 0 1st week 3rd week 1st week 1st week 1st week 1st week 3rd week 3rd week 3rd week 4th week 4th week 4th week 2nd week 4th week 2nd week 2nd week 2nd week May June July August September Week of onset

  40. Proportions of a total presenting selected characteristics • Breakdown of a total in proportions: • Pie chart • Breakdown of more than one total into proportion: • Juxtaposed bar charts cumulated to 100%

  41. Pie chart for the breakdown of a total in proportions Types of unintentional injuries, Tiruchirappalli, Tamil Nadu, India, 2003 • Incidence: 9.6 per 100 person-month (95% C.I. 8-11 Road 10% Minor injuries 35% Fall 32% Bites 16% Burns 7%

  42. Cumulated bar chart for the breakdown of many totals in proportions Estimated and projected proportion of deaths due to non-communicable diseases, India, 1990-2010 100% 90% 80% Injuries 70% 60% Communicable Proportion (%) 50% diseases 40% Non communicable 30% diseases 20% 10% 0% 1990 2000 2010 Year

  43. Comparing proportions across groups • No logical order: Horizontal bar chart • Sort according to decreasing proportions • Logical order: Vertical bar chart • Not a continuous variable: Do not display axis • Continuous variable: Display axis

  44. Horizontal bar chart Causes of non vaccination as reported by the mothers, Bubaneshwar, Orissa, India, 2003 Lack of awareness Child sick Irregularity by health staff Lack of motivation Lack of time Lack of facility Lack of money 0% 20% 40% 60% 80% 100% India FETP

  45. Vertical bar chart Prevalence of hypertension by age and sex, Aizawl, Mizoram, India, 2003 70 60 50 40 Male % 30 Female 20 10 0 30-39 40-49 50-59 60-69 70 + Age group (years)

  46. Spot map Cholera cases by residence, Kanchrapara, N-24 Parganas, West Bengal, India, 2004

  47. Incidence by area Incidence of acute hepatitis by block, Hyderabad, AP, India, March-June 2005 Attack rate per100,000 population 0 1-19 20-49 50-99 100+ Hypothesis generated:Blocks with hepatitis are those supplied by pipelines crossing open sewage drains Open drain Pipeline crossing open sewage drain

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