Basic epidemiology for disease surveillance IDSP training module

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

Elements included in the module 1. 2. 3. 4. Basic epidemiology relevant to surveillance Ratios, proportions and rates Incidence, prevalence and case fatality Data presentation • Tables • Graphs • Maps

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)

Comparing the job of a clinician and the job of an epidemiologist • • • 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

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

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

Place Attack rate of acute hepatitis by zone of residence, Baripada, Orissa, India, 2004 Attack rate 0 - 0. 9 / 1000 Chipat river 1 - 9. 9 / 1000 10 -19. 9 / 1000 20+ / 1000 Underground water supply Pump from river bed

Person Attack rate of acute hepatitis by age and sex, Baripada, Orissa, India, 2004 Cases Age Sex Population Attack rate per 1000 0 -4 1 1012 0. 1 5 -9 11 21802 2 10 -14 37 74004 5 15 -44 416 51358 81 45+ 73 56153 13 Male 341 102683 3. 3 Female 197 101646 1. 9

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

Uses of epidemiology 1. Examine causation 2. Study natural history 3. Description of the health status of population 4. Determine the relative importance of causes of illness, disability and death 5. Evaluation of interventions 6. Identify risk factors

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

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

3. Description of the health status of population Prevalence of anemia among adolescent girls, Mandla, MP, India 2005 Age in years 12 -13 Hemoglobin <12 g% Number (%) 71 93. 4 Total 76 14 -15 88 93. 6 94 16 -17 71 97. 3 73 18 -19 27 77. 1 31 Total 257 93. 8 274

4. Determine the relative importance of causes of illness, disability and death Disease Tuberculosis Measles Malaria DALYs* (000) Mortality (000) Included in IDSP 7577 6471 421 190 Yes 577 20 Yes * Disability-adjusted life years

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

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 Characteristics Univariate odds ratio (95% CI) Adjusted odds ratio (95% CI) Hookworm infestation 12 (5 -29) 10 (4 -24) Consumption of IFA < 90 days 4. 1 (2 -8) 2. 7 (1 -7) 4 (3 -7) 2. 3 (1 -4) 3. 6 (2 -6) 1. 9 (1 -4) Education below middle school * Number of pregnancy > 2 * Middle school = Seventh class in Orissa

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

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

Count, divide and compare: The basis of epidemiology 2. Divide the number of cases by the population New AIDS cases Number Year Population City A 58 2004 25, 000 City B 35 2004 -5 7, 000 City A: 58/25, 000/ 1 year City B: 35/7, 000/ 2 years

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

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

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

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%

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?

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 Observed in 2004 2 ----- = 0. 02 / year 100

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

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

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

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

Case fatality ratio • Divide § Number of deaths § Number of cases • Example: Measles outbreak § 3 deaths § 145 cases § Case fatality ratio: 2. 1%

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

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

Example of one way table: Data tabulated by one variable Age distribution of a sample of 100 villagers Age group (years) Number 0 -4 19 5 -14 25 15 -44 40 45+ 16 Total 100

Example of two way table: Data tabulated by two variable Age and sex distribution of a sample of 100 villagers Age group (years) Male Female Number 0 -4 10 9 19 5 -14 12 13 25 15 -44 20 20 40 7 9 16 49 51 100 45+ Total

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

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

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

Histogram Urinary iodine excretion status, 24 N Parganas, West Bengal, India, 2004 Percentage 80 60 40 20 0 0 -19. 9 20 -49. 9 50 -99. 9 100 -300 Urinary Iodine Excretion levels (µg/L) > 300

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

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%

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 Minor injuries 35% Road 10% Fall 32% Burns 7% Bites 16%

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

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

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% India FETP 20% 40% 60% 80% 100%

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 Age group (years) 60 -69 70 +

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

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