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 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?

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 1/1/04 1/3/04 1/5/04 1/7/04 1/9/04 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 2/2/04 2/4/04 2/6/04 2/8/04 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 3/1/04 3/3/04 3/5/04 3/7/04

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

Person Attack rate of acute hepatitis by age and sex, Baripada, Orissa, India, 2004 Cases Population Attack rate per 1000 Age 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 Sex Male 341 102683 3.3 Female 197 101646 1.9

Physical (injury, trauma) 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

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

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

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

Prevalence of anemia among adolescent girls, Mandla, MP, India 2005 3. Description of the health status of population Prevalence of anemia among adolescent girls, Mandla, MP, India 2005 Age in years Hemoglobin <12 g% Total Number (%) 12-13 71 93.4 76 14-15 88 93.6 94 16-17 97.3 73 18-19 27 77.1 31 257 93.8 274

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

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

Factors associated with anemia among pregnant women, Orissa, 2004 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 Hookworm infestation 12 (5-29) 10 (4-24) Consumption of IFA < 90 days 4.1 (2-8) 2.7 (1-7) Education below middle school * 4 (3-7) 2.3 (1-4) Number of pregnancy > 2 3.6 (2-6) 1.9 (1-4) * Middle school = Seventh class in Orissa This slide summarizes the factors that were associated with anemia in univariate and multivariate analysis. Factors that were associated with anemia included partial consumption of IFA, a number of pregnancy exceeding two, an education level below middle school and hook worm infestation.

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 58 City B 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: Question 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 Population at risk

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

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

Case fatality ratio Divide Example: Measles outbreak 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 40 45+ 7 16 Total 49 51 100

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

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

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 80 60 40 Percentage 20 0-19.9 20-49.9 50-99.9 100-300 > 300 Urinary Iodine Excretion levels (µg/L)

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 This graph represents the number of cases by week of onset between the month of May and September 2005 in the the tree villages. The first case in the area developed an illness during the first week of May. The outbreak peaked during the fourth week of July – when the investigated began – and the number of cases decreased during the early part of the month of August. The shape of the curve suggested a common source outbreak. You can see on this graph that before the outbreak, there were a number of initial cases that could be considered as index cases. 10 3rd week 1st week 1st week 2nd week 3rd week 4th week 1st week 2nd week 4th week 1st week 3rd week 2nd week 4th week 1st week 2nd week 3rd week 4th week May June July August September Week of onset

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%

Incidence: 9.6 per 100 person-month (95% C.I. 8-11 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% In this rural population of predominantly agricultural workers, despite the current low agricultural activities, the leading contributor to disability was due to agricultural and other occupational injuries. These findings were consistent with other studies.32, 34 A high rate of morbidity due agricultural injuries has also been reported by community-based surveys in India.7,10,11, 16, 57   Fall 32% Bites 16% Burns 7%

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

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

Vertical bar chart Prevalence of hypertension by age and sex, Aizawl, Mizoram, India, 2003 70 60 50 40 Male % 30 Female 20 Overall prevalence was 34.5% with a 95% Confidence interval of 32.3 and 36.7. As we can see from the graph below, prevalence was higher among men in all the age groups as compared to women. We observed that the prevalence of hypertension is increasing with increasing age. This was consistent for both the sexes. The trend was significant. We will now present the proportion aware of disease status among the hypertensive status. 10 30-39 40-49 50-59 60-69 70 + Age group (years)

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 1-19 20-49 50-99 100+ Open drain Hypothesis generated: Blocks with hepatitis are those supplied by pipelines crossing open sewage drains Pipeline crossing open sewage drain