Basic epidemiology for disease surveillance

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
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

5
The basic principles of descriptive epidemiology

• Time
• When did the event happen?
• Place
• Where did the event happen?
• Person
• Who was affected?

6
Cases of acute hepatitis by date of onset,
Baripada, January-March 2004
Time
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
Attack rate of acute hepatitis by zone of
residence, Baripada, Orissa, India, 2004
Place
Attack rate
0 - 0.9 / 1000
1 - 9.9 / 1000
10 -19.9 / 1000
20 / 1000
Chipat river
8
Attack rate of acute hepatitis by age and sex,
Baripada, Orissa, India, 2004
Person
Cases Population Attack rateper 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
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 Politi
cal
Genotype Nutrition Immunity Behaviour
ENVIRONMENT
HOST
10
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

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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
Age in years Hemoglobin lt12 g Hemoglobin lt12 g Total
Age in years Number () Total
12-13 71 93.4 76
14-15 88 93.6 94
16-17 71 97.3 73
18-19 27 77.1 31
Total 257 93.8 274
14
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 Yes
Malaria 577 20 Yes
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
Characteristics Univariate odds ratio (95 CI) Adjusted odds ratio (95 CI)
Hookworm infestation 12 (5-29) 10 (4-24)
Consumption of IFA lt 90 days 4.1 (2-8) 2.7 (1-7)
Education below middle school 4 (3-7) 2.3 (1-4)
Number of pregnancy gt 2 3.6 (2-6) 1.9 (1-4)
Middle school Seventh class in Orissa Middle school Seventh class in Orissa Middle school Seventh class in Orissa
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
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

19
Count, divide and compare The basis of
epidemiology
2. Divide the number of cases by the population
New AIDS cases New AIDS cases 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
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 0.550
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
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

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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

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Prevalence

• Number of people with
• the disease or condition
• at a specified time

X Factor
P
Total population at risk
29
Incidence rate
Number of people who get the disease or
condition in a specified time
X Factor
I
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
Age group (years) Number
0-4 19
5-14 25
15-44 40
45 16
Total 100
34
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
45 7 9 16
Total 49 51 100
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
Malaria in Kurseong block, Darjeeling District,
West Bengal, India, 2000-2004
Line graph for time series
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
Urinary iodine excretion status, 24 N Parganas,
West Bengal, India, 2004
Histogram
80
60
40
Percentage
20
0
0-19.9
20-49.9
50-99.9
100-300
gt 300
Urinary Iodine Excretion levels (µg/L)
39
Acute hepatitis by week of onset in 3 villages,
Bhimtal block, Uttaranchal, India, July 2005
Epidemic curve
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
4th week
3rd week
4th week
3rd 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
Types of unintentional injuries, Tiruchirappalli,
Tamil Nadu, India, 2003
Pie chart for the breakdown of a total in
proportions
Incidence 9.6 per 100 person-month (95 C.I.
8-11
Road
10
Minor injuries
35
Fall
32
Bites
16
Burns
7
42
Estimated and projected proportion of deaths due
to non-communicable diseases, India, 1990-2010
Cumulated bar chart for the breakdown of many
totals in proportions
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
Causes of non vaccination as reported by the
mothers, Bubaneshwar, Orissa, India, 2003
Horizontal bar chart
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
Prevalence of hypertension by age and sex,
Aizawl, Mizoram, India, 2003
Vertical bar chart
70
60
50
40
Male

30
Female
20
10
0
30-39
40-49
50-59
60-69
70
Age group (years)
46
Cholera cases by residence, Kanchrapara, N-24
Parganas, West Bengal, India, 2004
Spot map
47
Incidence of acute hepatitis by block, Hyderabad,
AP, India, March-June 2005
Incidence by area
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