DESCRIPTIVE EPIDEMIOLOGY

 Descriptive studies are usually the first phase of an epidemiological

investigation
 It concerned with observing the distribution of disease and health-related
characteristics in human population and identifying the characteristics
with which the disease question seems to be associated.
 Descriptive studies usually ask questions –
a. When the disease occurring -------- Time distribution
b. Where the disease occurring --------place distribution
c. Who is getting the disease ---------person distribution

Steps of descriptive epidemiology :

1. Defining the population
 At the outset, we have to define the population base i.e total
population and the characteristics of the population – age, sex,
occupation, cultural norms etc.
 The ‘defined population’ may be the whole population in a
geographic area or a specially selected group such as age group,
sex group, hospital patients, school children etc.
 The defined population needs to be large enough so that age, sex,
other specific rates are meaningful.
 The community chosen should be stable.
 It should be clear who does and who does not belong to the
population.
 Community participation is necessary.
 The population should not overtly different from the other
communities in the region.
 A health facility should be close enough to provide easy access for
patient requiring medical services.
 The ‘defined population’ provide the denominator for calculating
rates.

2. Defining the disease under study

 A working definition (operational definition) is required by which,
the disease or condition can be identified and measured in the
defined population with a degree of accuracy.
 Example -tonsillitis may be defined clinically as inflammation of
tonsils caused by infection. Where as according to operation
definition- tonsillitis may be defined as -enlarged, red tonsil with
white exudate, which on throat swab culture grow S. pyogenes.
 The diagnostic method used should be acceptable to the
population in which the study will be conducted.

3. Describe the disease by time, place and person

 Describe the occurrence and distribution of disease or health events
by time, place and person, and identifying those characteristics
associated with the presence or absence of disease in individuals. It
involves the systematically collected data and analysis.
  Usually the characteristics are described below

Time Place person

Year, season Climatic zone Age Birth order

Month, week Country, Sex Family size

region
Day, hour of Marital status Height, weight
onset Urban/rural
local Occupation Blood pressure
duration communities Social status Blood
education cholesterol
Town/cities Personal
habits

4. Measurement of disease
 It is mandatory to have a clear picture of the amount of disease
(‘disease load”) in the population.
 The information should be available in terms of Mortality, morbidity,
disability and so on, and should preferably be available for different
subgroups of the population.
 A longitudinal descriptive study help one to calculate Incidence rate
while in a cross-sectional study prevalence rate can be calculated.

5. Comparing with known indices

 By comparing between the sub-groups of population within the
same population, we can identify groups at increased risk of disease
and also possible clues regarding the aetiology.

6. Formulation of the hypothesis

 Descriptive study help to formulate hypothesis relating to disease
aetiology.
 An epidemiological hypothesis should specify the following –
a. The population – the characteristics of the person to whom the
hypothesis applies.
b. The specific cause being considered.
c. The expected outcome – the disease
d. The dose-response relationship – the amount of causes to lead to
a stated incidence of the effect.
e. The time-response relationship – the time period that will elapse
between the exposure to the cause and observation of the
effect.

Types of Descriptive study

a. Cross-sectional study
 It is based on a single examination of a cross-section of population
at one point in time- the result of which can be projected to the
whole population.
 Known as -prevalence study.
  More useful for chronic disease and short-lived disease.
 It tells about distribution of a disease in population rather than its
aetiology.
 It provides a very little information about the natural history of
disease or about the rate of occurrence of new cases (incidence).

b. Longitudinal study
 In this study, observations are repeated in the same population over
a prolonged period of time by means of follow-up examinations.
 It is useful in –
1. To study the natural history of disease and future outcomes.
2. For identifying of risk-factor of disease.
3. For finding out incidence rate in the community.
 Limitation – longitudinal study is difficult to organize and more-time
consuming.

Uses of descriptive epidemiology

1. Provide data regarding the magnitude of disease load and types of disease
problem in the community in terms of morbidity and mortality rates and
ratios.
2. Provide clue to disease aetiology and help in formulation of an aetiological
hypothesis.
3. Provide background data for planning, organizing and evaluating
preventive and curative services.
4. They contribute to research by describing variations in disease occurrence
by time, place and person.

EPIDEMIC CURVE
A graph of time distribution of epidemic cases is called an Epidemic curve. An
epidemic curve is plotted with the frequency of the disease along the vertical
(Y) axis and measure of time along the horizontal (X) axis.
Different epidemic curves –
a. Common source epidemic –
1. Single exposure or ‘point-source’ epidemics
2. continuous or multiple exposure epidemics.
b. Propagated epidemics
1. Person to person
2. Arthropod vector
3. Animal reservoir
c. Slow (modern) epidemics
Common source, single/point-source epidemics
a. The exposure to the disease is brief and all cases occur within one known
incubation period of the disease.
b. The epidemic curve has a sharp rise and equally abrupt decline with
only one peak.
c. The epidemic tends to be explosive and there is clustering of cases
within a narrow time interval.
d. Eg- exposure to infectious agent – food poisoning
Exposure to a contaminant in the environment – Bhopal Gas Tragedy.

Common source, continuous or repeated exposure

a. Cases occur over more than one incubation period as exposure to disease
agent is continuous, repeated or intermittent.
b. No explosive increase in cases- the epidemic curve rises slowly and falls
gradually.
c. The peak is not sharp but rather plateau like.
d. The duration of epidemic is stretched out.
e. Eg.- infectious hepatitis, prostitution carrying gonorrhoea, cholera etc.
 Propagated source epidemic
a. It is most often of infectious origin and result from person-to-person
transmission of an infectious agent.
b. Epidemic curve rises gradually and tails off over a much longer
period of time.
c. Transmission continues until the number of susceptible is depleted or
susceptible individuals are no longer exposed to infected persons or
intermediary vectors.
d. The speed of spread depends upon herd immunity, opportunities for
contact and secondary attack rate.
e. It is more likely to occur where large number of susceptible are aggregated,
or where there is a regular supply of new susceptible individual, lowering
herd immunity.

Modern or slow epidemic

a. It is usable to non-communicable diseases like cancer, hypertension,
diabetes etc.
b. These are slowly increasing in number compared to last decade because
of changes in lifestyle.

Uses of an epidemic curve

1. A time relationship with exposure to suspected source.
2. A pattern (cyclical or seasonal) suggestive of particular infection.
3. Whether a particular epidemic is common source or propagated
epidemics.