SPM -Park's 24th edition

1. DESCRIPTIVE EPIDEMIOLOGY
Arya. K. A

2. EPIDEMIOLOGY
 Defined by John M.Last in 1988.
 Definition:
The study of the distribution and determinants of health
related states or events in specified populations, and
the application of this study to the control of health
problems.

3. DESCRIPTIVE EPIDEMIOLOGY
• First phase of an epidemiological investigation.
• Concerned with observing the distribution of disease
or health-related characteristics in human population
and identifying the characteristics with which the
disease in question seems to be associated .
• When ,where ,who??

4. Procedures in descriptive epidemiology
1. Defining population to be studied
2. Defining disease under study
3. Describing the disease by:
 Time
 Place
 Person
4. Measurement of disease
5. Comparing with known indices
6. Formulation of aetiological hypothesis

5. 1)Defining the population
• The 'defined population' can be the whole
population in the geographic area or more often a
representative sample taken from it .
• The defined population needs to be large enough so
that age ,sex and other specific rates are
meaningful.
• Stable without migration

6. 2)Defining disease under study
 Both precise and valid, sufficient to differentiate diseased
from non diseased.
 Operational definition:
Definition by which the disease or condition can be
identified and measured in the defined population
with accuracy.
• Give criteria by which disease is measured

7. 3)Describing the disease
 Describe the occurence and distribution of disease
by time,place and person and identifying those
characteristics associated with presence or absence
of disease in individuals.
 Involves systematic collection and analysis of data.

8. Frequently examined characteristics

9. TIME DISTRIBUTION
3 kinds of time trends or fluctuations in disease
occurance:
 Short term fluctuation
 Periodic fluctuation
 Long –term or secular trends

10. Short term fluctuation
 Best known - Epidemic
 Epidemic is defined as “the occurance in a
community or region of cases of illness or other
health related events clearly in excess of normal
expectancy."

11.  Types of epidemics
A. Common source epidemics
a) Single exposure or point source epidemics
b) Continuous or multiple exposure epidemics
B. Propagated epidemics
a) Person to person
b) Arthropod vector
c) Animal reservoir
C. Slow(modern) epidemics

12. Epidemic curve

13. COMMON SOURCE EPIDEMICS
A)Single exposure epidemics
• Brief and simultaneous exposure to agent.
• Main features:
i. Epidemic curve rise and fall rapidly with no secondary waves
ii. The epidemic tends to be explosive,there is clustering of
cases in narrow interval of time.
iii. All cases develop within one incubation period of disease
 Median incubation period

14. BHOPAL GAS TRAGEDY

17. B)Continuous or repeated exposure
• Exposure from same source may be prolonged-
continuous,repeated or intermittent - not necessarily
at same point of time
Eg Outbreak of Legionnare’s disease in summer of
1976 in Philadelphia.
• Water borne cholera

18. PROPOGATED EPIDEMICS
• Infectious origin
• Person to person transmission of an infectious
agent.(Hepatitis A and polio).
• Transmission continues until the number of
susceptibles is depleted or susceptible individuals
are no longer exposed to infected persons or
intermediary vectors.
• Speed of spread depend on herd immunity
oppurtunity for contact and secondary attack rate.

19. Course of typical propagated epidemic

20. Periodic fluctuations
 Seasonal trend:
 Many communicable diseases-Measles, Varicella…
 Cyclic trend:
Diseases occur in cycles spread over short periods of time
Eg: Influenza pandemics are known to occur at intervals of
7-10 yrs,due to antigenic variations

21. Seasonal trend in dengue fever in india

22. Long term or secular trends
• Changes in the occurrence of disease over a long
period of time.
• Consistent tendency to change in a particular
direction or a definite movement in one direction.
• Coronary heart disease, lung cancer and diabetes.
Interpretation of time trends
 Epidemiologist seeks which diseases are increasing
,decreasing and which are the emerging health
problems and of the effectiveness of measures to
control old ones.
 Tries to formulate aetiological hypothesis.

23. PLACE DISTRIBUTION
Geographical comparisons
 International variation
 National variation
 Rural variation
 Local distribution:
 Inner and outer city variation
 Spot maps or shaded maps –Shows at a glance areas
of high or low frequency the boundaries and patterns of
disease distribution.

25. Migration studies
 To evaluate the role of the possible genetic and
environmental factors in the occurance of disease in the
population.
 Can be carried out in 2 ways:
 Comparison of disease and death rates for migrants with those
of their kin who have stayed at home.
 Comparison of migrants with local population of host country
provides information of genitically different group under same
environment.
Example: Japanese immigrants to USA

26. PERSON DISTRIBUTION
 Age
 Sex
 Ethinicity
 Marital status
 Occupation
 Social class
 Behaviour
 Stress
 Migration
Disease is further characterized by defining the persons
who develop the disease by:

27. Bimodality

28. 4)Measurement of disease
 Disease load in the population.
 Mortality ,morbidity,disability
Incidence(Longitudinal study)
 Morbidity
Prevalence(Cross-sectional studies)

29. CROSS SECTIONAL STUDIES
 Single examination of cross section of population at one
point of time.
 For chronic than short lived disease
 Study tells about distribution of a disease in population
rather than aetiology.
 Time sequence cannot be deduced from cross sectional
data.
LONGITUDINAL STUDIES
 Observations are repeated in the same population over a
prolonged period of time by means of follow up
examinations
 Useful to:
 Study natural history of disease and its future outcome
 Identifying risk factors
 Finding out incidence rate

30. 5)Comparison with known indices
 Arrive at clues to disease aetiology
 Identify groups at increased risk for certain diseases.

31. 6)Formulation of a hypothesis
 A hypothesis is a supposition arrived at from
observation or reflection.
 An epidemiological hypothesis should specify:
 The population
 The specific cause being considered
 The expected outcome
 The dose response relationship
 The time response relationship

32. Example:
 “Cigarette smoking causes lung cancer” is
incomplete hypothesis.
 “The smoking of 30-40 cigarettes per day causes
lung cancer in 10 per cent of smokers after 20 years
of exposure” is a complete hypothesis

33. Uses of descriptive epidemology
 Provides data regarding the magnitude of disease load
and types of disease problem in community in terms of
morbidity and mortality rates and ratios.
 Provides clues to disease aetiology and help in
formulation of an aetiological hypothesis.
 Provide background data for planning, organizing ,and
evaluating preventive and curative services.
 Contribute to research by describing variations in disease
occurence by time, place and person.