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Investigation of Epidemic

Epidemic is the unusual occurrence in a community or region of disease, specific health-related behavior (e.g. smoking) or other health related events clearly in excess of expected occurrence.

Epidemic Classification

A.    Common Source Epidemics
i.    Point Source epidemic
ii.    Continuous or multiple source epidemic

B.    Propagated (Progressive ) Epidemics
C.    Slow (Modern) Epidemics

If an Epidemic occurs, how much is the opportunity to control it?

outbreak detection and response 1Outbreak detection and response 2

The objectives of an epidemic investigation are:
1.    To define the magnitude of the epidemic and involvement in terms of time, place and person.
2.    To determine the particular conditions and factors responsible for the occurrence
3.    To identify the cause, source(s) of infection, and modes of transmission to determine control
4.    To make recommendations to prevent recurrence

Investigation of Epidemic

Investigation of epidemic involves the following steps:
1.    Verification of diagnosis
2.    Confirmation of the existence of an epidemic
3.    Defining the population at risk
4.    Rapid search for all cases and their characteristics
5.    Data analysis
6.    Formulation of hypotheses
7.    Testing of hypotheses
8.    Evaluation of ecological factors
9.    Further investigation of population at risk
10.    Writing the report

Verification of Diagnosis

Verification of diagnosis is done by clinical examination of samples of cases and laboratory investigations.

Confirmation of Existence of an Epidemic

Compare the present disease frequencies with the frequencies of the previous year during same period.

Defining the Population at Risk

Population at risk may be defined by obtaining the map of the area and counting the population. This is because for calculating the crude attack rate, we need the denominator ‘Population at Risk’.

Rapid Search for all cases and their characteristics

Cases may be identified by:
a)    Medical Survey
By taking history, questionnaire, etc.
b)    Epidemiological case sheet
Case interview form
c)    Searching for more cases

Data Analysis

Data is analyzed by using classical epidemiological parameters i.e.

  • Time -Epidemic curve, time relationship with exposure, seasonal or cyclic pattern
  • Place -Prepare ‘spot map’ of cases i.e geographical distribution
  • Person -Age, sex, occupation etc

Determine attack rate and case fatality rate

Investigation of an Epidemic

The purpose of data analysis is to determine;

  • Common event or experience   OR
  • To outline the group involved in common experience.

Formulation of Hypothesis

Explain factors which enabled epidemic to occur in terms of:
    Possible Source
    Causative Agent
    Possible mode of Spread
    Environmental Factor

Testing of Hypothesis

Reasonable hypothesis need to be considered and weighted by comparing the attack rates in various groups for those exposed and those not exposed to each suspected factor.
Relative Risk = Incidence among exposed/Incidence among non exposed

Evaluation of Ecological Factors

Relate the disease to environmental factors to know the:
    Source of Infection
    Reservoir
    Modes of Transmission
E.g. breakdown in water supply, eating habits, sanitary measures, atmospheric changes, etc.

Further Identification of Population at Risk

To obtain additional information from population at risk, following points shall be considered:
    Medical Examination
    Screening test
    Examination of suspected food, feces, blood, etc,
    Biochemical Studies
    Assessment of immunity status, etc.

Writing the Report

Report shall be convincing and complete. It shall include following points:
1.    Background
2.    Historical Data
3.    Methodology of Investigation
4.    Analysis of Data
5.    Control Measures

Background includes geographical Location, climate conditions, demographic status, socio-economic status, organization of health service, surveillance & early warning System and normal disease prevalence

Historical Data
Historical data should include previous occurrence of epidemics, Occurrence of related diseases and discovery of first case of present epidemic.

Methodology of Investigation
Methodology should include case definition, questionnaire used in epidemic identification, survey teams, type of survey, laboratory techniques, etc.

Analysis of Data
Analysis of data involves clinical data, epidemiological data, mode of transmission, laboratory data and interpretation of data

Control Measures

  • Control measures including definition of strategies, methodology of implementation, constraints & results
  • Evaluation including significance of results and cost effectiveness
  • Preventive Measures

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