This document outlines the steps involved in investigating an epidemic: 1. Verification of diagnoses and defining cases is the first step to understand the scope and characteristics of the epidemic. 2. Confirmation of an actual epidemic involves comparing case numbers to historical data to determine if there is unusual disease occurrence. 3. Defining the population at risk, rapidly searching for all cases, collecting data on characteristics, and analyzing patterns in time, place and person help identify potential causes and transmission routes.

1. INVESTIGATION OF AN
EPIDEMIC
MRS. NAMITA BATRA GUIN
ASSOCIATE PROFESSOR
DEPTT. OF COMMUNITY HEALTH NURSING

2. INTRODUCTION
• It is an emergency situation and provides opportunities to use
epidemiological knowledge for immediate control of disease.
• The primary motivation of any epidemic investigation is to
control the spread of disease within the population at risk.

3. EPIDEMIC INVESTIGATION
• An epidemic investigation calls for inference as well as description.
• Frequently, epidemic investigations are called for after the peak of the epidemic has
occurred;
• In such cases, the investigation is mainly retrospective.
• However, in investigating an epidemic, it is desired to have an orderly procedure or
practical guidelines as outlined below which are applicable for almost any epidemic
study.

4. OBJECTIVES OF EPIDEMIOLOGAL INVESTIGATION
a. To define the magnitude of the epidemic or outbreak involvement
in terms of time, place and person.
b. To determine particular conditions and factors responsible for the occurrence of the
epidemic.
c. To identify the causative agent, sources of infection, and modes of
transmission.
d. To make recommendations to prevent recurrence

5. STEPS IN
EPIDEMIC
INVETIGATION
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 hypothesis
7. Testing of hypothesis
8. Evaluation of ecological factors
9. Further investigation of population at risk
10. Writing the report

6. 1. Verification of diagnosis
 First step in the investigation.
 The report may be spurious due to misinterpretation of signs or symptoms by public
or health worker.
 It is not necessary to examine all the cases.
 A clinical examination of a sample of cases issufficient.
 Laboratory investigations wherever applicable, are most useful to confirm the
diagnosis but control measures should not be delayed until laboratory results are
available.

7. 1. Verification of diagnosis- contd..
Case definition is needed to identify and count the cases in order to determine who may
be affected by the epidemic.
Components of the case definition may include information about time and place of
exposure, clinical signs & symptoms and lab findings.

8. 2. Confirmation of existence of an epidemic
Done by comparing the number of cases with disease frequencies during the same period of previous
years.
In case of endemic diseases like cholera, typhoid hepatitis A, it is expected that some cases (few hundreds)
always present throughout the year. So these diseases to be consider as epidemic several hundreds or
thousands of cases have to occur in India.
Information may be collected from routine health service records e.g. OPD registers, in-patient registers
etc.
Information should be collected not only from modern medicine practitioners but also from other
systems.

9. 3. Defining the population at risk
1. Obtaining the map of the area: It should contain information about natural
landmarks, roads and location of all dwelling units along each road. The area may be
divided into segments, using natural landmarks as boundaries. This is again divided
into smaller sections. Within each section, the dwelling units may be designated by
numbers.
2. Counting the population: By doing census by house to house visits. The composition
should be known by age and sex. Eg: For population at risk. In case of food poisoning-
those who ate the food.

10. 4. Rapid
search for all
cases and
their
characteristics
• It is done through surveillance The continuous scrutiny of all
aspects of occurrence and spread of disease that are
pertinent to effective control“ is called surveillance.
• Medical survey: It should be carried out in the defined area to
identify all cases including those who have not sought medical care,
and those possibly exposed to risk.
• Epidemiological case sheet: This should be carefully designed to
collect relevant information. If the epidemic is large it may not be
possible to examine all the cases. In such cases random sample
should be examined.
• Search for more cases: The patient is asked about the similar cases
at home, family, neighbourhood, school, work place having an onset
within the incubation period of index case. The search for new cases
should be carried out everyday till the area is declared free of
epidemic.

11. 5. Data analysis
• It has to be done using the parameters – Time, Place And Person.
a. Time
• Prepare a chronological distribution of dates of onset of cases and construct an
“epidemic curve”. An epidemic curve suggests :
 Pattern of spread
 Magnitude
 Outliers
 Exposure and/or disease incubation period

12. 5. Data analysis
• Place
• Prepare a ‘spot map’ of cases and if possible their relation to the
sources of infection e.g. water supply, air pollution, foods eaten,
occupation etc.
• The map shows the boundaries and patterns of disease distribution.
Clustering of cases indicate a common source of infection.

13. Spot map- sample
Original map by Dr. John Snow showing the clusters of Cholera cases in the London
epidemic of 1854

14. 5. Data analysis
• Person
• Analyse the data by age, sex, occupation and other possible risk
factors.
• Determine the attack rates/case fatality rate.
• The purpose of data analysis is:
• To determine the modes of transmission and the source and the vehicle
of the agent, so that effective measures can be initiated.
• To determine the risk factors for disease.

15. 6. Formulation of hypothesis
• Hypothesis is a proposition or a tentative theory designed to explain the observed
distribution of the disease in terms of causal association of the direct nature.
• The hypothesis should explain the epidemic in terms of
1. Causative agent
2. The possible source
3. Possible modes of spread
4. The environmental factors which enabled it to occur

16. 7. Testing of hypothesis
All reasonable hypotheses need to be considered and weighed by comparing the attack
rates in various groups for those exposed and those not exposed to the each suspected
factor.
This will enable the epidemiologist to ascertain which hypotheses is consistent with all
the known facts.
Sometimes the hypothesis needed to be tested by the analytical study design (case control
study) to the statistical significance.

17. 8. Evaluation of ecological factors
• An investigation of the circumstances involved should be carried out to
undertake appropriate measures to prevent further transmission of the disease.
• The ecological factors which have made the epidemic possible should be
investigated such as sanitary status of eating establishments, breakdown in the
water supply system, changes such as temperature, humidity, and air pollution,
population dynamics of insects and animal reservoirs etc..
• In case of water-borne transmission is suspected(gastroenteritis) a sanitary
survey of water supply system from source to consumer

18. 9. Further investigation of population at risk
Needed to obtain further information.
This may involve medical examination, screening tests, examination of suspected food,
feces or food samples, biochemical studies, assessment of immunity status etc.
This will permit classification of all members as to
1. Exposure to specific potential vehicles
2. Whether ill or not

19. 10. Writing report
1. Background: Geographical location, Climatic conditions, Demographic status (population pyramid),
Socioeconomic situation, Organization of health services, Surveillance and early warning systems,
Normal disease prevalence.
2. Historical Data: Previous occurrence of epidemics of the same disease, locally or elsewhere.
Occurrence of similar diseases in the same area/ in other areas. Discovery of the first cases of the
present outbreak.
3. Methodology of investigation: Case definition, Questionnaire used in epidemiological investigation,
Survey teams, Household survey, Collection of laboratory specimens, Laboratory techniques.
4. Analysis of Data: Clinical data and epidemiological data, mode or transmission, laboratory data,
interpretation of the data.
5. Control measures: definition of strategies and methods, evaluation, preventive measures.

20. Uses of report writing
• Health Administrator/Manager: for planning and implementation of control/
eradication strategies.
• Clinician: to update clinical knowledge & make alterations in case management &
diagnostic methods.
• Research: For clues & basis for further need oriented research programmes.
• General Population: for sense of security, health educational tool and to develop
confidence in the health care delivery system.

21. THANK YOU