2. The concept….
Endemic
Disease occurring in a population regularly at an
usual level
Eg Tuberculosis, malaria
Epidemic
Unusual occurrence of disease clearly in excess
of its normal expectancy.
In a geographical location
At a given point of time.
Eg: Hepatitis, Measles, Typhoid
3. Outbreak versus epidemic
Occurrence of cases of illness in excess of the expected
numbers
Measure
Outbreak
Limited to a small area, within a district or a few blocks
Epidemic
Covers larger geographic areas
Linked to control measure instituted in the district/ state
Cluster
Merely a group of cases in a given area over a particular
period without regard to whether the number of cases is more
than expected.
No formal difference, you are the one
who decide how
much importance
you want to give to the event.
4. Investigating an
epidemic….why????
Epidemics or outbreaks signals some significant shift between
existing balance between agent, host and environment
Calls for thorough investigation to uncover the factors
responsible and…
…..Institute control measures to prevent further spread of
the disease
Role for research into a new laboratory method to diagnose
diseases
why some are more susceptible, what risk factors do they have?
To find out the limiting factor for outbreak of disease for which
an existing health programme is there.Keep the public’s confidence
5. Objectives of epidemic
investigation
Define the magnitude in terms of time, place and
person
Determine the particular conditions and factors
responsible
To identify cause/sources of infection and modes of
transmission to…
….Institute measures necessary to control the
epidemic
To make recommendations to prevent recurrence
6. Salient feature of an epidemic
investigation
Epidemic investigation is meant for Description
as well as Intervention.
Generally investigations are done at the peak of
the epidemic….hence largely Retrospective
The steps for investigation are not necessary to be
undertaken in the same sequence…..
7. 1. Verification of the
diagnosis
Initial report is not in the form of particular diagnosis but rather
in the from of a “syndromic” constellation of symptoms and
signs
It is therefore essential to verify the diagnosis on the spot, as
quickly as possible.
Clinical examination of a sample of cases should be done.
Laboratory investigation, where ever applicable boosts the
confirmation of diagnosis
Epidemiological investigation should not be delayed until lab
results available
It is not necessary to examine or
investigate all the cases to arrive at a
diagnosis
8. 2. Confirm the
existence of an
epidemic
Done by comparing the disease frequencies the
same period of previous years (generally past 3 years)
Epidemic confirmed when observed cases are clearly in
excess (more than 2SE) of that of previous experiences.
Some of the epidemics are easy to confirm eg. cholera, food
poisoning…..
…..whereas it is difficult to establish in case of
modern epidemics eg. HTN, CVD without comparison
Pragmatic considerations are also important as to whether to
investigate or not……
…a single case of a disease may be enough to call for
investigations eg. Polio, plague
9. 3. Defining the population at risk
(Initial line-listing)
a) Obtaining map of the area
Should contain information regarding natural landmarks,
roads and location of dwellings.
Areas divided according to the landmarks and the
dwellings numbered
a) Counting the population
Denominator is calculated of the affected area
Health workers can be engaged for this.
This is done to estabilish the population size of the area
Population censes helps in computing the attack rates in
the groups and rest of the population subsequently.
10. 4. Rapid search for all the cases
and their characteristics
a) Medical survey
Done to identify all the cases under medical attention and
also the exposed.
Trained Health workers employed to collect data by
administering “epidemiological case sheets”
b) Epidemiological case sheet
Designed based on rapid preliminary inquiry
Used to collect relevant information on
Identification
Occupation and social class
Travel and history of exposure
Time of onset and Signs and symptoms
Personal contacts
Events like attending parties or gatherings
History of reciept of any injections, blood products…etc
11. Cont….
c) Searching for more cases
Patients were enquired about other case in the vicinity
having onset within the incubation period of the index case
Cases admitted should also be taken into consideration
this data reveals the situation with finding of additional cases
and also explains person-to-person spread
Search of new (secondary) cases should be carried out
everyday, till the epidemic is over
This period is taken as the twice of the Incubation
period of the disease since the last case.
12. 5. Data analysis
Data so collected should be analyzed on ongoing basis
relating to….
a) Time
Epidemic curve designed and time clustering of cases sought
out, it suggests…..
i. Time relation with the exposure to source
ii. Type of epidemic( common source/ propagated)
iii. Trend of epidemic (cyclic/seasonal)
13. Cont…
b) Place
“Spot maps” prepared and if possible their relation to
the source of infection eg. Water supply
Geographic distribution (clustering) of cases
suggests source and spread of disease.
c) Person
Analysis of data according to age, sex, occupation
etc. is suggestive of susceptibility.
Attack rates/ case fatality rates for exposed and non-
exposed determined according to the host factors
Data analysis is to identify common events and
delineate the group involved
14. 6. Formulation of hypothesis
On the basis of Time-Place-Person distribution or…
…..Agent-Host-Environment model hypothesis is
formulated to explain….
Possible source
Causative agent
Environmental factors responsible
Formulation of tentative hypothesis
guides further investigation
15. 7. Testing of
hypothesis
All reasonable hypothesis should be considered
and weighed comparing attack rates with each
suspected factor
This helps to ascertain the hypothesis most
consistent with all known facts
Divergent hypothesis should also be checked out.
16. 8. Evaluation of the ecological
factors
Investigation of the circumstances should be made to prevent
further transmission
Possible ecological factors responsible for the epidemic
should be investigated…
Sanitary status of the eateries
Water and food supply
Population migration
Atmospheric changes like air and water pollution, temperature
and humidity
Vector bionomics and animal reservoir
Evaluation of the ecological factors helps to relate disease to
the environmental factors to know the sources of infection,
reservoir and mode of transmission
Out break can be studied in a case
control manner
17. 9. Further investigation of
population at risk
This involves study o a population at risk or a sample of it for
more information.
Involves medical examination, screening tests, biochemical
examination, examination of suspected food, faeces or blood
sample etc.
The approach may either be retrospective or prospective
Investigation of the healthy individuals will disclose the
classification of all members to…
a) Exposure to potential vehicle
b) Whether ill or not