INVESTIGATION
OF EPIDEMICS
By
Dr Sabah M.A.Abdelkader
Assist.Prof of public health
OBJECTIVES
After
completing this unit a
student will be able to:
Define the terms "epidemic"
and "outbreak"
Differentiate between pointsource and propagated
epidemics
Explain why health agencies
investigate reported epidemics
Discuss the steps of an
epidemic investigation
INTRODUCTION
 Occurrence
of an epidemic always
signals shift in existing balance
between agent, host and environment.
 This calls for a thorough investigation
of cases to uncover factors responsible
and to guide in control measures.
 Epidemiology has an important role in
investigation of epidemics.
OBJECTIVES OF INVESTIGATION
 A-
Define magnitude of outbreak in
terms of time, place, person.
 B- Determine particular conditions and
factors responsible for epidemic.
 C- Identify causes, sources of infection,
and modes of transmission to
determine control measures.
 D- Make recommendations to prevent
recurrence.
DEFINITIONS
 Epidemic:
 Is
the occurrence of more cases of
disease than would normally be
expected in a specific place or group of
people over a given period of time.
 One case of small pox today anywhere
in the world (except due to accidental
exposure in the labs which still store the
virus) will be considered an epidemic
because none is expected.
 Outbreak: Refers to a localized health
problem.
 Often used to denote local epidemic.
TYPES OF OUTBREAKS:
 Point
source: In which a simultaneous
exposure of many susceptible individuals
to a source of disease agent results in an
explosive increase in the number of cases
of the disease over a short period of time,
e.g. outbreak of salmonella food poisoning.


CONT.:
 Propagated:
In which an infectious
agent is propagated in a community by
passage from person to person, e.g.
outbreak of measles. Transmission
continues until the number of susceptible
is depleted or susceptible individuals are
no longer exposed to infected persons.

CONT.
Point source
Propagated
The epidemic curve rises Epidemic curve rises and
and falls rapidly.
falls gradually.
Explosive,
there
is Cases occur over a much
clustering
of
cases longer period.
within a narrow interval
of time.
All cases develop within Cases occur within more
one incubation period of than one incubation
disease.
period of the diseases.
STEPS OF AN OUTBREAK INVESTIGATION
Confirm diagnosis.
 2. Confirm existence of outbreak.
 3. Define a case and count cases.
 4. Orient data in person, time, place.
 5. Determine who is at risk.
 6. Develop a hypothesis and test it.
 7. Determine control measures.
 8. Plan a more systematic study.
 9. Implement control measures.
 10.Prepare a written report.

1.
CONT.
 N.B:
The steps are presented here in a
logical order.
 In practice, however, several may be
done at the same time, or they may be
done in a different order.
 For example, control measures should be
implemented as soon as the source and
modes of transmission are known, which
may be early or late in any particular
outbreak investigation.
CONT.
 Each
of these steps is important to
complete before moving to next steps.
 It is a process where each step is
dependent on successful completion in
previous steps.
STEP 1: VERIFY THE DIAGNOSIS
 First,
You must ensure that the problem has
been properly diagnosed- that it really is
what it has been reported to be.
 Second, You must to be certain that the
increase number of diagnosed cases is not
the result of a mistake in the laboratory.
 Verifying the diagnosis requires that you
review the clinical findings (the symptoms
and features of illness) and the laboratory
results for the people who are affected.
 Finally, you should visit several of the people
who became ill. You should see and talk to
some of them to gain a better understanding
of the diseases and in addition, you may be
able to gather critical information by asking
such questions as:
CONT.
 What
was their exposure before becoming
ill?
 What do they think caused their illness?
 Do they know anyone else with the
disease?
 Do they have anything in common with
others who have the disease?
 Conversations with patients are very
helpful in generating hypotheses about
the cause, source, and spread of disease.
STEP 2: ESTABLISH THE EXISTENCE
OF AN OUTBREAK:
 Compare disease frequency during same
period of previous years.
 For a notifiable disease, or you can use
health department surveillance records.
 For other disease, you can find data from
local sources such as hospital discharge
records, death (mortality) records, and
cancer or other health registries.
CONT.
 If local data are not available, you can
make estimates using data from:
A-National data.
 B-You might consider conducting a
telephone survey of physicians to
determine whether they have seen more
cases of the disease than usual.
 C- You can also conduct a survey of
people in the community to establish the
background level of disease.
 D- Epidemic threshold: any amount of
disease greater than the upper limit.

STEP 3: DEFINE AND IDENTIFY CASES
A
case definition includes four components:
 Clinical information about the disease. The
presence of fever of at least 38oC.
 Characteristics about the person who is
affected, You might restrict the definition to
those who attended a wedding, or ate at a
specific restaurant.
 Information about the location or place, e.g.
living in a certain area or working at a
particular plant.
 Specification of time during which the
outbreak occurred, the criterion might be
onset of illness within the past 2 months.
TYPES OF INFORMATION TO BE COLLECTED:
 Identifying
information: name, address, and
telephone number. Addresses also allow you to
map the geographic extent of the problem.
 Demographic information: age, sex, race,
and occupation. This provides the details that
you need to characterize the population at risk.
 Clinical information: to verify that the case
definition has been met. Date of onset allows
you to create a graph of the outbreak.
 Risk factor information: to tailor your
investigation to the specific disease in question;
e.g. in hepatitis A, you would look at exposure
to food and water sources.
A LINE LISTING FOR AN OUTBREAK OF
HEPATITIS A
Diagnostic
Signs and Symptoms
Lab
Physician
Case #
Initials
Report
Onset
Diagnosis
N
V
A
F
DU
J
HAIgM
Other
Age
Sex
1
JG
10/12
12/6
Hep A
+
+
+
+
+
+
+
SGOT
37
M
2
BC
10/12
10/5
Hep A
+
-
+
+
+
+
+
Alt
62
F
3
HP
10/13
10/4
Hep A
+
-
+
+
+
S*
+
SGOT
30
F
4
MC
10/15
10/4
Hep A
+
-
+
+
?
-
+
17
F
5
NG
10/15
10/9
NA
+
-
+
-
+
+
NA
32
F
6
RD
10/15
10/8
Hep A
+
+
+
+
+
+
+
38
M
7
KR
10/16
10/13
Hep A
+
-
+
+
+
+
+
43
M
HbS/
Ag-
NA
SGOT
=240
STEP 4: DESCRIBE AND ORIENT THE DATA
(DESCRIPTIVE EPIDEMIOLOGY)
A- Time:
 An epidemic curve provides a great deal of
information:
 1. outbreak’s magnitude. where you are in the
course of the epidemic, and possibly to project its
future course.
 2. Identifying disease and its usual incubation period,
you may be able to estimate a probable time period
of exposure and develop a questionnaire focusing on
that time period.
 3. Finally, you may be able to draw inferences about
the epidemic pattern, for example, whether it is an
outbreak resulting from a common source exposure,
from person-to-person spread, or both.
HOW TO DRAW AN EPIDEMIC CURVE?
 To
draw an epidemic curve, you first
must know the time of onset (or date of
onset) of illness for each person.
 The number of cases is plotted on the Yaxis; the unit of time on the X-axis.
 Show the pre- and post-epidemic period
on your graph to illustrate activity of
disease during those periods.
B – PLACE:
 Assessment
of an outbreak by place
provides information on the geographic
extent of a problem and may also show
clusters or patterns that provides clues to
the identity and origins of the problem.
 A simple and useful technique for looking
at geographic patterns is to plot, on a
“spot map” of the area where the
affected people live, work, or may have
been exposed.
 Based on map, calculate attack rate &
secondary attack rate ?????
C – PERSON:
 You
determine what populations are at risk for
the disease.
 Personal characteristics (e.g. age, race, sex, or
medical status) or exposures (e.g., occupation,
leisure activities, use of medications, tobacco,
and drugs) are important factors because they
may be related to susceptibility to the disease
and to opportunities for exposure.
 For example, if you were investigating an
outbreak of hepatitis B, you should consider the
usual high-risk exposures for that infection,
such as intravenous drug use, sexual contacts,
and health care employment.
STEP 5: DETERMINE WHO IS AT RISK.
 Where did cases come from?
 Identify population with same criteria of
cases:
 Geographic location.
 Time period.
 Population characteristics.
 Look for any cases in population at risk.
 Population at risk is important because:
 They will be used as controls to test
hypothesis.
 This
population will be target for
prevention measures.
STEP 6: DEVELOP HYPOTHESIS AND TEST IT
 You
can develop hypotheses in a variety of
ways:
First, consider what you know about the disease
itself.
 What is the agent’s usual reservoir?
 How is it usually transmitted?
 What vehicles are commonly implicated?
 What are the known risk factors?
 Test hypothesis using a suitable study design:

Retrospective study to compare cases to controls
 Possible prospective study if exposure is still present.

CONT.
Case-control studies compare people with a
disease (case-patients) with a group of people
without the disease (controls).
 You then can calculate measure of association –
called an odds ratio- to quantify the relationship
between exposure and disease.

Cohort studies compare groups of people who have
been exposed to suspect risk factors with groups who
have not been exposed.
 Calculate incidence rate, relative risk of exposure
to a certain suspected item can finally lead to
incrimination of the exposure responsible for the
epidemic.

STEP 7: DETERMINE CONTROL MEASURES
In common source epidemics, cause may no longer
exist. E.g ?????
 For an outbreak in which causative agent is still a
risk, it is important to consider and institute
control measures to stop current outbreak:

destroying implicated food,
 shutting down contaminated water source.


In propagated epidemic:
treating carriers,
 vaccinating population at risk.


Control measures are dependent on identified
source of outbreak.
STEP 8 : PLAN A MORE SYSTEMATIC STUDY
AND
REFINE HYPOTHESIS
Comparison of the hypotheses with the
established facts you would use this method
when your evidence is so strong that hypothesis
does not need to be tested.
 Analysis epidemiology used when the cause is
less clear. With this method, you test your
hypotheses by using a comparison group to
quantify relationship between various exposures
and the disease (cohort studies and casecontrol studies).
 When analytic epidemiological studies do not
confirm your hypotheses, you need to reconsider
your hypotheses and look for new vehicles or
modes of transmission.
 This is the time to meet with case- patients to
look for common links.

CONT.
Goal in step 6 was to quickly find source of
outbreak to be able to control.
 In outbreak, the quickest study design is
retrospective because there is access to a group of
cases from outbreak.
 However, investigator can perform prospective
study beginning with healthy individuals with
their exposure initially identified.
 Prospective
study allows testing control
measures working or not, to make additional
prevention interventions.

STEP 9: IMPLEMENTING CONTROL AND
PREVENTION MEASURES :
 Control
measures, which can be implemented
early if you know the source of an outbreak,
should be aimed at specific links in the chain of
infection, the agent, the source, or the reservoir.
 For example, an outbreak might be controlled by
destroying contaminated foods or exclusion of
food handler (diseased or carrier) from work until
he or she is well after treatment.
 In some outbreaks, you would direct control
measures at reducing susceptibility, example
immunization against rubella.
CONT.

Same activities as step 7 but these activities
should be ongoing and constant.
Conduct training programs.
 Keep surveillance for future disease.
 Communicate with population and health officials to
look for further symptoms of disease.

STEP
 An
10: PREPARE A WRITTEN REPORT
oral briefing for local health authorities
and people responsible for implementing control
and prevention.
 A written report: It serves as:
 A- A record of performance.
 b- A document for potential legal issues.
 c- A reference if the health department
encounters a similar situation in the future.
 d- Finally, it contributes to the scientific
knowledge of epidemiology and public health.
CONTENT OF REPORT
Description of settings
 Documenting methods
 Presenting results
 Documenting specific causative agent and source
 Listing recommendations for prevention
