The Principles of Outbreak
Epidemiology
By
Dr Abhinav Sinha MBBS, MD
Author
Currently doing MD in Community
Medicine at the Department of Community
Medicine, NSCB Medical College,
Jabalpur, India.
Passed MBBS from the same institute with
Gold Medals in Human Physiology and
Biochemistry in 2000.
Email- aspsm2000@yahoo.co.in
Interest & Motivation
During the last two and a half years of my
postgraduation, I had an opportunity to witness
and investigate 3 or 4 of different kinds of
outbreaks ranging from Food poisoning to
Gastrointestinal to Pyrexia of Unknown Origin to
Cholera. What aroused an interest in Outbreak
Investigation was the scientific cum artistic way of
handling such outbreaks. It is more of an art than a
pure science. When I came to know about the
Supercourse, I decided to contribute or rather share
my little experience with the world in the field of
Outbreak Epidemiology. I hope you will enjoy this
so called “lecture”.
Learning Objectives
The main motives behind this lecture
are to develop the basic concept in
investigating an epidemic, the need to
recognize the urgency behind it and
to orient the students toward the art
and science of outbreak investigation
and epidemiology.
Performance Objectives
After going through this lecture, the
students should be able to perform the
initial investigation of many smaller
outbreaks that occur so much frequently
in their vicinity that may often pass
unrecognized. Also, they should be able
to differentiate the epidemic and the
endemic fluctuations in the frequencies
of a disease.
Definitions
1. Outbreak: Sudden occurrence of an
epidemic
in relatively limited
geographic area. While an outbreak is
usually limited to a small focal area,
an
epidemic
covers
larger
geographical areas & has more than
one focal point.
2. Outbreak Epidemiology: Study of a
disease cluster or epidemic in order to
control or prevent further spread of
the disease in the population.
Field Epidemiology
A definition has been proposed by
Goodman. The essential elements are:
1. The problem is unexpected
2. An immediate response may be
necessary
3. Epidemiologists must travel to &
work on location in the field
4. The extent of investigation is likely to
be limited because of imperative for
timely intervention
Objectives of OI
1. Primary- to control the spread of
disease
2. To determine the causes of disease,
its source & mode of transmission
3. To determine who is at risk
4. To determine what exposures
predispose to disease
5. To know magnitude of the problem
Objectives continued….
To identify new agent
7. To determine the effectiveness of
control measures
8. To identify methods for present &
future prevention & control
9. Research & training opportunities
10. Public, Political and legal concerns
6.
Unique aspects of OI
1. There is a pressure & urgency to
conclude the investigations quickly
which may lead to hasty decisions.
2. Data sources are often incomplete &
less accurate.
3. Decreased statistical power due to
analysis of small numbers.
4. Publicity surrounding the
investigation – community members
may have preconceived ideas.
The pace & commitment of OI
There is often a strong tendency to
collect what is “essential” in the field &
then retreat to “home” for analysis. Such
premature departure reflects lack of
concern by the public, makes any further
data collection or direct contact with the
study population difficult. Once home,
the team loses the urgency & momentum
& the sense of relevancy of the
epidemic. Don’t leave the field without
final results & recommendations.
1.
2.
3.
4.
5.
6.
7.
Trigger events & Warning Signals
Clustering of cases/deaths in time/space
Unusual increase in cases/deaths
Shift in age distribution of cases
High vector density
Acute hemorrhagic fever or acute fever
with renal involvement/altered
sensorium
Severe dehydration following diarrhea
in patients above 5 years age
Unusual isolate
Diseases requiring investigations
1. Endemic diseases with epidemic
potential – malaria, cholera, measles,
hepatitis, meningococcal meningitis
2. Even a single case of diseases for which
eradication/elimination goals have been
set – polio, guineaworm and yaws
3. Rare but internationally important
diseases with high case fatality rates –
yellow fever
4. Outbreaks of unknown etiology
General lines of action
The basic general lines of action during
epidemics include – Preparedness and
Interventions (investigations). Success
in dealing with an epidemic depends
largely on the state of preparedness
achieved in advance of any action. It
would be an error to consider as an
epidemic, a hitherto unrecognized
endemic situation or a mere seasonal
increase in the incidence of a disease.
Preparedness
1. Identify a nodal officer at state/district
level
2. Strengthen routine surveillance system
3. Constitute rapid response teams
4. Train medical & other health personnel
5. Prepare a list of laboratories
6. List the “high risk” pockets
7. Establish rapid communication network
8. Undertake IEC activities
9. Ensure availability of essential supplies
10.Setup inter-departmental committees
Investigations
1. Recognition & response to a request for
assistance
2. Check initial information
3. Formulate a plan of action
4. Prepare for field work
5. Confirm the existence of epidemic
6. Verify the diagnosis
7. Identify & count cases/exposed persons
8. Orient data in terms of person, place &
time
Investigations continued…..
9. Choose a study design
10.Collect specimens for lab analysis
11.Conduct environmental investigations
12.Formulate & test hypotheses
13.Implement control measures
14.Conduct additional studies
15.Prepare a written report
16.Communicate the findings
Step 1 – Recognition & Response
If the local health officials request
assistance, the regional epidemiologist
should try to acquire as much
information about the disease and the
population at risk as possible. It is also
important to find out why the request is
coming – need extra hands?, unable to
uncover the details of the disease in
question?, share the responsibility?, or
legal or ethical issues?
Step 2 – Check initial information: As
soon as the initial information on an
outbreak reaches, the regional health
coordinator must determine whether the
information is correct.
Step 3 – Formulate a plan of action:
The plan should be based on situational
analysis & taking technical, economical
& political factors into account.
Step 4 – Prepare for field work:
Identify the team members & assign
responsibilities.
Composition of typical field team
Specialists
1. Epidemilogist
2. Clinician (pathologist)
3. Microbiologist
4. Veterinarian
5. Entomologist
6. Mammalogist
7. Sanitary engineer
8. Toxicologist
9. Information Specialist
Auxillaries
1. Nurses
2. Specialist assistants
3. Secretary/Interpreter
4. Driver
Step 5 – Confirm the existence
Are there cases in excess of the baseline
rate for that disease & setting? The
excess frequency should be found out
with Epidemic Threshold Curve. The
periodic frequency for previous 3 years
is plotted on a graph. Another graph at
mean + 2SD level is superimposed on it.
Any fluctuations beyond these 2 graphs
should be treated as epidemic
fluctuations (method of moving
averages).
Step 6 – Verify the diagnosis
The initial report may be spurious &
arise from misinterpretation of the
clinical features. This involves a review
of available clinical & lab findings that
supports the diagnosis. Do not apply
newly introduced, experimental or
otherwise not broadly recognized
confirmatory tests at this stage. 1520% of the suspected cases may be lab
confirmed.
Step 7 - Identify & count cases
Identify additional cases not known or
reported initially. The case definition
must be precise but not too exclusive..
Persons who meet the case definition
should be “line-listed”. Also, identify the
population at risk or the exposed
persons, places where the cases live,
work & have traveled to, & the possible
exposures that might have lead to the
disease.
Search for the source of infection
The main purpose here is to eliminate,
terminate or isolate the source. The
steps involved are – identify the time of
disease onset, ascertain the range of
incubation periods & look for the
source in time interval between the
maximum & the minimum IPs. In
outbreaks
with
person-to-person
transmission, all the contacts of the
index case are to be searched (contact
tracing).
Step 8 – Compile & Orient data
Identify when patients became ill (time),
where patients became ill (place) &
what characteristics the patients possess
(person). The earlier one can develop
such ideas, the more pertinent &
accurate data one can collect.
(a) Time: The epi-curve gives the
magnitude of outbreak, its mode of
spread & the possible duration of the
epidemic. The unit of time on X-axis are
smaller than the expected incubation
period of the disease.
(b) Place: It provides major clues
regarding the source of agent and/or
nature of exposure. Spot maps show a
pattern of distribution of cases.
(c) Person: Examine characters such as
age, sex, race, occupation or virtually
any other character that may be useful
in portraying the uniqueness of case
population.
Step 9 – Choose a Study Design
The design (Case-control, Cohort &
Case-cohort) is chosen based on size &
availability of the exposed population,
the speed with which the results are
needed & the available resources. The
study design that is chosen will then
dictate the appropriate analysis &
hypothesis testing.
Step 10 – Perform Lab analysis
It consists of collecting & testing
appropriate specimens. To identify the
etiologic agent, the collection need to be
properly timed. Examples of specimens
include - food & water, other
environmental samples (air settling
plates), and clinical
(blood, stool,
sputum or wound) samples from cases &
controls.
Step 11 – Environmental Investigation
A study of environmental conditions &
the dynamics of its interaction with the
population & etiologic agents will help
to formulate the hypothesis on the
genesis of the epidemic. Such actions
assist in answering How? And Why?
questions.
Step 12 – Formulate & Test Hypothesis
As soon as the preliminary data indicate
the magnitude & severity of the
outbreak, a hypothesis should be made
regarding time, place and person; the
suspected etiological agent & the mode
of transmission. Risk specific attack
rates are calculated & compared &
relative risk/odds ratio is calculated.
Important points
1. Rare disease assumption: The OR &
RR approximate each other if the attack
rates is less than 5% but the attack rates
are much higher in outbreaks.
2. To correct for multiple comparisons, the
most effective approach is to lower the
p-value according to the number of
comparisons made.
Step 13 – Control measures
Simultaneous to data collection &
hypothesis formation, steps should be taken
to contain the epidemic. These measures
depend upon knowledge of etiologic agent,
mode of transmission & other contributing
factors. Protective measures are necessary
for patients (isolation & disinfection), their
contacts (quarantine) and the community
(immunization, etc).
Step 14 – Additional studies
Because there may be a need to find
more patients, to define better the
extent of the epidemic, or because a
new lab method or case finding
method may need to be evaluated, the
epidemiologists may want to perform
more detailed & carefully executed
studies.
Step 15 – Prepare Written Report
The final responsibility of the investigative
team is to prepare a written report to
document the investigations, findings and
the recommendations. The written report
should be submitted, in a standardized
format, to the public health authorities
including the ministry of health & remain
confidential until it has been given official
permission.
Step 16 – Communicate findings
Communicating the investigative findings
clearly is essential. All public health
officers will benefit if the experience
acquired by the investigative team is
shared by the publication of an account of
the outbreak. As a rule, the epidemiologist
informs those who reported the first cases
of the epidemic first.
Step 17 – Post-epidemic Measures
The efficacy of control measures should
be assessed day by day during the
outbreak, a final assessment being made
after it has ended. This will provide a
logical
basis
for
post-epidemic
surveillance & preventive measures
aimed at avoiding the repetition of
similar outbreaks.
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