Outbreak
Management
1. Describe the occurrence of disease in a
population.
2. Discuss how an epidemic curve can help in
managing an outbreak.
3. Outline the steps in outbreak investigation.
December 1, 2013
Learning Objectives
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• 60 minutes
December 1, 2013
Time involved
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• To Identify the causal agent
• To find the source of infection by studying
the occurrence of the disease among
persons, place, or time, as well as
determining specific attack rates
• Formulate recommendations to prevent
further transmission
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Purposes of Outbreak
Investigation
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• Outbreak or epidemic
• An excess over the expected (usual) level of a disease
within a geographic area
• However, one case of an unusual disease may
constitute an epidemic
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Definitions - 1
• E.g., postsurgical group A streptococcus infection
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• Endemic
• The usual level of a disease within a geographic area
• E.g., a hospital
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Definitions - 2
• These ‘sporadic’ infections represent most preventable
healthcare-associated infections
• Baseline incidence
• Pandemic
• An epidemic that spreads in several countries
• Usually affecting many people
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Number of Cases
of Disease
Endemic and Epidemic
“Endemic”
“Epidemic”
Time
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• Majority of healthcare-associated infections
are endemic
• Outbreak of healthcare-associated infections
occur typically in a specific group of patients
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Healthcare-associated infections
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• Relative risk
• A measure of association between a disease or
condition and a factor under study
• Calculation
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Definitions - 3
• Divide the incidence rate of those exposed to the factor by
the incidence rate of those not exposed
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RR =1
Incidence in the exposed
group is the same as in the
non-exposed
RR >1
Denotes a larger incidence in Exposure seems to
the exposed than in the non- increase the probability of
exposed
developing the disease
RR <1
Denotes a smaller incidence
in the exposed than in the
non-exposed
There is no association
between exposure and
disease
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Interpretation of Relative Risk
Exposure seems to
decrease the probability of
developing the disease
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• Community-acquired
• Food-borne infections, measles
• Healthcare-associated
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Types of Outbreaks
• When two or more cases of infection
appear to be epidemiologically related
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1. Verify existence of the outbreak
2. Determine if there were changes in case
finding or diagnostics
3. Establish diagnosis of reported cases
4. Case finding
5. Characterise cases
6. Formulate hypothesis
7. Test the hypothesis
8. Institute control measures and follow up
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Steps of Outbreak Investigation
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• Compare the number of current cases with the
usual baseline incidence
• from previous months or years
• If local data are not available, compare to
information from national surveillance systems
or the literature
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Verify existence of an outbreak
• these data may not be applicable to the local situation
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• New techniques or laboratory tests may increase
identification when historically cases would not
have been identified
• Provides a new ‘baseline’ of disease
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Determine if there were
changes in case finding or
diagnostics
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• Typically involves identifying an agent
• Define cases based on the following common
factors:
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Define cases
• Population risk factors
• Age, race, sex, socioeconomic status
• Clinical data
• Onset of signs and symptoms
• Frequency and duration of clinical features associated with
the outbreak
• Laboratory results
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• The inclusion and exclusion criteria for cases
must be precisely identified
• The definition should differentiate between
infection and colonisation
• Includes a unit of time and place and specific
biological and/or clinical criteria
• A graded definition (definite, probable, or
possible) often helps
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Case Definition
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Collect critical data and specimen information
from:
•
•
•
•
•
Laboratory reports
Medical records
Patient charts
Physicians and nursing staff
Public health data
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Case Finding
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• Assemble and organise available
information for analysis
• From this information, the population at
risk can be accurately described
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Characterise cases
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• The exact period of the outbreak
• The probable period of exposure
• Date of onset of illness for cases; draw an
epidemic curve
• Is the outbreak common source or
propagated?
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Characterise cases - Time
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Place
• Service, ward, operating room
• Clustering of cases
Person
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Characterise cases – Place/Time
• Patient characteristics (age, sex, underlying disease)
• Possible exposures (surgery, nursing and medical
staff , infected patients)
• Therapy (invasive procedures, medications,
antibiotics)
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• Incidence rate
• The number of new cases occurring in the
population during a specified time / number of
persons exposed to the risk of developing the
disease during that time
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Calculate rates
• Attack rate
• The cumulative incidence rate of infection in a
group over a period of an epidemic
• Number of people at risk who are infected / Total
number of people at risk
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• Common source (single-point source)
• Same origin
• Propagated or continuing source (on-going
transmission)
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Source of Infection
• Infections are transmitted from person to person
• Both common and propagated source
• Intermittent exposure to a common source
• Produces an epidemic curve with irregularly
spaced peaks
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• Also called a histogram
• Determines the character of an epidemic
• Cases are plotted by date of onset of illness
• Time intervals (on the X axis) are based on
the incubation or latency period of the
disease and the length of the period over
which cases are distributed
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Epidemic curve - 1
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The reasons for constructing an epidemic curve
include:
 To determine whether the source of infection was
common, propagated, or both
 To identify the probable time of exposure of the cases
to the source(s) of infection
 To identify the probable incubation period
 To determine if the problem is ongoing
December 1, 2013
Epidemic curve - 2
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December 1, 2013
A.
B.
C.
D.
Propagated source: single exposure, no secondary cases (e.g., measles).
Propagated source: secondary and tertiary cases (e.g., hepatitis A).
Common source: point exposure .
Common source: Intermittent exposure.
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Helps to develop hypotheses
•
•
•
•
•
•
•
Incubation period
Etiological agent
Type of source
Mode of transmission
Propagated source
Common source
Time of exposure
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Epidemic curve
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Common Source Outbreak
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• A common source of infection
• Exposure over a short period of time
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An explosive epidemic of short
duration generally indicates
• A large numbers of susceptible exposed
• A high attack rate
• An incubation period with little variation
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• Cases occur over a long period
• Explosive epidemics due to person-to-person
transmission may occur
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Propagated Source Outbreak
• E.g., chickenpox
• If secondary and tertiary cases occur, intervals
between peaks usually approximate to the
average incubation period
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December 1, 2013
Propagated source - 1
Single exposure,
no secondary cases (e.g. measles)
8
7
6
Cases
5
4
3
2
1
0
1
2
3
4
5
6
7
Week
8
9
10
11
12
30
Propagated source - 2
10
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Secondary and tertiary cases
(e.g., hepatitis A)
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8
Cases
7
6
5
4
3
2
1
0
1
2
3
4
5
6
7
Week
8
9
10
11
12
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• Using the mean or median incubation period
• Identify the peak of the epidemic or the date of onset
of the median case
• Count back into one incubation period
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Determine Exposure Period
• Using minimum and maximum incubation
periods
• Start with the first case identified and count back in
time the minimum incubation period
• Then using the last case, count back in time the
maximum incubation period
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December 1, 2013
Exposure Period
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• Make a best guess to explain the observations
• The hypothesis should explain most cases
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Formulate hypothesis
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• Many investigations do not reach this stage
• Investigation may end with descriptive
epidemiology and then the problem goes
away without intervention or does not
require a special study
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Test the Hypothesis
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• Control measures are determined by the
results of the initial analysis in consultation
with appropriate professionals
• They will vary depending on the agent, the
mode of transmission, and observations
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Institute Control Measures
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Interventions commonly used to control an
outbreak are as follows:
• Control the source of the pathogen
December 1, 2013
Control Measures
• Remove the source of contamination, e.g., discard
contaminated food
• Remove persons from exposure
• Keep people from being exposed to mosquito bites to
prevent encephalitis
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Type of transmission suspected
Suggested action
Contact- Cross-transmission (transmission between
individuals)
Patient isolation and barrier precautions determined by infectious
agent(s)
Contact- Hand transmission
Improvements in hand hygiene (e.g., washing, disinfection, glove
use)
Airborne agent
Patient isolation with appropriate ventilation
Waterborne agent
Checking of water supply and all liquid containers
Use of disposable devices
Foodborne agent
Elimination of the at-risk food
December 1, 2013
Healthcare-Associated
Infections – Control Measures
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• Cases cease to occur or return to endemic
level
• No change (re-evaluate cases)
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Evaluate Efficacy of Control
Measures
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• During the investigation, up-to-date information
is communicated to administration and
appropriate authorities
• A final report should be prepared describing the
outbreak, interventions, and effectiveness, and
summarising the contribution of each team
member participating in the investigation
December 1, 2013
Communicate and Write
a Final Report
• It should include recommendations to prevent any
future occurrence
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• Outbreaks of infection should be clearly defined,
identified, and promptly investigated because of
their importance in terms of morbidity, cost,
improvement of patient care, and institutional
image
• Proper steps and effective techniques should be
used to investigate a suspected outbreak
• Clear recommendations should be formulated to
prevent further transmission and/or outbreaks
December 1, 2013
Summary
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• Flora IH, Manuel B. Disease Outbreak Investigation. The Young
Epidemiology Scholars Program (YES) supported by The Robert
Wood Johnson Foundation and administered by the College
Board, 2005.
http://www.collegeboard.com/yes/ft/iu/home.html
• Reingold AL. Outbreak Investigations—A Perspective. National
Center for Infectious Diseases, Centers for Disease Control and
Prevention. Emerg Infect Diseases 1998; 4 (1).
http://www.cdc.gov/ncidod/eid/vol4no1/reingold.htm
• Susan P. Outbreak Investigation. The University of Texas,
Harris county psychiatric center, 2010.
http://www.uth.tmc.edu/uth_orgs/hcpc/procedures/volume1
/chapter6/infection_control-51.htm
December 1, 2013
References
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• How to Investigate an Outbreak
• http://www.cdc.gov/excite/classroom/outbreak/index.
htm
• Communicable Disease Control: Outbreak Investigation
• http://www.healthknowledge.org.uk/public-healthtextbook/disease-causation-diagnostic/2gcommunicable-disease/outbreak-investigation
• Epidemiological Investigation of Outbreaks
• http://www.gov.mb.ca/health/publichealth/cdc/proto
col/investigation.pdf
December 1, 2013
Web Resource
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1. Outbreak investigations are performed primarily to:
a.
b.
c.
d.
Gather clinical information about a disease
Find the source of infections
Make sure patients are treated
All of the above
December 1, 2013
Quiz
2. Epidemic curves help determine if the problem is on-going.
T/F
3. Control measures for healthcare-associated infections
include all except:
a.
b.
c.
d.
Patient isolation
Use of disposables
Hand hygiene
Immunisation
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• IFIC’s mission is to facilitate international networking in
order to improve the prevention and control of
healthcare associated infections worldwide. It is an
umbrella organisation of societies and associations of
healthcare professionals in infection control and related
fields across the globe .
• The goal of IFIC is to minimise the risk of infection within
healthcare settings through development of a network of
infection control organisations for communication,
consensus building, education and sharing expertise.
• For more information go to http://theific.org/
December 1, 2013
International Federation of
Infection Control
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