Investigation of Outbreak
Objectives
1. Define the terms outbreak, epidemic and
pandemic
2. Discuss reasons that outbreaks occur
3. Understand the rationale for investigating
outbreaks and challenges associated with
investigating outbreaks
4. Describe the steps of an outbreak investigation
Case Study
• The investigation of 15 confirmed Salmonella cases in a
campus uncovered a nationwide outbreak associated
with consumption of ice cream that may have gone
undetected. This product was estimated to cause
224,000 cases of salmonellosis.
• Second, investigation may identify risk factors that are
associated with infection that are preventable.
Epidemiologic investigations of Escherichia coli
O157:H7 outbreaks have identified consumption of
foods such as pink hamburgers, unpasteurized apple
juice, that consumers may avoid to reduce their risk of
illness.
What do cantaloupe, frozen strawberries, ground
beef, milk, and oysters have in common??
They have all been implicated in foodborne
outbreaks in the past 5 years
E coli and spinach US 2006
E coli and cucumber Europe 2011
Salmonella and peanut butter US 2006-07
Vibrio and oysters US 2011
Listeriosis and cantaloupe US 2011
Don’t even need to be infectious…
Why Investigate Outbreaks?
• Prevention and control
• Research
• To learn more about a disease
• Public political legal concern
• Program considerations
• Training
Steps of an Outbreak Investigation
1. Establish existence of an outbreak
2. Verify the diagnosis
3. Define a case
4. Identify additional cases
5. Perform descriptive epidemiology
6. Develop and test hypothesis
7. Reconsider hypothesis
8. Perform additional studies if needed
9. Implement control measures
10.Communicate findings
• The steps described are in conceptual order.
• In practice, however, several steps may be done at the
same time, or the circumstances of the outbreak may
dictate that a different
• order be followed. For example, control measures
should be implemented as soon as the source and
mode of transmission are known, which may be early
or late in any particular outbreak investigation.
1- Establish existence of an outbreak
• The first thing you want to do is determine if the observed
numbers of cases exceed the expected level or baseline. This is
where your surveillance data comes in handy because it can give
you a good sense of the baseline
• You need to be aware of reasons that an increased number of
cases can be reported.
Be aware of artefactual causes of increases or decreases of reported
cases may be due to:
a change in reporting practice,
change in a case definition under surveillance,
and the availability of a new diagnostic test
2- Verify the diagnosis
• Diagnosis must be confirmed by reviewing the
clinical information available. This includes lab
results if available. Realize that not every case
has to have lab confirmation to be considered
part of the outbreak, but some of the initial
cases should have lab confirmation
• Interview cases and persons of interest
(potential cases)
3- Define a case
• Consists of standard elements
– Clinical information
– Time
– Place
– Person
• Can have varying degrees of certainty
– Suspect
– Probable
– Confirmed
Set of standard criteria for deciding whether an individual should be
classified as a case. Create a workable case definition with simple
and objective criteria
Case definition
• Can vary depending on the purpose
• Characterize the extent of the outbreak
• Determine risk factors or find a cause
Sensitive case definition to capture as many cases
as possible.
Narrow or specific case definition
Misclassification if case definition is not specific
non-cases counted as cases.
http://www.cdc.gov/osels/ph_surveillance/nndss/casedef/measles_2010.htm
4- Identify additional cases
• Mention active surveillance
• Regardless of the particular disease you are investigating, you should collect the following types of information
about every person affected:
• Identifying information: This may include name, address, and telephone number and allows you and other
investigators to contact patients for additional questions and to notify them of laboratory results and the outcome
of the investigation. Addresses also allow you to map the geographic extent of the problem.
• Demographic information: This may include age, sex, race, and occupation and provides the details that you need
to characterize the population at risk.
• Clinical information: This information allows you to verify that the case definition has been met. Date of onset
allows you to create a graph of the outbreak. Supplementary clinical information may include whether the person
was hospitalized or died and will help you describe the spectrum of illness.
• Risk factor information: Information about risk factors will allow you to tailor your investigation to the specific
disease in question. For example, in an investigation of hepatitis A, you would look at exposure to food and water
sources.
• Traditionally, we collect the information described above on a standard case report form, questionnaire, or data
abstraction form. We then abstract selected critical items in a table called a "line listing." In a line listing, each
column represents an important variable, such as name or identification number, age, sex, and case classification,
while each row represents a different case, by number. New cases are added to a line listing as they are identified.
This simple format allows the investigator to scan key information on every case and update it easily. Even in the
era of microcomputers, many epidemiologists still maintain a hand-written line listing of key data items and turn
to their computers for more complex manipulations of data. Here is a portion of a line listing that might have been
created for an outbreak of hepatitis A.
• In epidemiology, a Line Listing is one the most basic methods used to collect, display, and analyze outbreak
information.
• It is essentially a table that displays individual cases in rows, and their characteristics in columns. If you’ve ever
prepared a spreadsheet, you know the format.
5- Perform descriptive epidemiology
• data in terms of time
• data in terms of place
• data in terms of person
Data about Time
• Characterize the cases by plotting a graph or
epidemic curve that shows
– the number of cases (y-axis)
– over the time of onset of illness (x-axis) using an
appropriate interval
Epidemic Curve reflects:
• Size of the epidemic
• Relationship to endemic cases
• Outliers
• Time course
• Pattern of spread
• Where you are in the course of the epidemic
Epidemic Curve
• The epidemic curve represents in a graphic form the onset of cases of the
disease, either as a histogram, a bar graph, or a frequency polygon. The
frequency of new cases (or outbreaks) is plotted on the ordinate (y-axis) over a
time scale on the abscissa (x-axis).
A typical epidemic curve may have four segments:
1. the endemic level,
2. an ascending part,
3. a plateau,
4. a descending part, and, at times a secondary peak.
• The first limb of the curve which represents the endemic level, i.e. the expected
level of disease, should be drawn first. The actual epidemic curve is then
superimposed. An epidemic is said to prevail when the frequency of cases (or
outbreaks) in a population clearly exceeds the normally expected level for a
given area and season,
– x-axis displays time intervals
• Usually symptom onset but in this case on the above the dates
corresponded with date of hospital admission for pneumonia
• Choice of time interval is key
• ½ to ¼ the incubation period
http://www.wpro.who.int/NR/rdonlyres/7165D4BF-D820-4CB5-A1FD-267F905639D8/0/Slide1.jpg
Data about Place
• Geographic distribution of cases can provide
clues to source of the epidemic
– Water supplies
– Air flow patterns in buildings
– Ecologic habitat of vectors
• May help identify the vehicle or mode of
transmission
Data about Person
Look at the characteristics of cases regarding
• Personal characteristics
• Age
• Gender
• Race
• Medical status
Exposures
• Occupation
• Leisure activities
• Use of medications
• Tobacco
http://www.wpro.who.int/sites/csr/data/data_Graphs.htm
6- Develop and test hypothesis
• Develop hypothesis using descriptive epidemiology
– Person, place and time
– Clinical and laboratory findings
• Test hypothesis using analytic epidemiology
– Identify exposures significantly associated with illness
– Analytic epidemiologic study designs
*retrospective cohort study (relative risk )
* case-control study( Odds Ratio)
7- Reconsider hypothesis
• Confirm the hypothesis to clinical, laboratory and
epidemiologic facts
• If exposure histories for ill versus well are not
significantly different, develop a new hypothesis
8- Perform additional studies if needed
• Environmental studies
• Laboratory studies
• Narrowed case definition to look more closely at certain
risk factors
• Find more cases
• Define better the extent of the epidemic
• Evaluate new laboratory method
• Evaluate case-finding technique
– Improve sensitivity and specificity of case definition
– Degrees of exposure / dose response
• Environmental investigation
9- Implement control measures
• Prevent exposure
• Prevent infection
• Prevent disease
• Prevent death
Underlying purpose of all epidemic investigations is to control the spread
of disease and prevent morbidity and mortality
Quarantine and isolation of individuals to reduce spread
Removing contaminated food item
Vaccinate individuals / offer chemoprophylaxis if indicated
Control Measures for Influenza
10- Communicate findings
• Outbreak investigation staff
• Public health personnel
• Government officials
• Healthcare providers
• Media
• Community
Emergency Risk Communication
Principles
• Stop trying to allay panic
• Emphasize there is a process in place
• Don’t over reassure
• Acknowledge uncertainty
• Acknowledge people’s fears
• Express wishes
• Give people things to do
• Acknowledge the shared misery
• Give anticipatory guidance
• Address dreaded “what if” questions
• Be a role model and ask more of people
• Be regretful
Challenges in Outbreak Investigations
• Data sources
• Small numbers
• Specimen collection
• Publicity
• Reluctance to participate
Remember
• Steps of an outbreak investigation are a framework
• Not to be used as a “one size fits all” model
• Snow’s “shoe leather epidemiology” serves as a
model of critical thinking and public health action.
• Some steps may happen simultaneously
• Clear and consistent communication is key
References
• Gregg Field Epidemiology 3rd edition, 2008
• CDC
http://www.cdc.gov/foodborneoutbreaks/info
_healthprofessional.htm
• WHO
