Transcript Foodborne Disease Outbreak Investigation Team Training: Module 3 – Preliminary Investigation of an Outbreak Preliminary investigation.

Foodborne Disease Outbreak
Investigation Team Training:
Module 3 – Preliminary
Investigation of an
Outbreak
Preliminary investigation
1
Module Learning Objectives
At the end of this module, you will be able to
1. Describe the initial steps of an outbreak
investigation including
• Verify the diagnosis
• Search for additional cases
• Create a case definition
• Generate a hypothesis about the source
2. Develop a case definition for an outbreak.
3. Generate a hypothesis about the source of an
outbreak.
4. Prioritize an outbreak for further investigation.
Preliminary investigation > Learning objectives
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Step 1: Verify Diagnosis
Purpose
• Make sure illness properly diagnosed
• Rule out laboratory and reporting errors
• Determine that all cases suffer from same illness
Preliminary investigation > Verify diagnosis
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Ways to Verify Diagnosis
•
•
•
•
•
Obtain clinical samples for laboratory testing
Review medical records and laboratory results
Talk with health-care providers of cases
Interview (and even examine) cases
Consult a qualified microbiologist
Preliminary investigation > Verify diagnosis
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Laboratory Error – Example

• Increase in intestinal amebiasis (Entamoeba
histolytica infection) in Los Angeles
• 38 cases in 4 months (usual: one per month)
• Investigation showed no common exposures
• Diagnostic slides reexamined
E. histolytica
White blood cells
Preliminary investigation > Verify diagnosis
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Step 2: Search for Additional Cases
• Identified cases often “tip of the iceberg”
• Might not represent all cases associated with
outbreak
• Need to actively search for additional cases to
– Determine true magnitude of outbreak
– Characterize outbreak accurately
– Increase ability of epidemiologic studies to
link illness with true cause of outbreak
Preliminary investigation > Search for cases
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
Why Search for Additional Cases?
= Female
= Male
First cases detected (N=10)
Female
80%
All cases (N=20)
50%
Preliminary investigation > Search for cases
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Ways to Search for Additional Cases

• Contact implicated establishment to identify other ill
customers (e.g., reservations, credit card receipts)
• Contact event organizer for list of attendees
• Review foodborne illness complaints and notifiable
disease reports
• Examine laboratory reports and hospital medical
records
• Ask local health-care providers to look for cases
• Ask known cases if they know of other cases
• Review death certificates
• Make announcements through local media
Preliminary investigation > Search for cases
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Step 3: Develop a Case Definition
• Standard set of criteria used to classify ill people
as being cases associated with a particular
outbreak
• Criteria include
– Clinical findings (e.g., signs, symptoms,
laboratory results)
– Restrictions by time, place, and person
• Different from a clinical diagnosis, used for
epidemiologic purposes only
Preliminary investigation > Develop case definition
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Case Definition - Example
For outbreak of salmonellosis at child care center
Clinical criteria
• ≥3 loose stools in a 24-hour period OR
• stool culture that yielded Salmonella Javiana
Restrictions
• Time: onset from October 24-30
• Place: attended/worked at Child Care Center X
• Person: excludes siblings of initial case in a
family (if onset of illness after initial case)
Preliminary investigation > Develop case definition
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Case Definition Criteria
• Objective – Not open to interpretation
• Discriminating – Distinguish between individuals
with illness associated with outbreak and those
with illness not associated with the outbreak
Preliminary investigation > Develop case definition
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Class Question

Which criterion is more objective (not open to
interpretation)?
An oral temperature
“Fever”
OR
of 100.4 or higher
Which criterion is more discriminating (more likely to
distinguish between illness associated with an outbreak and illness not associated with that outbreak)?
≥3 loose stools in a
24-hour period
OR
A stool culture that
yielded Salmonella
Preliminary investigation > Develop case definition
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Case Definition

• Does not include suspected source of outbreak
(i.e., hypothesis you are trying to test)
• Can reflect different levels of certainty that
person has disease associated with outbreak
– Typical signs and symptoms only (“probable
case”)
– Inclusion of laboratory testing or subtyping
(“confirmed case”)
• Can change over time as more information
about the illness or outbreak is revealed
Preliminary investigation > Develop case definition
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Class Question
An outbreak of gastrointestinal illness occurred
following a birthday party at a private home on
June 10. At the party, a meal was served including
potato salad, hamburgers, and ice cream. Ten
guests reported becoming ill. Ill persons reported
vomiting, diarrhea (3 or more loose stools in a 24hour period), fever, and headache.
Develop a case definition for illness associated with
this outbreak. A line list of ill guests is provided at
the end of this module.
Preliminary investigation > Develop case definition
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Class Question (cont’d)
#
Sex Age Onset Vomiting Diarrhea* Fever
Headache
Salmonella
Positive Stool
Ice
Culture
Salad Burger Cream
1
F
14
6/12
Y
Y
Y
N
Y
Y
Y
Y
2
F
5
6/13
Y
Y
N
N
NA
Y
N
Y
3
M
16
6/12
Y
Y
Y
N
NA
Y
Y
Y
4
F
18
6/10
N
Y
Y
Y
N
N
Y
Y
5
M
16
6/14
Y
Y
N
N
NA
Y
Y
Y
6
M
15
6/11
Y
N
N
N
NA
N
N
Y
7
M
15
6/13
Y
Y
N
N
NA
Y
Y
Y
8
M
17
6/12
N
Y
N
N
NA
Y
N
Y
9
F
16
6/20
N
Y
N
N
Y
N
Y
Y
10
F
14
6/12
N
N
Y
Y
NA
Y
N
Y
*Three or more stools in a 24-hour period
NA=culture not performed
Preliminary investigation > Develop case definition
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Class Question (cont’d)

Looking at the line list, what symptoms are reported
among ill persons (and what is their frequency)?
Which signs and symptoms would you include in
the case definition for the outbreak?
Preliminary investigation > Develop case definition
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Class Question (cont’d)

Two ill people had stool cultures positive for
Salmonella. Would you include a positive stool
culture in the case definition?
Would you include restrictions by time, place, and
person in the case definition?
Preliminary investigation > Develop case definition
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Class Question (cont’d)

All of the ill people ate ice cream. Would you
include ice cream in the case definition?
Preliminary investigation > Develop case definition
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Step 4: Generate Hypothesis
• Using available information to make an educated
guess about the cause and source of an
outbreak
• Purpose
– To direct immediate control measures
– To narrow focus of subsequent studies
– To determine the need to involve others in
investigation
• Undertaken by entire team
Preliminary investigation > Generate hypothesis
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Hypothesis
• Includes likely causative agent, people at risk,
mode of transmission, vehicle, and period of
interest
• Example:
“The outbreak is due to a bacterium that was
spread during the first week of November by a
food commonly consumed by children.”
Preliminary investigation > Generate hypothesis
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Key Sources of Information
• Basic information about causative agent
• Information on implicated facility or food
• Descriptive epidemiology (i.e., describe cases by
time, place, and person)
• Case interviews (“hypothesis-generating
interviews”)
Preliminary investigation > Generate hypothesis
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Basic Information about Causative Agent
•
•
•
•
Common reservoirs of causative agent
Vehicles in past outbreaks
Growth requirements for causative agent
Incubation period (time from exposure to onset
of illness)
Preliminary investigation > Generate hypothesis
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References on Causative Agents
• APHA Control of Communicable Diseases Manual
• IAFP Procedures to Investigate Foodborne Illness
(Table B)
• AMA “Diagnosis and Management of Foodborne
Illnesses: A Primer for Physicians”
• FDA Bad Bug Book (online)
• CDC A-Z Index (online)
Preliminary investigation > Generate hypothesis
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Information on Implicated Facility
• Foods produced or served; production, processing,
distribution methods; past food safety problems
• Help identify high risk foods, likely causative agent,
and contributing factors
• Sources of information
– Online menus
– Paperwork from past inspections, Hazard
Analysis and Critical Control
Point risk assessment, facility
plan review
– Regulatory inspector
Preliminary investigation > Generate hypothesis
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Descriptive Epidemiology
• Simple characterization of outbreak by
– Time
– Place
– Person
• Can provide clues about the mode of
transmission and vehicle
• Comparison group usually needed to put findings
in perspective
Preliminary investigation > Generate hypothesis
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Time – Onset of Illness
Number of Cases
• Time or date of onset of symptoms
• Relates back to likely period of exposure
• Typically presented as epidemic curve (epi
curve)
6
= one case
5
4
3
2
1
1/11
1/13
1/15
1/17
Date of Onset of Illness
Preliminary investigation > Generate hypothesis
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Uses of Epi Curve
•
•
•
•
Grasp magnitude of outbreak
Clarify outbreak’s time course
Identify cases that are outliers
Draw inferences about pattern of spread
– Point source outbreak
– Continuous common source outbreak
– Propagated outbreak
Preliminary investigation > Generate hypothesis
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Point Source Outbreak

• Exposure to same source over brief time
• Cases rise rapidly to a peak and fall off gradually
• Majority of cases within one incubation period
Cryptosporidiosis cases by date of onset of illness, June
one incubation
period = 7 days
Preliminary investigation > Generate hypothesis
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Continuous Common Source Outbreak 
• Exposure to same source over prolonged time
• Epidemic curve rises gradually
• May plateau
Infections with Salmonella Enteritidis by date of onset
of illness, January and February.
one incubation
period = 2-3 days
January
February
Preliminary investigation > Generate hypothesis
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
Propagated Outbreak
• Spread from person to person
• Series of progressively taller peaks
• Peaks one incubation period apart
Measles cases by date of onset of illness, Aberdeen,
South Dakota, October 15 – January 16
one incubation
period = 10 days
October
November
December
Preliminary investigation > Generate hypothesis
January
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Class Question

Thirty seven cases of
campylobacteriosis
were identified
among children
attending a summer
camp. Based on the
epi curve, what is the
most likely means of
spread: point source,
continuous common source, or propagated? (average
incubation period 3-5 days)
Preliminary investigation > Generate hypothesis
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Place
• Residence of cases (typically)
• Distribution reveals clues about source of
outbreak
– Over broad area  commercial product with
wide distribution
– Clustering  locally sold product, point
source, or person-to-person spread
– Concentrated areas with outliers  travel to
affected area or importation of product
• Typically presented as spot map
Preliminary investigation > Generate hypothesis
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Place – Example

• Outbreak of Salmonella
Typhimurium with
unique PFGE pattern in
San Diego
• 50 cases among
Mexicans and Mexican
Americans
• Spot map of
households of cases
Preliminary investigation > Generate hypothesis
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Place – Example (cont’d)

Percent Hispanic
population
3-20%
21-39%
40-61%
62-97%
Preliminary investigation > Generate hypothesis
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
Place – Example
• Outbreak associated with Mexican-style soft
cheese sold by street vendors; four vendors cited
Case
Vendor #1 route
Vendor #1 stops
Vendor #1
Preliminary investigation > Generate hypothesis
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Person
• Age group, sex, and other characteristics
• Influence individual’s susceptibility to illness or
opportunities for disease exposure → can provide
clues to source of outbreak
• Typically presented as percentage of all cases or
rate among affected population
Preliminary investigation > Generate hypothesis
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Person – Example

• E. coli O104 outbreak in Germany with many
cases of hemolytic uremic syndrome (HUS)
• 88% of HUS cases ≥20 years (usually 1-10%)
• 71% of HUS cases female (usually around 50%)
• Think “girl food”
• Source of outbreak sprouts from fenugreek seeds
from Egypt
Preliminary investigation > Generate hypothesis
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Descriptive Epidemiology Clues
• Person-to-person transmission
– Clustering in social units
– Localized to one part of community
– Occurrence of cases in waves
• Transmission by public drinking water
– Widespread illness
– Affecting both sexes and all age groups
– Distribution consistent with public water system
• Transmission by food
– Increased risk among certain groups
– Distribution similar to distribution of foods
Preliminary investigation > Generate hypothesis
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Class Question

Based on the following case characteristics, what
mode of transmission seems likely in each outbreak?
Mode of
Transmission
Cases live in one community; most
<3 years of age; attend same child
care center; onset of cases in waves
Majority of cases live in city limits,
age range 1-75 years; 52% female
Cases live in two states; high
proportion 20-29 years; 65% male
Preliminary investigation > Generate hypothesis
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Hypothesis-Generating Interviews
• Extensive exploration of illness and exposures
with cases
• Purpose
– To identify a common location or activity
– To shorten list of foods and other exposures for
study
• Much more detailed than
interviews for foodborne illness
complaints or pathogen-specific
surveillance with hundreds
of exposures examined
Preliminary investigation > Generate hypothesis
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Hypothesis-Generating Interviews (cont’d)
• Extensive information on food and non-food
exposures within incubation period of illness
– Open-ended questions on exposures (e.g.,
restaurants, stores, events, food history)
– Lengthy list of specific foods
– Details on foods eaten (e.g., brands, where
purchased, purchase dates)
– Non-food exposures
• Use of standard questionnaire
www.cdc.gov/outbreaknet/references_resources
(example form in Appendix)
Preliminary investigation > Generate hypothesis
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Interpretation of Commonalities
If you find commonalities among cases:
• Are commonalities unique to cases or a
reflection of common exposures in the
community?
• Comparison group needed
– “Controlled” epidemiologic study
– Other cases of unrelated foodborne illness
interviewed for other reasons (case-case
comparisons)
– FoodNet Atlas of Exposures
Preliminary investigation > Generate hypothesis
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FoodNet Atlas of Exposures
• Population-based survey at
FoodNet sites
• Exposures among respondents
in previous 7 days
• Estimates of
background rate of
exposure
• Available at
www.cdc.gov/foodnet/studies_pages/pop.htm
Preliminary investigation > Generate hypothesis
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Prioritization for Further Investigation
• Ideally, all outbreaks are investigated further to
– Prevent others from becoming ill from the
outbreak source
– Identify problematic food preparation practices
or risky foods to prevent future outbreaks
– Add to our knowledge of foodborne diseases
• Given limited resources, not all outbreaks can be
investigated
Preliminary investigation > Prioritization for further investigation 44
High Priority Investigations
• High public health impact
– Life-threatening illness
– Population at high risk for complications of
illness affected
– Large number of affected persons
• Exposure likely to be ongoing
– Suspicion of continuing source of exposure
– Cases still rising
– Less than 2 incubation periods since onset of
symptoms in last case
• Intentional adulteration of food
Preliminary investigation > Prioritization for further investigation 45
Ability of Local Team to Respond?
• When to ask for help
– Scale of outbreak likely to overwhelm local
resources
– Nature of outbreak or response beyond the
experience of local staff
– Outbreak suspected to affect multiple
counties, states, or countries
• How to ask for help
– Call State Epidemiologist
– Be prepared to share outbreak information
Preliminary investigation
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Group Exercise
Divide into groups by table.
1. Read the brief description of an E. coli O157:H7
outbreak that occurred in Wisconsin.
2. Answer the questions.
3. Develop a hypothesis about the source of the
outbreak.
Be prepared to share your
answers with the class.
Time: 15 minutes
Environmental health investigation
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Group Exercise (cont’d)
On September 5, the WI Division of Public Health
received separate reports of clusters of laboratoryconfirmed E. coli O157:H7 infections in three noncontiguous counties: Manitowoc, Ozaukee, and
Dane. The Manitowoc County cluster involved five ill
persons, four of whom visited an animal exhibition
at a county fair.
On September 7, WI state epidemiologist was called
by the director of the Blood Center of Southeastern
Wisconsin regarding 5 adults who received plasma
exchanges during the prior 3 days to treat illnesses
consistent with hemolytic uremic syndrome. Three
had a lab-confirmed E. coli O157:H7 infection.
Environmental health investigation
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Group Exercise (cont’d)

A total of 30 E. coli O157:H7 infections were
reported in one week. (On average, 15 E. coli
O157:H7 infections are reported in WI each month.)
Question 1: Do these cases represent an outbreak?
What explanations might explain the increase?
What information might help you determine if the
increase represents an outbreak?
Environmental health investigation
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Group Exercise (cont’d)
Molecular subtyping of 8 of the E. coli O157:H7
isolates showed seven had PFGE patterns that
were indistinguishable. Infections with this strain
had only been reported sporadically in the past.
The PFGE patterns from the four Manitowoc
County fairgoers did not match this pattern.
Officials from the WI Division of Public Health
decided that the cases of E. coli O157:H7 infection
with the indistinguishable PFGE pattern
represented a possible outbreak and assembled
the outbreak investigation team to consider the
situation.
Environmental health investigation
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Group Exercise (cont’d)

Question 2: What are the modes of transmission
for E. coli O157:H7? What vehicles have been
associated with E. coli O157:H7 in past outbreaks?
What is the average incubation period of E. coli
O157:H7?
Environmental health investigation
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Group Exercise (cont’d)
To search for additional cases of E. coli O157:H7
infection, the WI Division of Public Health staff
used the WI Health Alert Network and e-mail to
notify local, regional, and tribal health departments;
laboratories; infection control professionals;
hospitals; emergency departments; and clinics of
the suspected outbreak.
For the investigation, a case was defined as a
patient who was infected with the outbreak strain of
E. coli O157: H7, who resided in WI and had onset
of symptoms since August 1.
Environmental health investigation
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Group Exercise (cont’d)
A total of 49 WI residents from 10 counties had
illness meeting the case definition. The most
frequently reported signs and symptoms among
cases included diarrhea (96%), abdominal cramps
(96%), bloody diarrhea (88%), fatigue (80%),
watery diarrhea (63%), and chills (57%).
Among cases, illness onset dates occurred from
August 20 through September 14. (Figure 1) Case
ages ranged from 1 to 84 years; 38 (78%) cases
were ≥ 20 years of age. Of the 49 cases, 35 (71%)
were female, 24 (49%) were hospitalized, and 9
(18%) had hemolytic uremic syndrome.
Environmental health investigation
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Group Exercise (cont’d)
Figure 1. Onset of illness among persons infected with
outbreak strain of E. coli O157:H7, Wisconsin.
Environmental health investigation
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Group Exercise (cont’d)

Question 3: Interpret the descriptive epidemiology
of the outbreak. Were symptoms among patients
consistent with infection with E. coli O157:H7?
Does the epi curve suggest a mode of transmission? Was clustering of cases by selected
demographic characteristics apparent?
Environmental health investigation
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Group Exercise (cont’d)

Staff at local health departments and the WI Division
of Public Health conducted telephone interviews of
cases using a standard comprehensive enteric
disease questionnaire. Of the initial 9 cases
interviewed, 7 (78%) ate potatoes, 5 (56%) ate
romaine lettuce, and 8 (89%) ate fresh spinach.
Question 4: Using the excerpt from the FoodNet
Atlas of Exposures, are these exposures unusual?
Environmental health investigation
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Group Exercise (cont’d)
Question 5: State your hypothesis about the
source of this outbreak including causative agent,
people at risk, mode of transmission, vehicle, and
period of interest.
Question 6: How would you prioritize this outbreak
for investigation? Why?
Environmental health investigation
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Quick Quiz
Preliminary investigation
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Quick Quiz
1. Why is it important to identify as many cases
associated with an outbreak as possible?
A. Determine true magnitude of outbreak
B. Characterize outbreak accurately
C. Increase the ability of epidemiologic studies
to link illness with true cause of outbreak
D. All of the above
Preliminary investigation
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Quick Quiz
2. A case definition might include all of the
following EXCEPT:
A. Symptoms of the illness
B. Laboratory test results
C. Food that is the suspected source of the
outbreak
D. Date of illness onset
Preliminary investigation
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Quick Quiz
3. Development of a hypothesis early in an
outbreak helps direct subsequent steps of an
outbreak investigation and should involve all
investigation team members.
A. True
B. False
Preliminary investigation
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Quick Quiz
4. A cluster of E. coli O157:H7 infections is
detected through PulseNet with cases from 30
states. Among cases, 66% were under 19
years of age and 71% were female. What is
the most likely mode of transmission in this
outbreak?
A. Person-to-person
B. Public water supply
C. Recreational water
D. Food
Preliminary investigation
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Quick Quiz
5. Which of the following characteristics would
make an outbreak the highest priority for further
investigation?
A. It has been 3 months since onset of illness
for the last case.
B. Affected persons are limited to staff who
attended an office potluck.
C. Listeria is the causative agent.
D. A total of 8 cases of campylobacteriosis
were identified after searching for additional
cases.
Preliminary investigation
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