Foodborne Disease Outbreak

1
Foodborne Disease Outbreak Investigation Team
Training
Module 3 Preliminary Investigation of an
Outbreak
2
Module Learning Objectives

• At the end of this module, you will be able to
• 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
• Develop a case definition for an outbreak.
• Generate a hypothesis about the source of an
  outbreak.
• Prioritize an outbreak for further investigation.

gt Learning objectives
3
Step 1 Verify Diagnosis

• Purpose
• Make sure illness properly diagnosed
• Rule out laboratory and reporting errors
• Determine that all cases suffer from same illness

gt Verify diagnosis
4
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

gt Verify diagnosis
5
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

White blood cells
gt Verify diagnosis
6
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

gt Search for cases
7
Why Search for Additional Cases?
?
Female
Male
Female
First cases detected (N10)
80
50
gt Search for cases
8
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

gt Search for cases
9
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

gt Develop case definition
10
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)

gt Develop case definition
11
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

gt Develop case definition
12
Class Question
?
Which criterion is more objective (not open to
interpretation)?
An oral temperature of 100.4? or higher
Fever
OR
gt Develop case definition
13
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

gt Develop case definition
14
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 24-hour 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.

gt Develop case definition
15
Class Question (contd)
Sex Age Onset Vomiting Diarrhea Fever Head- ache Salmonella Positive Stool Culture Salad Burger Ice 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
NAculture not performed
gt Develop case definition
16
Class Question (contd)
?
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?
gt Develop case definition
17
Class Question (contd)
?
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?
gt Develop case definition
18
Class Question (contd)
?
All of the ill people ate ice cream. Would you
include ice cream in the case definition?
gt Develop case definition
19
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

gt Generate hypothesis
20
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.

gt Generate hypothesis
21
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)

gt Generate hypothesis
22
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)

gt Generate hypothesis
23
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)

gt Generate hypothesis
24
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

gt Generate hypothesis
25
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

gt Generate hypothesis
26
Time Onset of Illness

• 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
Number of Cases
3
2
1
1/11
1/13
1/15
1/17
Date of Onset of Illness
gt Generate hypothesis
27
Uses of Epi Curve

• Grasp magnitude of outbreak
• Clarify outbreaks time course
• Identify cases that are outliers
• Draw inferences about pattern of spread
• Point source outbreak
• Continuous common source outbreak
• Propagated outbreak

gt Generate hypothesis
28
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
gt Generate hypothesis
29
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.
January
February
gt Generate hypothesis
30
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
October
November
December
January
gt Generate hypothesis
31
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)
gt Generate hypothesis
32
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

gt Generate hypothesis
33
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

gt Generate hypothesis
34
Place Example (contd)
?
gt Generate hypothesis
35
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
gt Generate hypothesis
36
Person

• Age group, sex, and other characteristics
• Influence individuals 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

gt Generate hypothesis
37
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

gt Generate hypothesis
38
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

gt Generate hypothesis
39
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 lt3 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
gt Generate hypothesis
40
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

gt Generate hypothesis
41
Hypothesis-Generating Interviews (contd)

• 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)

gt Generate hypothesis
42
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

gt Generate hypothesis
43
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

gt Generate hypothesis
44
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

gt Prioritization for further investigation
45
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

gt Prioritization for further investigation
46
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

47
Group Exercise

• Divide into groups by table.
• Read the brief description of an E. coli O157H7
  outbreak that occurred in Wisconsin.
• Answer the questions.
• Develop a hypothesis about the source of the
  outbreak.

Be prepared to share your answers with the class.
Time 15 minutes
48
Group Exercise (contd)

• On September 5, the WI Division of Public Health
  received separate reports of clusters of
  laboratory-confirmed E. coli O157H7 infections
  in three non-contiguous 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 O157H7 infection.

49
Group Exercise (contd)
?

• A total of 30 E. coli O157H7 infections were
  reported in one week. (On average, 15 E. coli
  O157H7 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?

50
Group Exercise (contd)

• Molecular subtyping of 8 of the E. coli O157H7
  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 O157H7
  infection with the indistinguishable PFGE pattern
  represented a possible outbreak and assembled the
  outbreak investigation team to consider the
  situation.

51
Group Exercise (contd)
?
Question 2 What are the modes of transmission
for E. coli O157H7? What vehicles have been
associated with E. coli O157H7 in past
outbreaks? What is the average incubation period
of E. coli O157H7?
52
Group Exercise (contd)

• To search for additional cases of E. coli O157H7
  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.
•  

53
Group Exercise (contd)

• 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.

54
Group Exercise (contd)
Figure 1. Onset of illness among persons
infected with outbreak strain of E. coli O157H7,
Wisconsin.
55
Group Exercise (contd)
?

• Question 3 Interpret the descriptive
  epidemiology of the outbreak. Were symptoms among
  patients consistent with infection with E. coli
  O157H7? Does the epi curve suggest a mode of
  trans-mission? Was clustering of cases by
  selected demographic characteristics apparent?

56
Group Exercise (contd)
?

• 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?

57
Group Exercise (contd)

• 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?

58
Quick Quiz
59
Quick Quiz

• Why is it important to identify as many cases
  associated with an outbreak as possible?
• Determine true magnitude of outbreak
• Characterize outbreak accurately
• Increase the ability of epidemiologic studies to
  link illness with true cause of outbreak
• All of the above

60
Quick Quiz

• A case definition might include all of the
  following EXCEPT
• Symptoms of the illness
• Laboratory test results
• Food that is the suspected source of the outbreak
• Date of illness onset

61
Quick Quiz

• Development of a hypothesis early in an outbreak
  helps direct subsequent steps of an outbreak
  investigation and should involve all
  investigation team members.
• True
• False

62
Quick Quiz

• A cluster of E. coli O157H7 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?
• Person-to-person
• Public water supply
• Recreational water
• Food

63
Quick Quiz

• Which of the following characteristics would make
  an outbreak the highest priority for further
  investigation?
• It has been 3 months since onset of illness for
  the last case.
• Affected persons are limited to staff who
  attended an office potluck.
• Listeria is the causative agent.
• A total of 8 cases of campylobacteriosis were
  identified after searching for additional cases.