Outbreak investigations
Biagio Pedalino & Domenico Martinelli
24th October 2012
Objectives for this session
• Describe
– the principles of outbreak investigation
– the steps in outbreak investigation
• Using practical example
– Foodborne outbreak during a birthday
party in a pub
What do you normally do on
Sunday afternooon?
Roberto, 41
Pub owner
Something funny.
We had lunch at
sibilla’s. We were 13
and now 7 are
vomiting…
Excellent. Do you
need me to be
there for the
“plasil”? Have you
eaten something
with cream?
What is it?
•
•
•
•
Friendly call
Need of medical assistance?
Coincidence?
What else?...
• Outbreak?
What is an outbreak ?
“The occurrence in a community or region of
cases of an illness (or an outbreak) with a
frequency clearly in excess of normal
expectancy”
[Heymann DL, CDC Manual – 19th Ed]
Outbreak investigation: tasks
•
•
•
•
•
•
•
•
•
Confirm outbreak and diagnosis
Form Outbreak Control Team
Define a case (case definition)
Identify cases and obtain information
Describe data by time, place, person
Develop hypothesis
Test hypothesis: analytical studies
Additional studies
Communicate results:
– outbreak report, publication
• Implement control measures
Confirm outbreak
Is this an outbreak?
• More cases than expected?
• Surveillance data
• Surveys: hospitals, labs, physicians
Caution!
• Seasonal variations
• Notification artefacts
• Diagnostic bias (new technique)
• Diagnostic errors (pseudo-outbreaks)
Confirm diagnosis
• Laboratory confirmation
– serology
– isolates, typing of isolates
– toxic agents
• Meet attending physicians
• Examine some cases
• Contact (visit) the laboratories
Not always necessary to confirm all the cases
but confirm a proportion
throughout the outbreak
Outbreak confirmed 
Immediate control
measures?
-
prophylaxis
exclusion / isolation
public warning
hygienic measures
others
Alert !!!
• Informal alert from the pub
– 16:43: Alert
• 7 participants were sick with vomit
• Formal intervention: a simple phone call !
Phone call 4:45 pm
• Talk to the pub owner
– How many people (list with contact details)
– Food items
– Symptoms/treatment
• “Impose” preventive closure of the pub
Outbreak confirmed 
Immediate control
measures?
- prophylaxis
- exclusion / isolation
- public warning
- hygienic measures
- others
Further
investigation?
- etiological agent
- mode of transmission
- vehicle of transmission
- source of contamination
- population at risk
- exposure causing illness
Steps of an outbreak investigation
•
•
•
•
•
•
•
•
•
Confirm outbreak and diagnosis
Form Outbreak Control Team
Define a case
Identify cases and obtain information
Describe data by time, place, person
Develop hypothesis
Test hypothesis: analytical studies
Additional studies
Communicate results:
– outbreak report, publication
• Implement control measures
My outbreak control team
Ale,
Preventive
medicine
school
student - II yr
Fra, PHD
student I yr
Case definition
A person presenting with vomit or
nausea or diarrhoea, within 24 hours
after participating at the meal in the
pub (Sunday January 22nd; 2pm)
Case definition
• Standard set of criteria for deciding
if a person should be classified
as suffering from the disease
under investigation
• Criteria
– clinical and/or biological criteria
– time
– place
– person
Case definition
• Simple, practical, objective
• Sensitive?
• Specific?
• Multiple case definitions
– confirmed
– probable
– possible
• CD can be adjusted, if new information
becomes available
Steps of an outbreak investigation
•
•
•
•
•
•
•
•
•
Confirm outbreak and diagnosis
Form Outbreak Control Team
Define a case
Identify cases and obtain information
Describe data by time, place, person
Develop hypothesis
Test hypothesis: analytical studies
Additional studies
Communicate results:
– outbreak report, publication
• Implement control measures
Descriptive epidemiology
- When did they become ill? (time)
- Where do they live? (place)
- Who are the cases? (person)
Identify &
count cases
notifications
hospitals, GPs
laboratories
schools
workplace, etc
Obtain
information
Identifying information
Demographic information
Clinical details
Exposures and known risk
factors
Monday 8 am
• Contact the participants:
– Symptoms
– Food consumption
– Stool specimen collection (n=5)
In the pub
• Food samples collection
• Info on food preparation
Information collected: symptoms
• 14:00: lunch (13 participants: birthday
party)
– 12 were sick
• Vomit (n= 10)
• Nausea (n=9)
• Diarrohea (n=5)
• Abdominal pain (n=6)
• Fever (<38°C; n=2)
Information collected: food consumption
(n=13)
•
•
•
•
Basmati rice: 92% (12)
Ragu’ : 77% (10)
Red rice: 69% (9)
Apple cake: 61% (8)
Possible cause of the outbreak?
• Virus ?
• Bacteria ?
• Toxins ?
• Parasite ?
Organize information: Line list
•
•
•
•
•
•
•
Names
Date of birth
Addresse
Onset of symptoms
Treating physician
Hospital stay
Laboratory results
Line List
Case
No.
Name
1
2
3
4
5
6
XY
AB
CD
…
…
…
Date
Addresse
of birth
Date of
onset
Lab
results
Identify &
count cases
Obtain
information
Descriptive study
Describe in
- time
- place
- person
Time: Epidemic Curve
• Histogram
• Distribution of cases by time of onset
of symptoms, diagnosis or identification
– time interval depends on incubation period
Cases
10
9
8
7
6
5
4
3
2
1
0
1
2
3
4
5
6
7
Days
8
9
10
11
12
Epidemic curve
• Describe
Cases
– start, end, duration
– peak
– importance
– atypical cases
• Helps to develop hypotheses
–
–
–
–
–
incubation period
etiological agent
type of source
type of transmission
time of exposure
10
9
8
7
6
5
4
3
2
1
0
1
2
3
4
Days
5
6
7
8
9
10
11
12
Examples of Epidemic curves
Common persistent
source
Common point source
cases
cases
10
9
6
8
7
6
5
4
5
4
3
2
3
2
1
1
0
0
1
2
3
4
5
6
7
8
1
hours
12
cases
10
9
10
8
7
6
8
6
2
5
4
3
2
0
1
0
4
1
2
3
4
5
6
7
weeks
3
4
5
6
7
8
9
10
11
12
days
Propagated source
cases
2
8
9
10
11
12
13
Common intermittent
source
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22
days
Hepatitis A by date of onset
Ogemaw county, Michigan, April - May 1968
Number of cases
15
one case
50 days
30 days
10
5
15 days
0
2
8
14
Exposure
20
26
2
8
Days
14
20
26
1
7
Hypothesis on the moment of infection
unknown pathogen and point source
2
~ median incubation period
(= duration of the epidemic)
15
5
0
3
Possible moment of
1 3 5 7 9
infection
Time
11 13 15 17 19 21 23 25 27 29
50%
50%
1 median
Epi curve
Incubation period:
•Median = 2 hrs
•Range = 1-5 hrs
Place
• Place of residence
• Place of possible exposure
– work
– meals
– travel routes,
– day-care
– leisure activities
Maps
– identify an area at risk
Person
• Distribution of cases
– age
– sex
– occupation, etc
• Distribution of these variables in population
• Attack rates
Roberto
41 yrs old
Massimo
40 yrs old
Marygiò
42 yrs old
The grandmother, 70 yrs old
Yasemin
35 yrs old
Barbara
34 yrs old
Steps of an outbreak investigation
•
•
•
•
•
•
•
•
•
Confirm outbreak and diagnosis
Form Outbreak Control Team
Define a case
Identify cases and obtain information
Describe data by time, place, person
Develop hypothesis
Test hypothesis: analytical studies
Additional studies
Communicate results:
– outbreak report, publication
• Implement control measures
Develop hypotheses
- Who is at risk of becoming ill?
- What is the disease?
- What is the source and the vehicle?
- What is the mode of transmission?
Hyp in the pub: summary
•
•
•
•
•
Close community
12 out of 13 were sick
Mainly vomit
Basmati rice mostly consumed food
Basmati rice left out of the fridge for > 24hrs
Compare hypotheses
with facts
Test specific hypotheses
Analytical studies
- cohort studies
- case-control studies
Testing hypothesis
• Cohort
- attack rate exposed group
- attack rate unexposed group
• Case control
- proportion of cases exposed
- proportion of controls exposed
Cohort study
• Among participants
• Telephone interviews
Description of the cohort
• 13 persons
– 54% F
– Median age: 42 (15 – 70)
• Overall Attack Rate (AR): 92%
• Food specific AR:
– Basmati rice: 100%
– Apple cake: 100%
– Ragu’: 91%
– Red rice: 90%
Steps of an outbreak investigation
•
•
•
•
•
•
•
•
•
Confirm outbreak and diagnosis
Form Outbreak Control Team
Define a case
Identify cases and obtain information
Describe data by time, place, person
Develop hypothesis
Test hypothesis: analytical studies
Additional studies
Communicate results:
– outbreak report, publication
• Implement control measures
Verify hypothesis
Additional investigations
• Microbiological investigation of food samples
• Environmental investigation
• Veterinarian investigation
• Molecular Typing
• Trace back investigations (origin of foods)
• Meteorological data
• Entomological investigations
Tuesday 10 am: laboratory results
Tuesday 10 am: laboratory results
2/5 faecal
specimens
Steps of an outbreak investigation
•
•
•
•
•
•
•
•
•
Confirm outbreak and diagnosis
Form Outbreak Control Team
Define a case
Identify cases and obtain information
Describe data by time, place, person
Develop hypothesis
Test hypothesis: analytical studies
Additional studies
Communicate results:
– outbreak report, publication
• Implement control measures
Outbreak report
• Regular updates during the investigation
• Detailed report at the end
– communicate public health messages
– influence public health policy
– evaluate performance
– training tool
– legal proceedings
Actions & Recommendations
• Pub closed for one day
• Appropriate food preparation (cold
chain, storage, etc)
• Timely alert for prompt intervention
• … To follow grandmother’s wisedom…
« no thanks, no rice for me… »
The reality….
time
Confirmation
Site visit
Recommendations
Case definition
Report
Publication
Organize Data
Confirm Diagnosis
Outbreak
suspected Form Outbreak
Control Team
Descripitve
Epidemiology
Analytical
Epidemiology
Line list
Control measures
Communication
Why investigate outbreaks?
•
•
•
•
•
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Stop the outbreak
Understand what happened and why
Prevent future outbreaks
Improve our knowledge
Improve surveillance and outbreak detection
Training
Acknowledgements
• Pub owner
• Participants of the party (special thanks
to the grandmother)
Questions ???