Transcript Outbreak Investigation Objectives for this session • Describe – the principles of outbreak investigation – the steps in outbreak investigation.

Outbreak Investigation
Objectives for this session
• Describe
– the principles of outbreak investigation
– the steps in outbreak investigation
What is an outbreak ?
• Occurrence of more cases of disease
than expected
– in a given area
– among a specific group of people
– over a particular period of time
Food-or waterborne outbreak
(WHO definition)
• two or more persons
• similar illness
• after ingestion of the same type of food or
water
• from the same source
• epidemiological evidence - the food or the
water - the source of the illness
Why investigate outbreaks?
• Stop the outbreak
– Find and neutralise the source (cause)
– Prevent additional cases
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Prevent future outbreaks
Improve surveillance and outbreak detection
Improve our knowledge
Keep the public’s confidence
Training
Retrospective investigation
• Often the outbreak exists since days,
weeks, months
• Many cases already occurred
• Count on the memory of people
• Many data already collected; use them or
start all over?
Never too late, but more difficult
Community wide outbreak
of Salmonellosis,
Jura, spring 1997
Context
• Alert: District medical officer
• 80 cases of salmonellosis in 5 weeks
• Salmonella Typhimurium
• No link identified between cases
• High political and media profile
• Local outbreak team set up
• Cases continued to occur
Specific demands
when investigating outbreaks
•
Unexpected event
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Act quickly
•
Rapid control
•
Bias caused by media reports
•
Legal and financial pressure
•
Interdisciplinary coordination
•
Work carried out in the field
Systematic approach
Investigation
Dead
Surveillance
Exposed
Prediction
Epidemiology
Vector
Reservoir
Clinicians
Co-ordination
Food safety
Supply
channels
Trace
back
Sick
Laboratory
Clinical
Specimen
transfer
Investigation
Diagnostic
Media
Authorities
Decisions
Infrastructure
Regulations
Vaccinations etc
Role of the Epidemiologist
• Systematic Description
• Identification of risk factors (by
descriptive or analytical means)
• Identification of interventions
• Work with others to implement control
measures
• Evaluate the impact of control measures
YOU MAY BE THE ONE
TO COORDINATE !
Steps of an outbreak
investigation
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Confirm outbreak and diagnosis
Define a case
Identify cases & obtain information
Describe data collected and analyse
Develop hypothesis
Test hypothesis: analytical studies
Special studies
Communicate results,
– including outbreak report
• Implement control measure
Detection
Routine surveillance
Clinical / Laboratory
General public
Media
Surveillance and outbreak detection:
Salmonella Goldcoast strains
by month of isolation, 1993-1996
Number of isolates
60
40
Goldcoast
Threshold
Epidemic threshold
facilitates detection
20
0
J M M J S N J M M J S N J M M J S N J M M J
1993
1994
Month
1995
1996
Confirm outbreak and diagnosis
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)
Cases of legionellosis by week of notification
France, January 1996 - August 1997
Number of cases
8
7
6
5
4
3
2
1
0
1 4 7 10 13 16 19 22 25 28 31 34 37 40 43 46 49 52 3 6 9 12 15 18 21 24 27 30 33
1996
Week of notification
1997
Cases of legionellosis by week of notification
France, January 1996 - August 1997
Number of cases
8
7
National meeting:
legionellosis diagnosis and
reporting
6
5
4
3
2
1
0
1 4 7 10 13 16 19 22 25 28 31 34 37 40 43 46 49 52 3 6 9 12 15 18 21 24 27 30 33
1996
Week of notification
1997
Confirm outbreak and diagnosis
• Laboratory confirmation
– serology
– isolates, typing of isolates
– toxic agents
• Contact (visit) the laboratories
• Meet attending physicians
• Examine some cases
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
Further
investigation?
- aetiological agent
- mode of transmission
- vehicle of transmission
- source of contamination
- population at risk
- exposure causing illness
Outbreak confirmed,
further investigations warranted
Form Outbreak
Control Team?
Team coordinates
field investigation
Epidemiologist
Microbiologist
Clinician
Environmentalist
Engineers
Veterinarians
Others
Descriptive epidemiology
- Who are the cases? (person)
- Where do they live? (place)
- When did they become ill? (time)
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
Case definition
Outbreak of salmonellosis in the Jura,
spring 1997
Confirmed case
Probable case
• diarrhoea (> 2 liquid stools • diarrhoea (> 2 liquid stools
per day) or fever (> 38°C,
per day)
at least one day)
and
• isolate of S. Typhimurium
and
• in a resident of Jura
or neighbouring
communities
• after May 1997
• contact (same household)
with confirmed case
• in a resident of the Jura
or neighbouring
communities
• after May 1997
Identify &
count cases
notifications
laboratories
hospitals, GPs
schools
workplace,
cases,
media, etc
Salmonellosis outbreak
in the Jura
case finding
• All laboratories in the Jura district
• Major laboratories in neighbouring districts
• National reference laboratory
• Food borne outbreak notifications
• Interview of cases
Identify &
count cases
Obtain
information
Identifying information
Demographic information
Clinical details
Exposures and known
risk factors
Salmonellosis in the Jura
Obtaining information
Trawling questionnaire:
• Attendance of events
• Places visited
• Food histories including, regional
products
Identify &
count cases
Obtain
information
Analysis of
descriptive data
Describe in
- time
- place
- person
Time
Epi 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
Epi curve
• Describe
– start, end, duration
– peak
– importance
– atypical cases
Cases
10
9
8
7
6
5
4
3
2
1
0
1
• Helps to develop hypotheses
–
–
–
–
–
incubation period
etiological agent
type of source
type of transmission
time of exposure
2
3
4
Days
5
6
7
8
9
10
11
12
Epicurves
Common point source
Common persistent 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
2
3
4
5
6
hours
7
8
9
10
11
12
days
Propagated source
cases
12
10
8
6
4
2
0
1
2
3
4
5
6
7
weeks
8
9
10
11
12
13
Estimation of time
or period of exposure
max incubation
cases
6
5
min
4
3
2
1
0
1
2
3
4
exposure
5
6
7
8
Outbreak of typhoid fever, Germany, 2004
(source, Marion Muehlen)
suspected case
probable case
confirmed case
Longest incubation period: 60
N° cases
Shortest incubation period:
6
15
25
April
May
26
27
28
29
30
31
1
2
June
Probable infection time period
3
4
5
6
7
8
9
10
11
5
2
3
1
4
12
13
14
15
Cases of Salmonella Typhimurium infection
by week of onset of symptoms, Jura, May - June 1997.
Number of cases
30
1 case
25
20
15
10
5
14
15
April
16
17
18
19
20
21
22
23
Mai
Week of onset of symptoms
24
June
25
26
27
28
July
Place
• Place of residence
• Place of possible exposure
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–
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–
–
work
meals
travel routes
day-care
leisure activities
• Maps
– identify an area at risk
Distribution of cases
of cholera, London 1854
PUMP B
X
WORK
HOUSE
PUMP A
X
X
BROAD STREET
PUMP
PUMP C
X
X
N
PUMP
W
E
S
Person
• Distribution of cases by age, sex,
occupation,etc (numerator)
– 60 female
– 50 male
• Distribution of these variables in population
(denominator)
– 600 females
– 350 males
• Attack rates
– female: 60/600
– Males: 50/350
S.Typhimurium infection
distribution of cases by age group,
Jura, May - June 1997
Age groups
(years)
<1
No cases
2
1-5
36
6 - 14
22
15 - 64
29
> 65
9
Total
98
S.Typhimurium infection
attack rates by age group,
Jura, May - June 1997
Age groups
(years)
<1
nr of cases population
2
3,200
Attack rate per
100,000
63
1-5
36
16,000
225
6 - 14
22
30,300
72
15 - 64
29
159,500
18
> 65
9
39,100
22
Total
98
248,100
40
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?
Outbreak of S. Typhimurium infections, Jura, spring 1997
Descriptive data
 No common place, event
Hypotheses
Most likely food borne
 S.Typhimurium
Meat (beef), pork meat and
meat products, poultry, dairy
products, etc
Regional product, local
 South of Jura district
distribution
 Young childen most affected Product eaten by children
 Several cases moslim
Pork products less likely
 Period of warm weather
Barbeque, take away grilled
chicken
Outbreak linked to take away
grilled chicken
 Publications
Food consumption by cases, outbreak of
Salmonellosis, Jura, spring 1997
Food
Chipolatas
Take away
chicken
Chicken
bought raw
Minced beef
burger
Pork
Veal
Comté
Morbier
Bleu de Gex
have eaten
6
5
Total
replies
15
17
% of cases
exposed
40
29
7
16
44
7
17
41
9
8
13
14
6
17
17
17
16
10
53
47
77
88
60
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
- % of cases exposed
- % of controls exposed
Food consumption by cases and controls,
outbreak of salmonellosis, Jura, spring 1997
Foods
nr (%) of cases nr (%) of controls
OR
matched
CI 95%
17 (40)
0,5
0,2 - 1,3
24 (57)
28 (67)
0,7
0,3 - 1,6
Beef
32 (78)
33 (79)
1
0,3 - 3,5
Pork
23 (59)
29 (76)
0,5
0,2 - 1,5
Veal
22 (54)
19 (46)
1,4
0,6 - 3,4
Chicken
30 (71)
34 (81)
0,6
0,2 - 1,7
Munster
4 (10)
1 (2)
4,0
0,5 - 35.8
Bleu de Gex
12 (35)
10 (24)
3,0
0,6 - 14,9
Comté
36 (86)
37 (88)
0,8
0,3 - 2,7
Morbier
33 (83)
23 (55)
6,5
1,4 - 28,8
have eaten
have eaten
(n=42)
(n=42)
Pâtés
11 (26)
Sausages
Verify hypothesis
Special investigations/studies
• Microbiological investigation
• Environmental investigation
• Veterinarian investigation
• Trace back investigations (origin of foods)
• Meteorological data
• Entomological investigations
Special complementary investigations,
outbreak of S. Typhimurium infections, Jura, spring 1997
• Microbiological investigations
– cheese samples cases homes, retail shops
– human and food isolates by phage and molecular
typing
• Trace back investigation supply channels
• Production plant
– veterinarian (herds supplying milk)
– occupational medicine
– environmental
Investigation of supply channels
Whole saler
Production
plant
CREMERIE
Morbier vehicle of infection?
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•
•
•
Raw cows’ milk
Eaten by children
Regional product
Morbier sampled at the residence of cases
– S. Typhimurium isolated from 3 Morbiers leftovers
– all other cheeses sampled tested negative
• Trace back: single producer
• No source of contamination identified
Implement control measures
May (must) occur at any
time during the outbreak!!
At first, general measures
According to findings,
more specific measures
1) Control the source of pathogen
2) Interrupt transmission
3) Modify host response
Control measures,
Outbreak of S. Typhimurium infections, Jura, 1997
At the beginning of the outbreak
• Personal hygiene advice
• Thorough cooking of meat and poultry
After the investigation
• Recall of the incriminated batches
of Morbier cheese
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
Steps of an outbreak
investigation
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Confirm outbreak and diagnosis
Define case
Identify cases and obtain information
Descriptive data collection and analysis
Develop hypothesis
Analytical studies to test hypotheses
Special studies
Communication,
including outbreak report
• Implement control measures
The Tasks !