Outbreak Investigation Methodological aspects

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Outbreak InvestigationMethodological aspects

• Jurgita Bagdonaite, Jolita Mereckiene
• Jurmala, Latvia, 2006
• Based EPIET material

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Objectives for this session

• Describe
• the principles of outbreak investigation
• the steps in outbreak investigation
• Using practical examples
• Salmonella outbreak in Jura, France
• Next lecture
• some operational and logistical aspects
  of outbreak investigation

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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

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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

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Why investigate outbreaks?

• Stop the outbreak
• Find and neutralise the source (cause)
• Prevent additional cases
• Prevent future outbreaks
• Improve surveillance and outbreak detection
• Improve our knowledge
• Keep the publics confidence
• Training

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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

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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

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Specific demands when
investigating outbreaks

• Unexpected event
• Act quickly
• Rapid control
• Bias caused by media reports
• Legal and financial pressure
• Interdisciplinary coordination
• Work carried out in the field

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Dead
Sick
Surveillance
Investigation
Exposed
Prediction
Clinicians
Epidemiology
Vector Reservoir
Food safety
Laboratory
Clinical
Supply channels
Specimen transfer
Trace back
Investigation
Diagnostic
Authorities
Media
Decisions Infrastructure Regulations Vaccinations
etc
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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 !
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Steps of an outbreak investigation

• 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

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Routine surveillance Clinical /
Laboratory General public Media
Detection
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Surveillance and outbreak detection Salmonella
Goldcoast strains by month of
isolation, 1993-1996
Number of isolates
Epidemic threshold facilitates detection
1993
1994
1995
1996
Month
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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)

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Number of cases
3
6
9
12
15
18
21
24
27
30
33
1996
1997
Week of notification
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Number of cases
National meeting legionellosis diagnosis and
reporting
3
6
9
12
15
18
21
24
27
30
33
1996
1997
Week of notification
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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
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Outbreak confirmed ?
- prophylaxis - exclusion / isolation - public
warning - hygienic measures - others
- aetiological agent - mode of transmission -
vehicle of transmission - source of
contamination - population at risk - exposure
causing illness
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Outbreak confirmed, further investigations
warranted
Epidemiologist Microbiologist Clinician Environmen
talist Engineers Veterinarians Others
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Descriptive epidemiology

• Who are the cases? (person)
• - Where do they live? (place)
• - When did they become ill? (time)

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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

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Case definition

• Simple, practical, objective
• Sensitive?
• Specific?
• Multiple case definitions
• confirmed
• probable
• possible

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Case definitionOutbreak of salmonellosis in the
Jura, spring 1997

• Confirmed case
• diarrhoea (gt 2 liquid stools per day) or fever
  (gt 38C, at least one day)
• and
• isolate of S. Typhimurium
• in a resident of Jura or neighbouring
  communities
• after May 1997

• Probable case
• diarrhoea (gt 2 liquid stools per day)
• and
• contact (same household) with confirmed case
• in a resident of the Jura or neighbouring
  communities
• after May 1997

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Identify count cases
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Salmonellosis outbreak in the Juracase finding

• All laboratories in the Jura district
• Major laboratories in neighbouring districts
• National reference laboratory
• Food borne outbreak notifications
• Interview of cases

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Identify count cases
Obtain information
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Salmonellosis in the JuraObtaining information

• Trawling questionnaire
• Attendance of events
• Places visited
• Food histories including, regional products

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Identify count cases
Obtain information
Analysis of descriptive data
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TimeEpi Curve

• Histogram
• Distribution of cases by time of onset
  of symptoms, diagnosis or identification
• time interval depends on incubation period
• Cases

Days
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Epi curve
Cases

• Describe
• start, end, duration
• peak
• importance
• atypical cases
• Helps to develop hypotheses
• incubation period
• etiological agent
• type of source
• type of transmission
• time of exposure

Days
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Epicurves
Common point source
Common persistent source
cases
cases
hours
days
Propagated source
cases
weeks
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Estimation of time
or period of exposure
cases
exposure
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Outbreak of typhoid fever, Germany, 2004(source,
Marion Muehlen)
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Cases of Salmonella Typhimurium infection
by week of onset of
symptoms, Jura, May - June 1997.
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Place

• Place of residence
• Place of possible exposure
• work
• meals
• travel routes
• day-care
• leisure activities
• Maps
• identify an area at risk

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Distribution of cases of
cholera, London 1854
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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

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S.Typhimurium infection distribution of cases by
age group, Jura, May - June 1997
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S.Typhimurium infection attack rates by age
group, Jura, May - June 1997
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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?

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Outbreak of S. Typhimurium infections, Jura,
spring 1997
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Food consumption by cases, outbreak of
Salmonellosis, Jura, spring 1997
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Compare hypotheses with facts
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Testing hypothesis

• Cohort
• attack rate exposed group
• attack rate unexposed group
• Case control
• of cases exposed
• of controls exposed

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Food consumption by cases and controls,
outbreak of salmonellosis, Jura, spring 1997
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Verify hypothesis Special investigations/studies

• Microbiological investigation
• Environmental investigation
• Veterinarian investigation
• Trace back investigations (origin of foods)
• Meteorological data
• Entomological investigations

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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

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Investigation of supply channels
Whole saler
Production plant
CREMERIE
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Morbier vehicle of infection?

• 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

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Implement control measures
At first, general measures According to findings,
more specific measures
May (must) occur at any time during the
outbreak!!
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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

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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

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Steps of an outbreak investigation

• 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

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The Tasks !
Meet minister
Outbreak control team meeting - urgent!
TV inter view
Laboratory confirmation
Source
Control measures ?