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PHRAMONGKUTKLAO COLLEGE OF MEDICINEDepartment of Military and Community Medicine
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PHRAMONGKUTKLAO COLLEGE OF MEDICINEDepartment of Military and Community MedicineOutbreak
Investigation
COL.POTE AIMPUN, MD., Dr.PH
Medical SpecialistOffice for Medical Services
Office of Permanent Secretary of Defense
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Contents Definition of outbreak Purpose of outbreak investigation Principles and steps of an outbreak investigation
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Definition of outbreak The occurrence of cases of an illness, specific
health-related behavior, or other health-related events clearly in excess of normal expectancy. (>Mean +2SD) The area and the period in which the cases occur are specified precisely
Adapted from WHO recommended surveillance standards, 2nd edition, 1999
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Outbreak Investigation Processes of finding cause(s) of an epidemic by
collecting, analyzing the data to propose the control program(s) of this outbreak and to prevent the future one.
The prevention and control program is the outcome of the finding about time place and person of what when where why and how.
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Judged to be an outbreak
A greater number of cases than normally occur in the same placecompare to the same period of time
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Meningococcal Epidemic Curve July '96 - June '98, Ireland
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Judged to be an outbreak
A cluster of cases which can be linked to the same exposure.
The number of meningococcal cases associated with pilgrims who have traveled for the Haj has increased, and the total number of cases to date is 14 including 4 death
Judged to be an outbreak
A single case of disease that has never been occurred .
A 3 year old boy, case of Avian flu (HSN1), alerted the public health people around the world to start a full scale investigation
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Routine Surveillance
Health personnel
Laboratory
General public
Media
Detection ofOutbreak
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Purpose of outbreak investigation To control current outbreak To prevent occurrence of future outbreak Research for more knowledge of the disease To evaluate the effectiveness of prevention
program To evaluate the effectiveness of the existing
surveillance To train health professional
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Special circumstance for outbreak Unexpected event Emergency situation Urgency for control Field work Systematic approach
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Types of outbreak Individual case investigation Outbreak investigation
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Individual case investigation Objectives
1. Confirm an outbreak
2. Prevent spreading of disease
3. Natural history of the disease in individual
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Individual case investigation Steps in investigation
1. Patient data collection
2. Determine the spreading of the disease
3. Specimen collection
4. Disease control
5. Writing a report
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Patient data collection History taking Diagnosis Laboratory Environmental survey Other epidemiological
factors
Environment
HostAgent
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Determine the spreading of the disease Contact
Family Villagers Work place
Other patients
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Specimen collection Contacts Environment Index case
What specimen? Where to collect? How to send the
specimen?
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Specimen collection What to collect Where to collect When to collect What kind of container Media Transportation Patient data detail
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Disease control
Area of contamination Contactors
Pathogen destruction Control of spreading
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Writing report(s)1. Introduction
2. Investigation report
3. Control activities that have been done
4. Tendency of epidemics
5. Important and emergency to Public health
6. Recommendations
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Diseases Cholera Severe acute diarrhea Polio, Diphtheria , Whooping cough, Tetanus Rabies Measles in IPD Other interesting disease
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Types of outbreak Individual case investigation Outbreak investigation
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Outbreak investigation1. More than mean (>2 SD)
2. Common exposure
3. Never occur in the area before
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Types of epidemics Common source epidemics Propagated source epidemics
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Common source epidemics1. Group of people
2. Common exposure of pathogen
3. Short incubation period
Etc. food poisoning, hepatitis for common needle user
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Common source outbreak
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Epidemic curve
Sick date
Exposure date
Nu
mb
er
of
pati
en
ts
0
1
2
3
4
5
6
7
8
9
10
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Propagated source epidemics1. Person
2. Individual contact
3. Spreading in community
Etc. influenza in household, tuberculosis in factory
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Propagated source epidemics
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Epidemic Curve Propagated source epidemics
0
5
10
15
20
25
30
Sick date
Nu
mb
er
of
pati
en
ts
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Epidemic curve
Sick date
Exposure date
Nu
mb
er
of
pati
en
ts
0
1
2
3
4
5
6
7
8
9
10
Axis X: Times Axis Y: Patients
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Epidemic curve Type of epidemics Estimate exposure period to calculate incubation
period
No. Pt.
Sick date(Point source outbreak)
Min. IP
Max. IPMedian.
IP
0123456789
10
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Compare Common Source Propagated source
1.Curve Bell curve Scatter bell
2.First and last pt. < 1 IP > 1 IP
3.Transmission One source From person to person
4.Duration Short Long
5. control Eradicate source Health education
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Steps of an outbreak investigation1. Field work preparation2. Confirm outbreak and diagnosis3. Define case and start case-finding4. Descriptive data collection and analysis5. Develop hypothesis6. Analytic study to test hypotheses7. Special study (environmental study)8. Communicate the conclusion and recommend control
measures9. Follow-up the control implementations10. Outbreak investigation Report
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Field work preparation Knowledge of the disease Team
Epidemiologist Health educator Laboratory technician Specialist
Coordinate with local authorities
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Confirm outbreak and diagnosis Signs, symptoms and laboratory In undiagnosed disease
Definite diagnosis must be done
Confirm that this is a real outbreak Must gather information from local authorities to
conclude that this is an outbreak and must start an investigation
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Define case and start case-finding Must comply to clinical Easy to diagnose Sense and Specificity Finding cases
1. Passive case detection: From hospital or clinic Seriously ill
2. Active case detection From community Subclinical
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Iceberg Phenomenon
Hosp. visit w symptomsNo symptomsInfected
Risk
Diagnosed
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Case definition Standard set of criteria to identify cases Clinical criteria restriction of time, place, person Simple, practical, objective Sensitivity vs. specificity
ExamplePatient older than 5 years with severe dehydration or
dying of acute watery diarrhea in town “A” between 1 January and 1 June 2002
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Case definiteion Possible/Suspected
Symptoms/signs not clear
Probable Dx from Hx and PE
Confirmed Dx from Hx, PE and laboratory testing
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Case definiteion Possible/Suspected
Severe diarrhea
Probable 5 y/o with severe diarrhea and dehydration
Confirmed Positive culture for Vibrio cholera 0139
Normally case definition will not contain risk factors
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Sensitivity and specificity
Sensitivity Specificity
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Tes
t res
ult
dcNegative
baPositive
NoYes
Disease
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Accuracy Sensitivity
Ability of the test to identify correctly those who have the disease
Probability of a positive test in people with the disease
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Tes
t res
ult
dcNegative
baPositive
NoYes
Disease
Sensitivity = P(T+|D+) = a/(a+c)
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Tes
t res
ult
dcNegative
baPositive
NoYes
Disease
False-negative rate = P(T-|D+) = c/(a+c)TPF + FNF = 1
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Accuracy Specificity
Ability of the test to identify correctly those who do not have the disease
Probability of a negative test in people without the disease
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Tes
t res
ult
dcNegative
baPositive
NoYes
Disease
Specificity = P(T-|D-) = d/(b+d)
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Tes
t res
ult
dcNegative
baPositive
NoYes
Disease
False-positive rate = P(T+|D-) = b/(b+d)TNF + FPF = 1
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Tes
t res
ult
900200Negative
100800Positive
NoYes
Disease
Specificity = 800/1000 = 0.8Specificity = 900/1000 = 0.9Accuracy = (800+900)/2000 = 0.85
1000 1000
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Sens. > Spec. Sens. < Spec. Many false positive Many specimens to test Low % tested specimen
+VE Overload work
Few false positive Fewer specimens to test Higher % tested
specimen +VE Too small number of
cases
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Confirm outbreak and diagnosis
Is this anoutbreak?
What is the diagnosis?
Link between cases?Higher than expected?
Clinical manifestationLaboratory result
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Scenario Many adults in a remote village were sick with
fever, severe joint and muscle pain and rash over the body
Is this an outbreak?What is likely diagnosis?Which intervention should be start?Shall we start the investigation?
Outbreak confirmedMeasles, rubella, dengueInvestigation warrantedShall we start vaccine or spray mosquitoes?
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Outbreak confirmed,further investigation warranted
Form outbreakInvestigation & control
team
EpidemiologistMicrobiologistClinicianEnvironmentalistGovernmentMediaetc.
Team coordinatesField investigation
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2. Define case and start case-finding
3. Descriptive data collection and analysis
Descriptive epidemiology
PersonPlaceTime
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Identify &count cases
Obtain Information
Identifying information
Demographic data
Clinical details
Risk factors
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Identify &count cases
Obtain Information
Orient cases in
-Time
-Place
-Person
Analysis ofDescriptive data
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Demographic data Stratified population data according to gender, age group,
jobs Calculate specific attack rate to identify population at risk
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Age group( year )
Number of pt. Medical personnel Attack rate (%)
male female male female male female
20 - 24 0 1 0 6 0 16.67
25 - 29 0 3 8 29 0 10.34
30 - 34 2 2 8 22 25 9.09
35 - 39 0 1 1 11 0 9.09
40 – 44 0 1 4 4 0 25.00
45 – 49 0 0 3 0 0 0
50 + 0 0 3 2 0 0
Total 2 8 27 74 7.4 10.81
Dz and rate of measles
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Time data Epidemic curve Incubation period Type of epidemics
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Place data Mapping Locations Distribution Spreading of the disease
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Cases
0
5
10
15
20
25
1 2 3 4 5 6 7 8 9 10
0
200
400
600
800
1000
1200
0-4 '5-14 '15-44 '45-64 '64+
Age Group
Evaluate information
Pathogen?Source?Transmission?
Person Place Time
Hypothesis setup : from all the data
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5. Develop hypothesis Who is at risk of becoming ill? What is the source and the vehicle? What is the mode of transmission?
ExampleTattoo was the risk of getting hepatitis A because 13 of
15 cases had new tattoos.
A shallow well was he source of shigellosis because most of cases use water from there.
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6. Analytic study to test hypotheses Compare hypotheses with facts Prove hypotheses from descriptive studies Test specific hypotheses with analytic studies
between cases and none cases Case-control studies Cohort studies
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7. Special studies Laboratory Serology Environmental studies Etc.
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8. Conclusion and recommend control measures Control pathogens/agents/causes
Destroy sources Emigrate people from sources Diagnoses and identify patients then treatment
Stop transmission Vector control Sanitation improvement Health education and information
Modified host Vaccination Prophylaxis drug
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9. Follow-up the control implementations Assess prevention and control programs Surveillance the future outbreak
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10. Outbreak investigation Report Format
1. Introduction
2. Materials and methods
3. Results
4. Prevention and control programs that have been conducted
5. Recommendations
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10. Outbreak investigation Report Return the information back to
Prevention and control authorities Health personnel Population