Outbreak Investigation

สำ��นั�กง�นัแพทย์� สำ��นั�กง�นัสำนั�บสำนั�นั สำ��นั�กง�นัปลั�ดกระทรวงกลั�โหมOffice for Medical Services of The Permanent Secretary for Defence (OMSPSD)

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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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


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

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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

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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

Health & Medicine

Outbreak Investigation