Outbreak Investigation Tran Minh Nhu Nguyen Technical Adviser WHO Vietnam [email protected].

Outbreak InvestigationOutbreak Investigation

Tran Minh Nhu Nguyen

Technical Adviser

WHO Vietnam

[email protected]

What is an outbreak ?

Occurrence of more cases of disease than expected

– in a given area – over a particular period of time – among a specific group of people

“Endemic” vs “Epidemic”

Time

“Endemic” “Epidemic”

Number of cases

Objectives for this session

• Highlight a systematic approach to outbreak investigation

• Define steps of an outbreak investigation

• Provide an overview of the course content

Scenario• It’s a late Friday afternoon at the local health

department. You were about to leave when the phone rings.

• On the line, a panicked voice explains that there have been 2 patients from a local garment factory admitted to the hospital with severe acute diarrhea, and you need to do something about it.

• You hang up the phone. “Right, right,” you say to yourself, “I need to do something about this problem! PROBLEM?”

• But what should you do?

DAY

Delayedresponse

CASES

Latedetection

First case

Potentiallypreventable cases

Outbreak with timely detection and

response

Outbreak with delayed detection

and response

DAY

CASES

Earlydetection

Potentiallypreventable cases

First case

Promptresponse

Objetives of your investigation

Determine the etiologic agent involved

Determine the potential source

Determine the mode of transmission

Optimize the case management

Interrupt the chain of transmission

Prevent future outbreaks

Investigation must meet these OBJETIVES:

To implement these INTERVENTIONS:

Epidemiology

Food safety

Clinicians

Laboratory

Media AuthoritiesDiagnostic

Clinical

Specimen transfer

Dead Sick

Exposed

SurveillanceInvestigation

Prediction

Supply channels

Trace back

DecisionsInfrastructureRegulationsVaccinations etc

VectorReservoir

Investigation

Co-ordination

Specific demands when investigating outbreaks

• Unexpected event

• Act quickly

• Rapid control

• Interdisciplinary coordination

• Work carried out in the field

Systematic approach

Steps of an outbreak investigation

1. Confirm outbreak and diagnosis2. Form Outbreak Control Team3. Define a case 4. Identify cases and obtain information5. Describe data by time, place, person6. Develop hypothesis7. Test hypothesis: analytical studies 8. Additional studies9. Implement control measures10. Communicate results

Confirm outbreakIs 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

• Review the clinical findings• Examine some cases• Meet attending physicians• Review the lab findings• Contact the laboratories• Obtain additional lab specimens

Not always necessary to confirm all the casesbut confirm a proportion throughout the outbreak

Outbreak confirmed

Immediate control measures?

Further investigation?

- prophylaxis- exclusion / isolation- public warning- hygienic measures- others

- causative agent- mode of transmission- vehicle of transmission- source of contamination- population at risk- exposure causing illness

EpidemiologistMicrobiologistClinicianEnvironmentalistEngineersVeterinariansOthers

Team coordinatesfield investigation

Outbreak confirmed, further investigations warranted

Form Outbreak Control Team

Descriptive epidemiology

- Who are the cases? (person)

- Where do they live? (place)

- When did they become ill? (time)

Case definition

1.1. PersonPerson2.2. PlacePlace3.3. TimeTime

Epidemiologic Epidemiologic triangletriangle

Standard set of criteria for deciding if a person should be classified as suffering from the disease under investigation

Case definition

• Simple, practical, objective

• Sensitive?

• Specific?

• Multiple case definitions

– confirmed

– probable

– possible

Identify & count cases

notificationshospitals, GPslaboratoriesschoolsworkplace, etc

Identify & count cases

Obtain information

Identifying information

Demographic information

Clinical details

Exposures and known risk factors

Organize information: Line list

• Names• Date of birth• Address• Onset of symptoms• Treating physician• Hospital stay• Laboratory results

Line List

Case

No. Name

Date

of birthAddress

Date of onset

Lab

results

1

2

3

4

5

6

XY

AB

CD

…

…

…

Identify & count cases

Obtain information

Analysis of descriptive data

Describe in

- time

- place

- person

Time: Epi curve

• 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

0

1

2

3

4

5

6

7

8

9

10

1 2 3 4 5 6 7 8 9 10 11 12

Cases

Days

Examples of Epicurves

0

1

2

3

4

5

6

1 2 3 4 5 6 7 8

0

1

2

3

4

5

6

7

8

9

10

1 2 3 4 5 6 7 8 9 10 11 12

0

2

4

6

8

10

12

1 2 3 4 5 6 7 8 9 10 11 12 13

hours

cases

cases

cases

days

weeks

Common point source Common persistent source

Propagated source

0

1

2

3

4

5

6

7

8

9

10

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22

cases

days

Common intermittent source

Outbreak of typhoid fever, Dong Thap 2002

5 2

6 3 1 4

15 25 26 27 28 29 30 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15

April May

Probable time period of infection

J une

N° c

ases

Shortest incubation period:

Longest incubation period: 60 dayssuspected caseprobable caseconfirmed case

Place

• Place of residence• Place of possible exposure

– work– meals– travel routes, – day-care– leisure activities

• Maps– identify an area at risk

Using maps

Person

• Distribution of cases – age– sex– occupation, etc

• Distribution of these variables in population • Attack rates

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?

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

Verify hypothesisAdditional investigations

• Microbiological investigation of food samples

• Environmental investigation

• Veterinarian investigation

• Molecular Typing

• Trace back investigations (origin of foods)

• Meteorological data

• Entomological investigations

Implement control measures

1) Control the source of pathogen

2) Interrupt transmission

3) Modify host response

May (must) occur at any time during the outbreak!!

At first, general measures

According to findings, more specific 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

Steps of an outbreak investigation

1. Confirm outbreak and diagnosis2. Form Outbreak Control Team3. Define a case 4. Identify cases and obtain information5. Describe data by time, place, person6. Develop hypothesis7. Test hypothesis: analytical studies 8. Additional studies9. Implement control measures10. Communicate results

The reality….

Info:Outbreak suspected

time

Confirmation

Form Outbreak Control Team

Confirm Diagnosis

Site visit

Case definition

Line list

Organize Data

Descripitve Epidemiology

Control measures

AnalyticEpidemiology

Recommendations

ReportPublication