Epidemic Epidemic

InvestigationInvestigation

Ma. Eufemia M. Collao, MD

Learning Objectives

At the end of the lecture the student should be able to:

• Cite reasons why health agencies investigate reported outbreaks

• Describe the different steps in investigating outbreaks;

• Determine whether an epidemic exists or not; • Differentiate common source and propagated

epidemics• Interpret an epidemic curve

Definition of 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

or

• Two or more linked cases of the same illness

Objectives of Outbreak Investigations

• To determine the cause of the outbreak

• To control ongoing outbreaks

• To prevent future outbreaks

• To provide statutorily mandated services

• To strengthen surveillance at local level

• To advance knowledge about a disease

• To provide training opportunities

Specific Demands when Investigating Outbreaks

• Unexpected event• Need to act quickly• Need for rapid control• Work carried out in the field

Systematic approach

Steps of an Outbreak Investigation

• Confirm outbreak and diagnosisCase definition and identificationDescriptive data collection and analysisDevelop hypothesisAnalytical studies to test hypotheses• Special studies• Implementation of control measuresCommunication, including outbreak report

Detection ?

DetectionRoutine surveillanceClinical / LaboratoryGeneral publicMedia

Is this an outbreak?

DetectionRoutine surveillanceClinical / LaboratoryGeneral publicMedia

Is this an outbreak?

Diagnosis verified ? clinical + laboratory

Link between cases?Expected numbers?

DetectionRoutine surveillanceClinical / LaboratoryGeneral publicMedia

0

10

20

30

40

50

60

70

80

90

1 4 7 10 13 16 19 22 25 28 31 34 37 40

Sequence of Events 1 Outbreak Detection and Confirmation

DAY

CA

SE

S

PrimaryCase

1st case at HC

Report to DMO

Lab result

Samplestaken

Responsebegins

Opportunity for control

0

10

20

30

40

50

60

70

80

90

1 4 7 10 13 16 19 22 25 28 31 34 37 40

Sequence of Events 2 Outbreak Detection and Confirmation

PRIM HC REP RESSAMPResponse

begins

DAY

CA

SE

S Potential Cases Prevented

Outbreak confirmed

Further investigation?

Immediate control measures?

Unknown etiology(pathogen/source/transmission)Cases seriousCases still occurringPublic pressureTraining opportunity Scientific interest

Outbreak confirmed

Further investigation?

ProphylaxisExclusion / isolationPublic warningHygienic measures

Immediate control measures?

Unknown etiology(pathogen/source/transmission)Cases seriousCases still occurringPublic pressureTraining opportunity Scientific interest

Outbreak confirmed

Further investigation?

ProphylaxisExclusion / isolationPublic warningHygienic measures

Assistance ?

Immediate control measures?

Outbreak Investigation

Team??

Outbreak Investigation

Team?

EpidemiologistMicrobiologistEnvironmental specialistMinistry / GovernmentPress officerOthers

FIELD

Outbreak Investigation

Team?

EpidemiologistMicrobiologistEnvironmental specialistMinistry / GovernmentPress officerOthers

Assess situationExamine available information

Preliminary hypothesis ?Case definition

Case finding

Outbreak Investigation

Team?

EpidemiologistMicrobiologistEnvironmental specialistMinistry / GovernmentPress officerOthers

Assess situationExamine available information

Preliminary hypothesis ?Case definition

Case finding

Descriptive epidemiology

Case Definition

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

• Clinical criteria, restrictions of time, place, person

• Simple, practical, objective

• Sensitivity versus specificity

Example of Case Definition

Patient older than 5 years with severe

dehydration or dying of acute watery

diarrhea in town “X” between 1 June

and 20 July 2006

Examples of Case Definition

• Possible– Patient with severe diarrhea

• Probable

– Patient older than 5 years with severe dehydration or dying of acute watery diarrhea in town “x” between 1 June and 20 July 2006

• Confirmed

– Isolation of Vibrio cholerae from stool of patient

Identify & count cases

Obtain information

Perform descriptive epidemiology

Clearly identifiable groups

Hospitals

Laboratories

Schools

Workplace, etc

Identify & count cases

Obtain information

Perform descriptive epidemiology

Identifying information

Demographic information

Clinical details

Risk factors

Identify & count cases

Obtain information

Perform descriptive epidemiology

Orient cases by

- time

- place

- person

Person

Place

Time

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?

Cases of viral meningitis by date of admission (n=416) Cyprus, 5 July - 5 November 5 1996

0

5

10

15

20

25

5 11 17 23 29 4 10 16 22 28 3 9 15 21 27 3 9 15 21 27 2

Date of admission

Cas

es

July August September October November

0

5

10

15

20

6am

7am

8am

9am

10am

11am

12nn

1pm

2pm

3pm

4pm

5pm

6pm

7pm

8pm

“Binalot” lunches distributed

Supreme Court Employees Association

General Assembly

Distribution of Food Poisoning Cases by Onset of Illness at the Philippine Supreme Court on August 20, 2004 (n=41)

Sandwiches distributed

0

5

10

15

20

25

1 3 5 7 9 11 13 15 17 19

0

5

10

15

20

0

5

10

15

20

Examples of epidemic curves

Point source

Continuing common source Multiple waves -person to person or further outbreak

Admissions per 100,000 population for viral meningitis by age group. Cyprus, 5 July - 5 November 1996

0

50

100

150

200

250

300

350

400

450

500

0-4 5-9 10-14 15-19 20-24 25-29 30-34 35-39 40-44 45+Age Group

Ad

mis

sio

ns/

100,

000

po

p

Spot Map of Cholera in London, 1854

Develop hypotheses

Compare hypotheses with facts

• Who is at risk of becoming ill?• What is the disease causing the

outbreak? • What is the source and the vehicle?• What is the mode of transmission?

Case-controlCohort

Analytical epidemiological studies

Prospective Retrospective

Test specific hypotheses

Logistic Regression Analysis of Potential Risk Factors for Food Poisoning at the Philippine Supreme Court on

August 20, 2004

Exposure Odds Ratio p value 95% CI

Tuna Sandwich 4.814 .121 .662 – 35.028

Ham Sandwich 4.748 .100 .741 – 30.402

Chicken Sandwich 1.368 .741 .214 – 8.749

Egg 11.046 .001 2.522 – 48.377

Tomato .706 .566 .215 – 2.317

Banana 1.727 .434 .440 – 6.773

Zesto (snack) .523 .496 .081 – 3.384

Zesto (lunch) 1.349 .641 .384 – 4.747

Implement control measures

May occur at any time during the outbreak!!

Prevent recurrence

Control the source of the pathogen

Interrupt transmission

Modify host response

Control the Source of Pathogen

• Remove source of contamination

• Remove persons from exposure

• Inactivate / neutralize the pathogen

• Isolate and/or treat infected persons

Interrupt Transmission

• Interrupt environmental sources

• Control vector transmission

• Improve personal hygiene

Modify Host Response

• Immunize susceptibles

• Use prophylactic chemotherapy

Source / Transmission

known unknown

Eti

olo

gy

kn

ow

nu

nkn

ow

n

Investigation +

Control +++

Source: Goodman 1990

Source / Transmission

known unknown

Eti

olo

gy

kn

ow

nu

nkn

ow

n

Investigation +

Control +++

Investigation +++

Control +

Source: Goodman 1990

Source / Transmission

known unknown

Eti

olo

gy

kn

ow

nu

nkn

ow

n

Investigation +

Control +++

Investigation +++

Control +

Investigation +++

Control +++

Source: Goodman 1990

Source / Transmission

known unknown

Eti

olo

gy

kn

ow

nu

nkn

ow

n

Investigation +

Control +++

Investigation +++

Control +

Investigation +++

Control +++

Investigation +++

Control +

Source: Goodman 1990

At the End

• Prepare written report

• Communicate public health messages

• Influence public health policy

• Evaluate performance

Cessation of an outbreak occurs when there is:

- elimination of source of contamination

- interruption of transmission

- reduction/exhaustion of susceptibles

- modification of the effect of primary

pathogen

An investigator should have knowledge on the following:

• various types of etiologic agents and the resultant disease

• epidemiological characteristics of outbreaks associated with different etiologic agents of resultant diseases

• clinical symptoms of resultant diseases• environmental and food sanitation

practices

An investigator should have knowledge on the following:

• sources of exposure which are vulnerable to contamination with an infectious or chemical agent

• laboratory test available to determine causative agents

• read/search for additional data on studies, prevention and control

• Acknowledgement: – Dr. Jose Ramoncito P. Navarro

Epidemiologist, CPH, UP-Manila