EPIDEMIC INVESTIGATIONEPIDEMIC INVESTIGATIONEPIDEMIC INVESTIGATIONEPIDEMIC INVESTIGATION

Dr. Azmawati Mohammed NawiDr. Azmawati Mohammed NawiDept. Of Community HealthDept. Of Community Health

Definition of outbreakDefinition of outbreak

O f f di th• Occurrence of more cases of disease than expected in a given area among a specific group of people over a particular period ofgroup of people over a particular period of time.

oror

• Two or more linked cases of the sameTwo or more linked cases of the same illness.

DefinitionsDefinitions

• Outbreak- more cases in a time and placep(or population) than expected.

• Epidemic- same as outbreak or morewidespread/prolonged, more politicalwidespread/prolonged, more political

• Cluster- a group of cases in a certainCluster- a group of cases in a certainplace and time suspected to be greaterthan expectedthan expected.

DefinitionsDefinitions• Vehicle- non-living intermediary (food,

t f it )water, fomite)

V t li i i t di (i t• Vector- living intermediary (insect,arthropod): mechanical or biologicaltransmission (part of life cycle)

• Reservoir- habitat where the agent growsand multiplies (humans, animals,environment)

Definitions• Modes of transmission– Direct:Direct contact (mucous membranes, skin, fecal-oral)Droplet spread– Indirect:AirborneVehicle borne food water or fomiteVehicle borne- food, water or fomiteVectorborne- arthropod

• Portals of Entry- ingestion, inhalation,percutaneousp

Objectives of outbreak investigationsinvestigations

To control ongoing outbreaks• To control ongoing outbreaks,• To prevent future outbreaks,• To advance knowledge about a

disease.

10 Steps in an Outbreak Investigation

A) Preliminary analysis: 1 Prepare for field workp2 Verify the diagnosis3 Establish the existence of an outbreak4 Define and identify casesy5 Describe the data in terms of time (epidemic curve),

place, and person6 Develop hypotheses7 Test hypotheses: (compare attack rates between exposed

and non exposed)B) further investigation:8 Carry out additional studies9 Implementing control and prevention measures

10 Outbreak report

Step.1 Prepare for field workStep.1 Prepare for field work

• Review literature • Prepare the supplies and equipmentsp pp q p• Consult laboratory staff• Arrange for portable computer, camerag p p ,• Consult local staff

Outbreak

? Investigation Team??

Outbreak EpidemiologistMicrobiologistEnvironmental specialist Investigation

Team?

Environmental specialistMinistry / GovernmentPress officerOthersOthers

FIELDFIELD

Roles in Foodborne Outbreak Investigations

Graphic developed by Terry Rabatsky-Ehr, Regional Epidemiologist, CT DPH

Dead SickSurveillanceInvestigationVectorReservoir

Epidemiologist CliniciansExposedPrediction

Coordination

g

Coordination

Education L b tEducation Laboratory Clinical

Specimen t f

Health personnel

Media AuthoritiesDiagnostic

transferSpecialgroups

Generalpopulation

DecisionsInfrastructureRegulationsRegulationsVaccinations etc

Step 2 Verify the diagnosisStep 2 Verify the diagnosis• Describe cases clinically

Obtain a complete listing of foods served• Obtain a complete listing of foods served• Collect specimens of feces and vomits and send

for laboratory• Submit suspected food for laboratory• Look for the possible source of contamination and

periods of inadequate refrigeration and heatingperiods of inadequate refrigeration and heating• Inquire about the origin of the incriminated food,

manner of its preparation and storage before servingserving

• Search for food handlers with skin infections. Culture all purulent lesions and collect nasal swabs from all food handlers

Step 3 Establish Existence of an Outbreak

) fRoutine surveillanceClinical / Laboratory

Step 3. Establish Existence of an Outbreak

3)Detection of outbreak

Clinical / LaboratoryGeneral publicMedia

3)D t ti fRoutine surveillanceCli i l / L b t3)Detection of

outbreakClinical / LaboratoryGeneral publicMedia

Is this an outbreak?

3)D t ti fRoutine surveillanceCli i l / L b t3)Detection of

outbreakClinical / LaboratoryGeneral publicMedia

Is this an outbreak?

•Compare the current number of cases withthe number from the previous weeks or months •Check health department recordsCheck health department records•Consult local data sources

St 4 D fi d Id tif CStep 4. Define and Identify Cases• Epidemiologists establish a case p de o og sts estab s a case

definition: a standard set of criteria for deciding whether a person should be

l ifi d h i th diclassified as having the disease or condition under study.

• Usually includes :• Usually includes :1. Clinical information about the disease2. Characteristics about the people who2. Characteristics about the people who

are affected3. Information about the location or place4. A specification of time during which

the outbreak occurred.

St 4 D fi d Id tif CStep 4. Define and Identify CasesInvestigators often classify cases as one ofInvestigators often classify cases as one of

the following:• Confirmed: usually has laboratory• Confirmed: usually has laboratory

verification• Probable: usually has clinical features• Probable: usually has clinical features

without lab verification• Possible: usually has fewer of typical• Possible: usually has fewer of typical

clinical features

Example case definition• Possible or suspect

Patient with severe diarrhoea– Patient with severe diarrhoea

• Probable– Patient older than 5 years with severe

dehydration or dying of acute watery diarrhoea in town “x” between 1 Junediarrhoea in town x” between 1 June and 20 July 1998

• Confirmed• Confirmed– Isolation of Vibrio cholerae from stool

of patientof patient

Step 4. Define and Identify Cases

The following information is collected:

Identifying informationRisk factors Identifying information

Name, address,

Telephone no, …..

ClinicalInformation:Date of onset,

t i

Risk factors Information:Specific Exposures, Telephone no, …..symptoms, signs,

ttt, Hospitalization, death

Exposures,Immunity status

Demographic information:Age, sex, race,

ti

death

occupation

Case Report Form

Line ListingLine Listing

Next, selected critical items are ,abstracted into a table called a “line listing”g– Each column represents an important

variable, such as age and sexg– Each row represents a different case,

by number• This simple format allows the investigator

to scan key information on every case and update it easily

Example of a Line ListingExample of a Line Listing

Step 5. Describe the DataStep 5. Describe the Data• Characterize the outbreak by time,

place, and person (descriptive epidemiology)

Characterizing By Time: Epidemic CCurve

Epidemic curve or “epi curve” = a graph of the number of cases by their

date of onset.• Number of cases is plotted on y axis• Number of cases is plotted on y-axis• Time is plotted on the x-axis Advantages:Advantages:• Shows course of epidemic• May enable estimation of probable time• May enable estimation of probable time

period of exposure

Epidemic CurveEpidemic Curve

• X axis- time (of onset, of exposure)• Y axis- number of cases• No gap between data points (X axis iscontinuous))• X axis unit= ¼ as long as the incubationperiodpe od• Each box usually= 1 case

Epidemic CurveEpidemic Curve

• Magnitude over time• Shape: point source vs propagatedp p p p g• Evaluation of intervention• Outliers recognizedg

Examples of epidemic curvesExamples of epidemic curves• Point source

epidemicepidemic– Shape – a steep

up slope a peakPoint source

up slope, a peak and a gradual down-slopep

– Interpretation -people are exposed to the same source over

l ti l b i fa relatively brief period

Examples of epidemic curvesExamples of epidemic curves• Continuous

common source epidemic

Shape curve will

Continuing common source

– Shape - curve will have a plateau instead of a peakinstead of a peak

– Interpretation -people are p pexposed to the same source over an extended period

Examples of epidemic curvesExamples of epidemic curves

Multiple waves -person to person • Propagated

epidemic

p p p

– Shape - a series of progressively t ll ktaller peaks

– Interpretation -tperson-to-

person spread

Characterizing By Place: Spot MMap

• Assessment of an outbreak by place provides information on the geographic extent of a problem A t f i• A spot map of cases in a community may show clusters or patterns thatclusters or patterns that reflect water supplies, wind currents, or ,proximity to a restaurant or grocery store.

Characterizing By PersonCharacterizing By Person• Determine the populations at risk by

characterizing the outbreak by person• Define populations by

– Personal characteristics (Examples: age, race, sex, or medical status)

(– Exposures (Examples: occupation, leisure activities, use of medications, tobacco, drugs)

Step 6 Develop HypothesesStep 6. Develop Hypotheses• Hypotheses based on

– Descriptive epidemiology - person, place and time

Cases

Person Time

Place

Evaluate informationEvaluate information

Pathogen? Source? Transmission?

StStep 7. Test Hypotheses

– Use analytic epidemiology to test hypotheses by using a comparison

t tif l ti higroup to quantify relationships between various exposures and the diseasedisease.

– Calculate and compare attack ratesamong those exposed And those notamong those exposed And those not exposed.

Example:Example:

• Best for analyzing an outbreak in a small y gwell-defined population – Example: gastroenteritis among people p g g p p

who attended a wedding• Ask each attendee the same set of

questions about potential exposures• Then, the attack rate can be calculated for

people who ate a particular item (were exposed) and an attack rate for those who did not eat that item (were not exposed)did not eat that item (were not exposed).

Attack RatesAttack Rates

For the exposed group, the attack rate equals the number of people who ate item and became ill divided by (÷) the total number of people who ate that item.

Total # of people who ate the item and became illExposed Group =

Total # of people who ate that itemExposed Group

Attack RatesAttack Rates

For the not exposed group, the attack rate equals the number of people who did not eat item but still became ill divided by (÷) the total number of people who did not eat that itemthat item.

Total # of people who did not eat the item but still became illthe item but still became ill

Total # of people who did not eat that item

Not Exposed Group =

• To identify source of outbreak, look forTo identify source of outbreak, look for– High attack rate among those exposed and– Low attack rate among those not exposedLow attack rate among those not exposed

andIn additionIn addition– Most of the people who became ill should

have consumed the item

Relative RiskRelative Risk

Relative risk is calculated by dividing (÷) the attack rate for people who were exposed to the item by the attack rate for those who were not exposed.

Attack rate for people who were exposed to the item

Relative Risk = Attack rate for those who were not exposed

Relative Risk =

Attack Rate Table 1Attack Rate Table 1

Example: Attack RatesExample: Attack Rates

Food Exposed GroupFood• Baked Ham• Mashed potatoes

Exposed Group 29/46 = 63%23/37 = 62%• Mashed potatoes

• Spinach• Cabbage salad

23/37 = 62%26/43 = 60%18/28 = 64%• Cabbage salad

• Milk• Ice Cream (Van)

18/28 = 64%2/4 = 50%

43/54 = 80%• Ice Cream (Van)• Ice Cream (Choc)

Fruit salad

43/54 = 80%25/47 = 53%

4/6 67%• Fruit salad 4/6 = 67%

Example: Attack RatesExample: Attack Rates

Food Not Exposed GroupFood• Baked Ham• Mashed potatoes

Not Exposed Group17/29 = 59%23/37 62%• Mashed potatoes

• SpinachC bb l d

23/37 = 62%20/32 = 62%28/47 60%• Cabbage salad

• MilkI C (V )

28/47 = 60%44/71 = 62%3/21 14%• Ice Cream (Van)

• Ice Cream (Choc)F it l d

3/21 = 14%20/27 = 74%

• Fruit salad 42/69 = 61%

Attack Rate Table 2Attack Rate Table 2

Attack Rate Table 3Attack Rate Table 3

Step 7: Testing the hypotheses

– Carry out statistical tests to determinethe proper source of the outbreak ( e.gchi-square test)

– look for “statistical significance”P <0.05 ( i.e the difference between thegroups is not due to chance)

Step 8Step 8. Carry Out Additional Studies

Laboratory and environmental studiesy• While epidemiology can implicate vehicles

and guide appropriate public health action, laboratory evidence can clinch the findings

• Environmental studies often help explain why an outbreak occurred and may be very important in some settingsvery important in some settings

Step 9. Implementing Control and P ti MPrevention Measures

• Remove source of contamination

• Remove persons from exposureRemove persons from exposure

• Isolate and/or treat infected persons

• Interrupt transmission

• Eliminate the susceptibility of individuals• Eliminate the susceptibility of individuals by vaccination or prophylactic

M t ti d i th tb k!!chemotherapyMay occur at any time during the outbreak!!

St 10 O tb k tStep 10. Outbreak report

• Prepare written report

• It include discussion of factors• It include discussion of factors leading to outbreak

• Evaluation of the methods used for the control of outbreakthe control of outbreak

• Recommendations for prevention of similar outbreak

Thank youThank you