Outbreak invest. last

Outbreak investigation & Outbreak investigation & management management

Shikur Mohammed Shikur Mohammed ((BScBSc, MPH in Epidemiology, MPH in Epidemiology))

What is an outbreak ?

� Occurrence of more cases of disease than

expected

• in a given area

• among a specific group of people• among a specific group of people

• over a particular period of time

2/2/2015 2Prepared by Shikur Mohammed (BSc, MPH)

Food or waterborne outbreak

(WHO definition)

� Food/waterborne outbreak: two or more persons

with similar illness after ingestion of the same type of

food or water which are from the same source

� Three to five years data are required to establish

epidemicity


endemicity(example malaria)

� Assessed by splenomegally and asymptomatic

parasitemia rate in children under 10 yr

• Holoendemic: >75%

• Hyperendemic: 51-74%

• Mesoendemic: 10-50%• Mesoendemic: 10-50%

• Hypoendemic: <10%

Types of outbreak

1. Point source (common source):- it is an outbreak

caused by simultaneous exposure of a group of

susceptible persons to a common source of

pathogenic organisms or chemicals or any other

noxious things/events

2/2/2015 Prepared by Shikur Mohammed (BSc, MPH) 5

Point source …

� They are usually characterized by explosiveness of

onset and limited/localized in time, place and person

� This type of pattern is typical of water and foodThis type of pattern is typical of water and food

borne diseases


2. propagated/progressive outbreak

� it is an outbreak in which an organism is propagated

in the community by passage from person to person

so that the initial rise in the number of cases isso that the initial rise in the number of cases is

usually gradual


propagated/progressive outbreak…

� This type of out break occurs where the agent is

spread through contact between infected and

susceptible individuals by means of:

• Respiratory(e.g. measles),

• Anal-oral (e.g. shigellosis),• Anal-oral (e.g. shigellosis),

• Genital (e.g. syphilis) or

• By insects and arthropods (e.g. Malaria)


What is the importance of knowing types of outbreaks?

� The classification of outbreaks by their mode of

transmission provides basis for considering possible

measures to prevent occurrence of outbreaks in themeasures to prevent occurrence of outbreaks in the

community


Why do we investigate outbreaks?

� To control continuing outbreaks

� To take appropriate control & prevention measures

� To prevent future outbreaks

� To strengthen surveillance at local level� To strengthen surveillance at local level

� To advance knowledge about a disease

� Opportunity for research & training

� Evaluation of control programs example EPI.


How do we detecting outbreaks

� Surveillance

� Report from health workers/ clinicians

� From community groups and others


Steps of an outbreak investigation

1. Prepare for field work

2. Confirm outbreak and diagnosis

3. Define case

4. Identify cases and obtain information

5. Descriptive data collection and analysis

6. Develop hypothesis6. Develop hypothesis

7. Analytical studies to test hypotheses

8. Conduct special investigations/ studies

9. Implement control measures

10. Communication, including outbreak report


1. Prepare for field work

• Investigation related

• Administration related

• Establish local contactsEstablish local contacts


2. Confirm outbreak and diagnosis

• Rule: “When you receive a report of epidemic, it is always

epidemic unless disproved.” Background rates of most

conditions show random variation. How can we measure if

there is an increase in incidence is significant?

a) Compare expected cases vs observed/verify

the diagnosis (by taking sample cases)/

b) Excess cases may be actual or artifactual


2. Confirm diagnosis….

a) Actual b) Artifactual

Change in incidence

Change in age composition

Catastrophes

Change in diagnostic methods

Change in case detection

Change in reportingCatastrophes

Duration of illnessChange in disease classification

Change in population count


3. Define cases (develop case definition)

� Case definition is defined as standard set of

criteria for deciding if a person should be classified

as suffering from the disease under investigation

� It should be simple, practical and objective

� Decide its sensitivity versus specificity


Case Definition: Categorisation

� Possible

• Patient with severe diarrhoea

� Probable

• Patient older than 5 years with severedehydration or dying of acute watery diarrhoeadehydration or dying of acute watery diarrhoeain town “x” between 1 June and 20 July 1998

� Confirmed

• Isolation of Vibrio cholerae from stool of patient


4. Identify & count

cases

4. Obtain information

Clearly identify groups from

Hospitals, Laboratories, Schools

and Workplace …

•Identifying information

(Demographic information)

•Clinical details

•Risk factors

5. Perform descriptive

epidemiology

•Risk factors

Orient cases in

• 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+


1 2 3 4 5 6 7 8 9 1044 64

Age Group

Evaluate information

Pathogen?Source?

Transmission?

6. Develop hypotheses

• 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?


Compare hypotheses with facts

• What is the mode of transmission?

7. Test specific hypotheses

Cohort

- attack rate exposed group

- attack rate unexposed group


- attack rate unexposed group

Case control

-% of cases exposed

-% of controls exposed

7. Special complementary investigations

• Microbiological investigation (examination of blood

samples, faces) and biochemical studies and doing

screening tests

• Environmental investigation (risk factors)

• Investigation of supply channels (suspected foods

and drinks)


8. Implement control measures

May occur at any time during the outbreak!!

Control the source of the pathogen

Interrupt transmission

Modify host response


Prevent recurrence

Modify host response

8.1 Control 8.1 Control the source of pathogenthe source of pathogen

� Remove source of contamination

� Remove persons from exposure

� Inactivate / neutralise the pathogen� Inactivate / neutralise the pathogen

� Isolate and/or treat infected persons


8.2 Interrupt 8.2 Interrupt TransmissionTransmission

� Interrupt environmental sources

� Control vector transmission

� Improve personal sanitation� Improve personal sanitation


8.3 Modify Host Response

� Immunise susceptible (Active immunization and

Passive immunization)

� Use prophylactic chemotherapy� Use prophylactic chemotherapy


9. Communicate findings

� Prepare outbreak report (Follow scientific format:

introduction, methods, result etc.)

� Communicate public health messages

Convince public health policy� Convince public health policy

� Evaluate performance


Challenges

1. Ongoing outbreak- urgent intervention

2. Pressure from public decision makers to conclude

investigation within short time

3. Number of cases available for study is limited

4. Early media reports-bias of response of people who4. Early media reports-bias of response of people who

are interviewed

5. If decision is delayed: slow clinical activity,

environmental and feedback


Health & Medicine

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