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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
2/2/2015 3Prepared by Shikur Mohammed (BSc, MPH)
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
2/2/2015 Prepared by Shikur Mohammed (BSc, MPH) 6
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
2/2/2015 Prepared by Shikur Mohammed (BSc, MPH) 7
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)
2/2/2015 Prepared by Shikur Mohammed (BSc, MPH) 8
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
2/2/2015 Prepared by Shikur Mohammed (BSc, MPH) 9
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.
2/2/2015 Prepared by Shikur Mohammed (BSc, MPH) 10
How do we detecting outbreaks
� Surveillance
� Report from health workers/ clinicians
� From community groups and others
2/2/2015 Prepared by Shikur Mohammed (BSc, MPH) 11
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
2/2/2015 12Prepared by Shikur Mohammed (BSc, MPH)
1. Prepare for field work
• Investigation related
• Administration related
• Establish local contactsEstablish local contacts
2/2/2015 Prepared by Shikur Mohammed (BSc, MPH) 13
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
2/2/2015 Prepared by Shikur Mohammed (BSc, MPH) 14
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
2/2/2015 Prepared by Shikur Mohammed (BSc, MPH) 15
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
2/2/2015 Prepared by Shikur Mohammed (BSc, MPH) 16
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
2/2/2015 Prepared by Shikur Mohammed (BSc, MPH) 17
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+
2/2/2015 Prepared by Shikur Mohammed (BSc, MPH) 19
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?
2/2/2015 Prepared by Shikur Mohammed (BSc, MPH) 20
Compare hypotheses with facts
• What is the mode of transmission?
7. Test specific hypotheses
Cohort
- attack rate exposed group
- attack rate unexposed group
2/2/2015 Prepared by Shikur Mohammed (BSc, MPH) 21
- 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)
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8. Implement control measures
May occur at any time during the outbreak!!
Control the source of the pathogen
Interrupt transmission
Modify host response
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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
2/2/2015 Prepared by Shikur Mohammed (BSc, MPH) 24
8.2 Interrupt 8.2 Interrupt TransmissionTransmission
� Interrupt environmental sources
� Control vector transmission
� Improve personal sanitation� Improve personal sanitation
2/2/2015 Prepared by Shikur Mohammed (BSc, MPH) 25
8.3 Modify Host Response
� Immunise susceptible (Active immunization and
Passive immunization)
� Use prophylactic chemotherapy� Use prophylactic chemotherapy
2/2/2015 Prepared by Shikur Mohammed (BSc, MPH) 26
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
2/2/2015 Prepared by Shikur Mohammed (BSc, MPH) 27
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
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