7 Lecture 8 Outbrek invesitgation.pptx

Outbreak Investigation & Management
Erean Sh. (MPH/E)
1

Learning objectives
After the end of this session, students will
be able to:
o State different level of disease occurrences
o List the rationale to investigate outbreak occurrence
o Discuss steps in the investigation of an outbreak occurrence
o Describe types of outbreak occurrence
o Discuss the outbreak controlling strategies
2

Level of disease occurrences
Endemic
o Presence of a disease condition at more or less stable level
o It can be defined as ‘the constant presence of a disease or
infectious agent within a given geographic area or population
group’
Sporadic
o The occurrence of individual case or outbreak of disease at
occasional, irregular and unpredictable intervals over time
Periodic (cyclic changes)
o The occurrence of disease outbreaks at regular intervals, in
cycles
3

Epidemic
o The occurrence of more cases of disease than expected in a
given area among a specific group of people over a particular
time
Outbreak
o Epidemic of shorter duration covering a more limited area
affecting small proportion of populations
E.g. in a village, town or closed institution
Cluster
o An ‘aggregation of relatively uncommon health outcomes in
space and/or time in amounts that are believed to be greater
than could be expected by chance’
4

Pandemic
o An epidemic of disease involving several countries or
continents affecting a large proportion of populations
o If the epidemic crosses many international boundaries
Secular trends
o Slow and gradual changes of a disease over long period
of time such as decades (long term trends of disease
occurrence like cancer)
5

Enzootic
o An “endemic” disease occurring in animal populations
Epizootic
o An “epidemic” of disease occurring in animal
populations
6

Endemic versus Epidemic
Endemic
Epidemic
Number
of
Cases
of
a
Disease
Time
7

Common conditions for the occurrence of an
outbreak
Agent–host–environment states may change and
precipitate an outbreak occurrences:
o The new appearance or sudden increase of an infectious
agent
o An increase in susceptibles in an environment that has an
endemic pathogen
o The introduction of an effective route of transmission
from source to susceptible host
8

Outbreak occurrence…
o Are there cases in excess of the baseline rate for that
disease and setting?
o The excess frequency should be found out with
epidemic threshold curve
9

The following points are worth noting about
epidemic declaration:
ƒ
The term epidemic can refer to any disease and health related
condition
The minimum number of cases that fulfils the criteria for
epidemic is not specific and the threshold may vary
Knowledge of the expected number is crucial to label an
occurrence of a particular event as an epidemic
The expected level varies for different diseases and different
geographic locations
10

Thresholds
Thresholds are markers that indicate when something
should happen or change
They help surveillance and program managers answer the
question, “When will you take action, and what will that
action be?”
11

Types of thresholds
 An alert threshold suggests to health staff that further
investigation is needed and preparedness activities
should be initiated
 An action threshold triggers a definite response
12

What is outbreak occurrence?
14

What does outbreak investigation & control?
It is the process of identifying:
o the cause of the epidemic
o the source of the cause
o the mode of transmission
o taking of preventive and control measures
15

Ways of Outbreak Detection
o One of the uses of public health surveillance is detecting
an outbreak occurrence
o Outbreak is detected when a routine surveillance data
reveals an increase in reported cases of a disease
o It can also be detected when the outbreak come to
attention of health care providers
o Members of affected group are another important sources
for outbreak information and concerned citizens
o Media like TV, newsletters and radio
16

Deciding whether to investigate a possible
outbreak
Factors related to the problem itself include:
o the severity of the illness/ virulence
o the number of cases
o the source
o mode of transmission/communicability
o availability of preventive and control measures
17

Objectives for outbreak
investigation
1) To initiate control & preventive measures
The most important public health reason for
investigating an outbreak occurrence is to guide disease
prevention and control strategy
Before the investigators do a control strategy, they
should identify where the outbreak is in its natural
course:
ois the outbreak continuing?
ois the outbreak just over?
18

To initiate control & prevention measures …
o If the outbreak is continuing, the major goal should be to
prevent additional cases
o If the outbreak is almost over, the goal should be to
identify risk factors to prevent future episodes of similar
outbreak occurrence
19

Outbreak Detection and Response Without Preparedness
0
10
20
30
40
50
60
70
80
90
Delayed Response
Days
Cases
Opportunity
for control
Late Detection
Index Case
20

Outbreak Detection and Response With Preparedness
0
10
20
30
40
50
60
70
80
90
1
3
5
7
9
11
13
15
17
19
21
23
25
27
29
31
33
35
37
39
Rapid Response
Days
Cases
Early Detection
Potential
cases prevented
21

o Choosing between control measures versus further
investigation depends on how much is known about:
the cause
the source of the outbreak
the mode of transmission
o If the investigators know only little about the outbreak,
further investigation is needed
o If investigators know well about the outbreak, control
measures should be initiated immediately
22

o Decision regarding how extensively to investigate an
outbreak occurrence is influenced by:
– Severity of the illness
– Knowledge of the source of the agent
– Mode of transmission of the agent
– Availability of effective preventive and control
measures
23

2) Research and training opportunity
o Each outbreak should be viewed as an experiment
waiting to be analyzed
o It presents a unique opportunity to study the natural
history of the disease
o It could be a good opportunity to gain additional
knowledge by assessing:
– The impact of prevention and control measures
– The usefulness of new epidemiology and laboratory techniques
24

Research and training opportunity …
o For a newly recognized disease, field investigation
provides an opportunity to define the natural history of a
disease including:
 causative agent
 mode of transmission
 incubation period
 clinical spectrum of the disease
 Outcome of the disease
o Investigators also attempt to characterize the populations
at greatest risk and to identify specific risk factors for
prevention and control 25

3) Public, political and legal obligations
o Politicians and leaders are usually concerned with control
of the epidemic
o Politicians and leaders may sometimes override scientific
concerns
o The public are more concerned in cluster of disease and
potentials of getting medication
o Such investigations almost never identify a causal link
between exposure and disease of interest
o It’s advantage could be educational for the community
26

4) Program considerations (evaluation)
o Occurrence of an outbreak notifies the presence of a
program weakness
o This could help program directors to change or
strengthen the program’s effort in the future to prevent
potential episodes of similar outbreak occurrence
27

Steps of outbreak investigation and control
1. Prepare to field work
2. Establish the existence of outbreak
3. Verifying the diagnosis
4. Case definition and case findings
5. Perform descriptive epidemiology
6. Formulate hypothesis
7. Evaluate hypothesis
8. Refine hypothesis and conduct additional studies
9. Intervention and follow up strategies
10. Communicate findings and design post-outbreak surveillance
system
28

First spot
o When we got information of an epidemic:
– We should identify where the outbreak is in its natural
course?
– Is it starting ? (Early recognition)
– Is it just about over? (Late recognition)
29

Time that
come
to attention of
Investigators
Time
Early recognition
Major aim:
To prevent additional disease occurrence
(i.e. initiation of preventive and control measures)
30

Time that come
to attention of
Investigators
Time
Late recognition
Major aim:
to prevent future similar outbreak
occurrence
(i.e. further investigation of risk factors) 31

Rapid Response Team (RRT)
o A good field investigator must be a good manager,
collaborator and epidemiologist
o It is not only health professionals but also it may need
involvement of others including:
 An epidemiologist
 A clinician
 A laboratory technician
 Environmental health specialist
 Public health officer
 Microbiologist
 A representative of the local health authority etc…
32

Step 1: Prepare for field work
Before leaving for the field, an investigator must be well
prepared to under take the investigation:
o Investigation (Knowledge in epidemiology and the disease of
concern is important)
o Administrative (Logistics, administrative procedures, travel
arrangements)
o Consultation (Health workers should know their role, and should
participate in the planning phase)
33

Step 2: Confirm outbreak occurrence
o An outbreak is the occurrence of more cases of disease
than expected level
o But be careful, excess cases may not always indicate an
outbreak occurrence rather it may be because:
 Change in population size
 Change in case definition
 Change in reporting procedure
 Improvement in diagnostic procedure
 Increase local awareness or interest etc…
34

Step 3: Verify the diagnosis
o The initial report may be spurious and arise from
misinterpretation of the clinical features
o Review clinical and laboratory findings to establish
diagnosis
o Goals in verifying the diagnosis includes:
 To ensure that the problem has been properly diagnosed
 To rule out laboratory error as a basis for the increase in
diagnosed cases
 To ensure the diagnosed disease is possibly epidemic
35

Step 4: Workable case definition & case findings
o Prepare “case definition” before starting identification
of cases
o It’s aim is to count all cases of the illness
o It includes clinical criteria restricted by time, place and
person
o Use the case definitions objectively
o Do not include an hypothesised exposure to risk
factor(s) in your case definition
36

Step 4: Workable case definition …
o The clinical criteria should be simple having objective
measures
o Whatever the criteria, they must be applied consistently
to all persons under investigation
o Use sensitive or "loose case definition” early in the
investigation using descriptive epidemiology to identify
the extent of the problem
o But, during testing the hypothesis generated from this
process using analytic epidemiology, specific or "tight or
strict case definition” must be used
37

Step 4: Workable case definition …
o Direct the case finding to take place both in health
institutions and outreach sites
o If a localized form of epidemic, case finding should go
to the epidemic area
o Finally, you can ask case patients if they know anyone
else with the same signs and symptoms
38

Two ways of case findings:
Stimulated/enhanced passive surveillance
o Sending a letter describing the situation and asking for reports
o Alerting the public directly through local media to visit health
facility if they have symptoms compatible with the disease in
question
o Asking cases if they know anyone else with the same signs and
symptoms
Active surveillance
o Making telephone call or visit the health facilities to collect
information on cases
o Conducting a survey of the entire population
39

Step 5: Performing Descriptive
Epidemiology
o Once data is collected, it should be analyzed by time,
place and person
o The tools to be used when characterizing the epidemic
are epidemic curve, spot map and attack rate
o The characterization often provides clues about etiology,
source and modes of transmission that can be turned into
testable epidemiologic hypothesis
40

Analysis of epidemic by time
 One can distinguish several types of epidemics
according to the mode of transmission and duration:
o Analysis by time of onset
o The epidemic curve can help to identify the type of
epidemic
o An epidemic curve provides a simple visual display of
the outbreak’s magnitude & time trend
41

Types of epidemic
o Epidemics can be classified according to the mode of
spread or propagation, nature and length of exposure to
the infectious agent and duration
o There are three principal types of epidemic
42

1. Common source epidemic
 It occurs as a result of the exposure of a group of
population to a common source (etiological agent)
o It can result from a single exposure of the population to
the agent
E.g: contaminated water supply, or the food in a
certain restaurant
43

A) Point common source epidemic
o If the exposure is brief and simultaneous
o All exposed hosts will develop the disease within one
incubation period
o The epidemic usually decline after a few generations,
either because the number of susceptible hosts fall below
some critical level, or because intervention measures
become effective
o A rapid rise and gradual fall of an epidemic curve
suggests a point source epidemic occurrence
44

E.g. Food borne outbreak of “AGE” in a wedding
feast
45

Point Source Epidemic: Meningitis in A.A Ethiopia, 2000
46

B) Continuous common source epidemic
o If the duration of exposure is prolonged
o The epidemic is continuous common source epidemic
and the epidemic curve has a plateau (multimodal epi
curve)
47

Continuous common source epidemic
48

Continuous common source
epidemic…
49
Usual rate
Number of cases
Time
Flat top

C) Intermittent common source epidemic
o An intermittent common source epidemic (exposure to
the causative agent is sporadic over time)
o Usually produces an irregularly jagged epidemic curve
reflecting the intermittence and duration of exposure and
the number of persons exposed
E.g. waterborne outbreak
50

Intermittent common source
epidemic
51

Intermittent common source epidemic…
52

2. Propagative epidemic
o It occurs as a result of transmission of an infectious
agent with a multiple sources
o It could be transmission from one person to another
directly or indirectly
o The epidemic curve in a progressive epidemic is usually
presence of successive several peaks, a prolonged
duration, and usually a sharp fall
53

Propagative source epidemic
54

3. Mixed Epidemic
o It shows the features of both types of epidemics
o It begins with a common source of infectious agent with
subsequent propagated spread
o For example a common source outbreak may be followed
by secondary person-to-person spread
E.g. Food borne outbreaks
56

Analysis of epidemic by place
– Using spot map you may ascertain localized epidemic
by place (cluster epidemic)
– Area map if large area is affected
– It is identified by intensity of shading corresponding to
incidence of a disease
– A spot map is a simple and useful technique for
illustrating where cases live, work or may have been
exposed
– It is important to indicate source of outbreak
occurrence
57

Spot Map (Cholera Outbreak in London )
58

Analysis of epidemic by place…
 Spot map does not take in to account underlying geographic
differences in population density.
 Therefore the spot map needs to be supplemented by
calculations of place specific attack rates.
 Arti-factual differences in disease occurrence between
places can occur because of regional differences in
diagnostic capacity, case detection procedures and compliance
with reporting etc.
60

Analysis of epidemic by person
o Characterizing an outbreak occurrence by person is how
we determine what populations are at risk for the disease
o Host characteristics: age, race, sex, or medical status and
exposures-occupation, leisure activities, use of
medications, tobacco and drug use etc…
o These influence susceptibility to disease and
opportunities for exposure to risk factors
o We use attack rates to identify high risk groups
61

Example: Attack Rate by age group per 100,000
populations
Age
Groups
(years)
Cases Populations Attack Rates
<1 2 522 383
1 – 5 36 16,014 225
6 – 14 22 30,385 72
15 – 64 29 157,989 18
> 65 9 41,948 22
Total 98 246,858 40
62

Step 6: Formulating epidemiological
hypothesis
Depending on the outbreak, the hypothesis may address:
o the source of the agent
o the mode of transmission
o the exposures that caused the disease
 The hypothesis should be workable & testable!
63

Step 7: Testing of the hypothesis
o After a hypothesis has been developed, the next step
is to evaluate the plausibility of the hypothesis
o Association between the postulated exposure to risk
factor(s) and the disease will be tested using analytic
studies:
 Retrospective cohort study
 Case-control study
64

Step 8: Refining hypothesis & conduct
additional studies
o Search for additional cases: Locate unrecognised or
unreported cases
o Environmental studies are equally important in some
settings
65

Step 9: Intervention and follow up
strategies
o Simultaneous to data collection and hypothesis formation, steps
should be taken to control the epidemic occurrence
o These measures depend upon knowledge of etiologic agent, mode
of transmission and other contributing factors
o In most outbreak investigations, the primary goal is control of the
outbreak and prevention of additional cases
o Implementing control measures should be done as soon as possible
if the source and/or mode of transmission is known
o It should go in parallel to investigating the outbreak or even before
investigation
66

 Control measures are aimed at the weak link in the
chain of infection:
o It may be aimed at the specific agent, source or
reservoir
o Interrupting the transmission or exposure to risk factors
o Instructing (educating) people to reduce their risk of
contacting possible exposure to the risk factors
o Reducing host susceptibility
67

Management of outbreak occurrences
o Measures directing against the reservoir
o Measures interrupting the mode of transmission
o Measures reducing host susceptibility
68

Measures directed against the reservoir
Domestic animals as reservoir
oImmunization
oTesting of herds
oDestruction of infected animals
Wild animals as reservoir
oPost-exposure prophylaxis
Humans as reservoir
oRemoval of the focus of infection
oIsolation of infected persons
oTreatment to make them non-infectious
oDisinfections of contaminated objects 69

Measures interrupting the mode of
transmission
Actions to prevent transmission of disease by ingestion:
o Purification of water
o Pasteurisation of milk
o Inspection procedures designed to ensure safe food supply
o Improve housing conditions
Attempts to reduce transmission of respiratory infections
Action to interrupt transmission of diseases whose cycles
involve an intermediate host
E.g. Clearing irrigation farms from snails to control
schistosomiasis
70

Measures to reduce host susceptibility
Control measures also include strengthening the host’s
immunity to resist disease through the following
activities:
o Active immunization: E.g. EPI preventable diseases
o Passive immunization: E.g. TAT, Rabies
o Chemoprophylaxis: E.g. Tuberculosis, Malaria , HIV (PEP)
71

Step 10: Communicating findings of
investigation
 The final responsibility is to prepare a written
report to document the investigations, findings
and the recommendations
 An oral briefing for local authorities and
implementers of control and preventive
activities:
o What was done
o What was found
o What should be done
72

A written report
At the end, prepare a comprehensive report and submit to
the appropriate responsible bodies
The report should follow the scientific reporting format
which includes:
o introduction
o objectives
o methods
o results
o discussion
o conclusion, and
o recommendations
o references
73

The report should be discussed in detail:
o Factors leading to the epidemic occurrence
o Measures taken for the controlling of the epidemic
o Recommendations for the prevention of similar episodes
of outbreaks in the future
74

Post-Epidemic Surveillance
o It is epidemic prevention strategy
o The efficacy of control measures should be assessed day
by day during the outbreak, a final assessment being made
after it has ended
o This will provide a logical basis for post-epidemic
surveillance, and preventive measures aimed at avoiding
similar outbreaks in the future
o Develop long term early warning system
o Monitor environmental risk factors for the disease
occurrence
75

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