2. Definition of outbreak
• O
Occurrence of more cases of disease than
f f di th
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.
3. Definitions
• Outbreak- more cases in a time and place
p
(or population) than expected.
• Epidemic- same as outbreak or more
widespread/prolonged, more political
• Cluster- a group of cases in a certain
place and time suspected to be greater
than expected.
expected
4. Definitions
• Vehicle- non-living intermediary (food,
water, fomite)
t f it )
• V t
Vector- li i intermediary (insect,
living i t di (i t
arthropod): mechanical or biological
transmission (part of life cycle)
• Reservoir- habitat where the agent grows
and multiplies (humans, animals,
environment)
5. Definitions
• Modes of transmission
– Direct:
Direct contact (mucous membranes, skin, fecal-oral)
Droplet spread
– Indirect:
Airborne
Vehicle borne- food water or fomite
borne food,
Vectorborne- arthropod
• Portals of Entry- ingestion, inhalation,
p
percutaneous
6. Objectives of outbreak
investigations
• To control ongoing outbreaks
outbreaks,
• To prevent future outbreaks,
• To advance knowledge about a
disease.
7. 10 Steps in an Outbreak Investigation
A) Preliminary analysis:
1 Prepare for field work
p
2 Verify the diagnosis
3 Establish the existence of an outbreak
4 Define and identify cases
y
5 Describe the data in terms of time (epidemic curve),
place, and person
6 Develop hypotheses
7 Test hypotheses: (compare attack rates between exposed
and non exposed)
B) further investigation:
8 Carry out additional studies
9 Implementing control and prevention measures
10 Outbreak report
8. Step.1 Prepare for field work
• Review literature
• Prepare the supplies and equipments
p pp q p
• Consult laboratory staff
• Arrange for portable computer, camera
g p p ,
• Consult local staff
10. Epidemiologist
Microbiologist Outbreak
Environmental specialist
Ministry / Government
Investigation
Press officer Team?
Others
FIELD
11. Roles in Foodborne Outbreak
Investigations
Graphic developed by Terry Rabatsky-Ehr,
Regional Epidemiologist, CT DPH
12. Vector
Reservoir Dead Sick
Investigation Surveillance
Prediction Exposed
Clinicians
Epidemiologist
g
Coordination
Education Laboratory
L b t Clinical
Health
personnel
Specimen
transfer
t f
Special General Diagnostic
groups population
Media Authorities Decisions
Infrastructure
Regulations
Vaccinations etc
13. Step 2 Verify the diagnosis
• Describe cases clinically
• 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 heating
• Inquire about the origin of the incriminated food,
manner of its preparation and storage before
serving
• Search for food handlers with skin infections.
Culture all purulent lesions and collect nasal
swabs from all food handlers
14. Step 3 Establish Existence of an Outbreak
3.
Routine surveillance
Clinical / Laboratory
3)Detection of
) f General public
outbreak Media
16. Routine surveillance
3)Detection f
3)D t ti of Clinical L b
Cli i l / Laboratory
t
General public
outbreak Media
Is this an outbreak?
•Compare the current number of cases with
the number from the previous weeks or months
•Check health department records
Check
•Consult local data sources
17. Step 4. Define d Identify C
St 4 D fi and Id tif Cases
• Epidemiologists establish a case
p de o og sts estab s
definition: a standard set of criteria for
deciding whether a person should be
classified as having the disease or
l ifi d h i th di
condition under study.
• Usually includes :
1. Clinical information about the disease
2. Characteristics about the people who
are affected
3. Information about the location or place
4. A specification of time during which
the outbreak occurred.
18. Step 4. Define d Identify C
St 4 D fi and Id tif Cases
Investigators often classify cases as one of
the following:
• Confirmed: usually has laboratory
verification
• Probable: usually has clinical features
without lab verification
• Possible: usually has fewer of typical
clinical features
19. Example case definition
• Possible or suspect
– Patient with severe diarrhoea
• Probable
– Patient older than 5 years with severe
dehydration or dying of acute watery
diarrhoea in town “x” between 1 June
x”
and 20 July 1998
• Confirmed
– Isolation of Vibrio cholerae from stool
of patient
20. Step 4. Define and Identify Cases
The following information is collected:
Risk factors Identifying information
Clinical
Information:
Information: Name, address,
Specific
Date of onset,
Exposures, Telephone no, …..
symptoms, signs,
t i
Immunity
ttt,
status
Hospitalization,
death
Demographic
information:
Age, sex, race,
occupation
ti
22. Line Listing
Next, selected critical items are
,
abstracted into a table called a “line
g
listing”
– Each column represents an important
variable, such as age and sex
g
– 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
24. Step 5. Describe the Data
• Characterize the outbreak by time,
place, and person (descriptive
epidemiology)
25. Characterizing By Time: Epidemic
Curve
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
y-axis
• Time is plotted on the x-axis
Advantages:
• Shows course of epidemic
• May enable estimation of probable time
period of exposure
26. Epidemic Curve
• X axis- time (of onset, of exposure)
• Y axis- number of cases
• No gap between data points (X axis is
continuous) )
• X axis unit= ¼ as long as the incubation
pe od
period
• Each box usually= 1 case
27. Epidemic Curve
• Magnitude over time
• Shape: point source vs propagated
p p p p g
• Evaluation of intervention
• g
Outliers recognized
28. Examples of epidemic curves
• Point source
epidemic
– Shape – a steep Point source
up slope, a peak
slope
and a gradual
down-slopep
– Interpretation -
people are
exposed to the
same source over
a relatively brief
l ti l b i f
period
29. Examples of epidemic curves
• Continuous
common source
Continuing common source
epidemic
– Shape - curve will
have a plateau
instead of a peak
– Interpretation -
p p
people are
exposed to the
same source over
an extended
period
30. Examples of epidemic curves
Multiple waves -person to person
p p p
• Propagated
epidemic
– Shape - a series
of progressively
taller
t ll peaksk
– Interpretation -
person-to-
t
person spread
31. Characterizing By Place: Spot
Map
• Assessment of an
outbreak by place
provides information on
the geographic extent of
a problem
• A spot map of cases i a
t f in
community may show
clusters or patterns that
reflect water supplies,
wind currents, or
,
proximity to a restaurant
or grocery store.
32. Characterizing 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)
33. Step 6 Develop Hypotheses
6.
• Hypotheses based on
– Descriptive epidemiology - person,
place and time
34. Cases
Person Time
Place
Evaluate information
Pathogen? Source? Transmission?
35. Step
St 7. Test Hypotheses
– Use analytic epidemiology to test
hypotheses by using a comparison
group t quantify relationships
to tif l ti hi
between various exposures and the
disease.
disease
– Calculate and compare attack rates
among those exposed And those not
exposed.
36. Example:
• Best for analyzing an outbreak in a small
y g
well-defined population
– Example: gastroenteritis among people
p g gp 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).
exposed)
37. Attack 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 ill
Exposed Group =
Total # of people who ate that item
38. Attack 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 item.
item
Total # of people who did not eat
the item but still became ill
Not Exposed Group =
Total # of people who did not eat
that item
39. • To identify source of outbreak, look for
– High attack rate among those exposed and
– Low attack rate among those not exposed
and
In addition
– Most of the people who became ill should
have consumed the item
40. Relative 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
43. Example: Attack Rates
Food Not Exposed Group
• Baked Ham 17/29 = 59%
• Mashed potatoes 23/37 = 62%
• Spinach 20/32 = 62%
• Cabbage salad
C bb l d 28/47 = 60%
• Milk 44/71 = 62%
• Ice C
I Cream (V )
(Van) 3/21 = 14%
• Ice Cream (Choc) 20/27 = 74%
• Fruit l d
F it salad 42/69 = 61%
46. Step 7: Testing the hypotheses
– Carry out statistical tests to determine
the proper source of the outbreak ( e.g
chi-square test)
– look for “statistical significance”
P <0.05 ( i.e the difference between the
groups is not due to chance)
47. Step 8.
8
Carry Out Additional Studies
Laboratory and environmental studies
y
• 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 settings
48. Step 9. Implementing Control and
Prevention Measures
P ti M
• Remove source of contamination
• Remove persons from exposure
• Isolate and/or treat infected persons
• Interrupt transmission
• Eliminate the susceptibility of individuals
by vaccination or prophylactic
May
M occur at any
t time during the outbreak!!
ti d i th tb k!!
chemotherapy
49. Step 10 Outbreak report
St 10. O tb k t
• Prepare written report
• It include discussion of factors
leading to outbreak
• Evaluation of the methods used for
the control of outbreak
• Recommendations for prevention of
similar outbreak
