Transcript 02. EPIDEMIC Investigation

INVESTIGATION OF AN EPIDEMIC
Dr. Amna Rehana Siddiqui
Department of Family and Community Medicine
February 28, 2011
OBJECTIVES
 List and explain the steps of investigation of
an epidemic
 Draw & interpret the epidemic curve
 Compare food specific attack rates to identify
possible vehicles
 List reasons for investigating an epidemic
WHAT IS AN EPIDEMIC ?
The occurrence of more cases in a
place (or population) and time
than expected (can be 1 case if endemicity was 0)
The terms outbreak and epidemic are
used interchangeably
It is due to breaks in a system that
needs to be identified & corrected
Types of Epidemics/Outbreaks
Propagated
Indicative of person to person
transmission e.g. Measles
Point-source
Indicative of a common exposure
to a contaminated vehicle or
reservoir e.g. food poisoning
ENDEMIC SITUATION
One that is usually present in a given
geographical area of a population
group in comparison with other areas
or populations
FROM ENDEMIC TO EPIDEMIC ?
 Observation
 Increase in disease events
 Increase in risk factors
 Surveillance (ongoing) reports
 Predefined Threshold value
WHY INVESTIGATE?
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Identify source of infection/s urgently
Detect weakness in existing system
To prevent mortality and morbidity
To highlight public health needs
Could result in policy changes
To improve public health
To be well informed about agent,
host, and environment.
During an outbreak u need to devise a best strategy utilizing all steps e.g. u will also
be controlling an outbreak (e.g. advice to take precautionary measures) while you r
still defining cases; collecting data and at the same time communicating each day to
those who need to know ; e.g. newspapers; medical director, ministry of health etc.
HOW TO INVESTIGATE ?
Example 1:
An undiagnosed case of
Crimean Congo hemorrhagic fever
(CCHF) infects a hospital worker
who gets moderately sick
Example 2:
You are requested to investigate food
poisoning cases after a gathering at a
local community hall
STEP 1.
Prepare for fieldwork
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Identify potential investigation team
Research the disease
Make administrative arrangements
Clarify your and team members’ role
Identify & arrange for resources
Develop communication strategies for
team, hospital employees, and public
STEP 2: Establish the existence
of epidemic
Does the observed number of cases
exceed the existing number ?
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Literature search
Consult the experts / Government
Surveillance records
Previous reports
Is there a confirmed case ?
Are there additional cases?
STEP 3
Verify the diagnosis
 Speak directly with the persons who
are affected
 Clinical criteria
 Laboratory methods available ?
 Arrange to send samples to relevant
laboratories
 Initial reports correct ?
 Possible modes of spread
PS: STEPS ARE NOT IN ANY ORDER
Step 3 Continued
Possible Exposures
 Possible Cause of illness
 Try best to know your agent/disease
 Clinical picture
 Pathogenesis
 Mode of transmission
 Natural Reservoir
 Common Vehicle or Vector
Step 3 Continued
Consider Chain of Infection
STEP 4
Define and identify cases
 Develop a case definition, identify and
count cases
 Develop a line listing for all
 Create a working case definition for
CCHF
 For hospital workers/Contacts
Any one with fever &/ sore throat
 For Cases
Anyone with undiagnosed fever, or has
any type of bleeding
PS: STEPS ARE NOT IN ANY ORDER during outbreak
Step 4a:
Establishing a Case Definition
 A case definition is a standard set of
criteria for deciding whether an
individual should be classified as
having the health condition of interest.
 A case definition includes clinical
criteria and--particularly in the
setting of an outbreak
investigation--restrictions by time,
place and person.
 Apply them consistently and without
bias to all persons under investigation.
CASE DEFINITION
 To be classified as confirmed, a
case usually must have laboratory
verification. A case classified as
probable usually has typical
clinical features of the disease
without laboratory confirmation.
A case classified as possible
usually has fewer of the typical
clinical features.
DEFINING CASES
 Early in an investigation, investigators
often use a WORKING case definition
which includes confirmed, probable,
and even possible cases.
 Later on, when hypotheses have come
into sharper focus, the investigator may
“tighten” the case definition
ACCORDINGLY /by dropping the
possible category.
Step 4 continued
Case finding and Line listing
/Spread Sheet
Each row represents data for a subject and
columns represent following
 Identification
 Symptom onset date and time
 Symptoms present
 Suspected exposure date if known
 Residence
 Age
 Gender
 Laboratory tests
 Confirmed case
Line Listing: example
ID
Date
Surg
Sym age
onset
sex
C/S
001
Jan 1 Fev
002
Case
defn
OT #
33
M
Await poss/ 1
prob
clean Hern
Jan 1 Pain
25
M
Staph CON
FIRM
2
clean Hern
003
Jan 1 Fev
28
F
E coli ?2nd 1
await
clean Biop
004
Jan 1 Red
24
M
Staph CON
FIRM
clean Hern
2
Surg
Type
Surg
Each row represents data for a subject
Change order by any column and evaluate
STEP 5
Descriptive epidemiology
Find cases systematically
Epidemic / Outbreak curve
Map
Identify demographic and other
characteristics of persons at risk
 Tabulate and orient the data in terms of
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 Person
 Place
 Time
PS: STEPS ARE NOT IN ANY ORDER
STEP 5 Continued
Epidemic Curve
 Graph showing number of cases on
‘Y’ axis and time line on ‘X’ axis
 Indicates the magnitude of
epidemic over time
 Can distinguish epidemic from an
endemic situation
 Indicates pattern of spread
 Outliers
EPIDEMIC CURVE for CCHF 2000
Propagated Outbreak
3
2
1
0
Sep17
Sep1
5-21
Sep2
9-5
Oc
t13-19
Oc
t27-2
TIME IN WEEKS
No
v1016
No
v2430
EPIDEMIC CURVE
Point Source outbreak
Onset of Gastrointestinal Illness
7
Number of Cases
6
5
4
3
2
1
0
2/27/01
18:00
2/28/01 0:00 2/28/01 6:00 Noon Feb 28
2/28/01
18:00
3/1/01 0:00
Date and Time
3/1/01 6:00
Noon Mar 1 3/1/01 18:00 3/2/01 0:00
STEP 5 Continued
Person: who is getting sick ?
 Age
 Sex
 Race/Ethnicity
 Socio-Economic Status
 Behavior related
STEP 5 Continued
Place: where are the cases
coming from ?
Geographic
Distribution
Homes
Work
School
Orphanages
Hospitals/Clinics
STEP 5 Continued
Time: when are they getting sick?
 Onset of symptoms
 Incubation Period
 Infectious Period
 Seasonality
 Baseline vs. epidemic
STEP 6
DEVELOP HYPOTHESES
Following things help
 Open ended and wide ranging
interviews/history of travel/foods eaten
 Descriptive epidemiology
 Occupation/residence/raw meat use
 Line listing of all subjects
 Epidemic curve
 Existing knowledge of disease
Step 6:
Developing Hypotheses
 All aspects of the investigation should be
addressed:
- source of the agent,
- mode of transmission, (vehicle/vector),
- exposures that caused the disease,
- any additional time, person, & place factors
e.g. CCHF transmitted by blood and body fluids what other means as there were
patients from occupations who were not exposed to blood and body fluids?
First, consider what you know about
the disease itself:
 What is the agent’s usual reservoir?
 How is it usually transmitted?
 What vehicles are commonly
implicated?
 What are the known risk factors?
DO EXTENSIVE Literature Search;
Be familiar with the disease
STEP 7
EVALUATE HYPOTHESES
 Further analyze existing data
 Compare ill populations with not ill
populations
 Compare attack rates
 Conduct statistical tests
 Recommend Control Measures
Step 8
Execute additional Studies
 Refine hypothesis
 Calculate & compare attack rates (AR)*
 Compare AR in Occupational exposure to
blood/body fluids with occupations not exposed
to blood/body fluids; e.g. HCW vs Non HCW;
butchers vs non butchers
 2. Which food item caused poisoning? Compare
attack rates (e.g. RR) in those who ate meat
and in those who did not eat meat
AR (a) in meat eaters=# who got sick / Total # who ate meat
AR (b) in not meat eaters=# who got sick / Total # who did not eat meat
Step 8
Execute additional Studies
 Environmental & Laboratory tests
(e.g. home visit; left over food samples)
 Do additional epidemiologic studies;
 Case control;
 Retrospective cohort studies;
 Follow up to evaluate control measures;
Basic measure of infectivity
Attack rate =
# of new cases of specific disease in a time
population at risk during the same period
Define Population
Retrospective cohort
meat
Ate food item
Did not eat RR=
a/b
(unexposed)
(exposed)
Ill
well AR
Ill well AR
RR
% (a)
% (b)
29 17 63
17 12 59
spinach
26
17
60
20 12
62
potato
23
14
62
23 14
62
salad
13
11
54
28 19
60
Ice cream
43
11
80
3
14
Food
Items at
dinner
18
Retrospective cohort
meat
Ate food item
Did not eat RR=
a/b
(unexposed)
(exposed)
Ill
well AR
Ill well AR
RR
% (a)
% (b)
29 17 63
17 12 59
1.07
spinach
26
17
60
20 12
62
0.97
potato
23
14
62
23 14
62
1.00
salad
13
11
54
28 19
60
0.90
Ice cream
43
11
80
3
14
5.71
Food
Items at
dinner
18
Case control
Exposure
Case
Control
Total
Ate at A
Yes
restaurant
No
30
36
66
10
70
80
Total
40
106
146
Calculate the OR
Case control
Exposure
Case
Control
Total
Ate at A
Yes
restaurant
No
30
36
66
10
70
80
Total
40
106
146
OR= (30 x 70) / (36 x 10) = 5.83
Example of Surgical Wound
Infections
 Investigating Post operative Surgical Wound
Infections
 Confirm; case definition, Determine outbreak
period, Epidemic curve, Line listing for risk
factors
 Risk factors; OT, Doctors, Staff, Anesthetists,
type of surgery, ASA status, Duration of
surgery, aseptic conditions, etc.
 Calculate attack rate for all
 Evaluate hypothesis
RR of illness for the Theatre
Disease
Exposure
OT 1
OT 2
SWI
Yes
9
7
SWI
No
Attack
Rate
7
(9/16)
56%
11
(7/18)
38%
Risk Ratio: Attack rate in OT1 = 56/38 = 1.4
Attack rate in OT2
Relative Risk= 56/38=1.4 (95% CI 0.70 – 2.98); (Chi sq) p=0.50
RR of illness for Surgery Duration
Disease
Exposure
Duration
> 2 hrs
Duration
<=2 hrs
SWI
Yes
14
5
SWI
No
Attack
Rate
6
(14/20)
70%
12
(5/12)
29%
Risk Ratio: Attack rate Duration > 2 hrs = 70/29 = 2.4
Attack rate Duration <= 2 hrs
Relative Risk= 70/29 = 2.4, 95% CI 1.08-5.25; p value 0.03
STEP 9: Take Immediate
Control Measures
Surveillance for hospital employees
Surveillance for cases
Health education
Reduce panic
How to avoid exposures/Contacts
How and where to report
Appropriate quarantine and isolation
measures
 Cohorting
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PS: STEPS ARE NOT IN ANY ORDER
An outbreak might be controlled by :
- destroying contaminated foods,
- sterilizing contaminated water, or
- destroying mosquito breeding sites
- vaccinating animals/tick prevention
-An infectious food handler could be
removed from the job and treated.
-Cancelling restaurant permit and
reviewing inspecting rules
Control Measures
reducing host susceptibility ,
- Immunization ,
- chemoprophylaxis for travelers
STEP 10
COMMUNICATE FINDINGS
 Summarize investigation for
requesting authority
 Prepare written report
 Prevention of future similar outbreaks
The report should include:
 Introducing the identification of epidemic
 Methods; data collection, analyses, and
interpretations for environmental, laboratory,
and epidemiological methods
 Results; descriptive as well as analytic
 Implemented preventive and control measures
 Effectiveness of control measures.
 Impacts relevant to prevention and control.
 Make recommendations regarding future
surveillance and control.
 Distribute report to others in disease control
programs as well as publish by doing studies.
The report:
 A record of performance and a document
for potential legal issues.
 Reference if the health department
encounters a similar situation in the
future.
 Broader purpose for contributing to the
knowledge base of epidemiology and
public health.
STEP 10
FOLLOW UP
 May execute further additional studies
 Maintain surveillance for a pre-defined
period of time
 Evaluate interventions and control
measures
 Formulate recommendations
Figure . Meningococcal disease during the 2000 Hajj: Jeddah, Mecca,
and Medina, January 24–June 5, 2000.
The number of cases of sero group specific meningococcal disease is shown by
date. The duration of the 2000 Hajj is indicated.
Cholera epidemic: evolution of Epidemiology
John Snow and the Broad Street Pump, 1854
Snows dot map of London