1. INVESTIGATION OF AN EPIDEMIC
Dr. Amna Rehana Siddiqui
Department of Family and Community Medicine
February 28, 2011
2. 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
3. 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
4. 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
5. ENDEMIC SITUATION
One that is usually present in a given
geographical area of a population
group in comparison with other areas
or populations
6. FROM ENDEMIC TO EPIDEMIC ?
■ Observation
■ Increase in disease events
■ Increase in risk factors
■ Surveillance (ongoing) reports
■ Predefined Threshold value
7. WHY INVESTIGATE?
■ 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.
8. 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.
9. 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
10. ■ 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 1.
Prepare for fieldwork
11. STEP 2: Establish the existence
of epidemic
Does the observed number of cases
exceed the existing number ?
■ Literature search
■ Consult the experts / Government
■ Surveillance records
■ Previous reports
■ Is there a confirmed case ?
■ Are there additional cases?
12. ■ 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
STEP 3
Verify the diagnosis
PS: STEPS ARE NOT IN ANY
13. 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
15. 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
16. 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.
17. □ 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.
CASE DEFINITION
18. □ 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.
DEFINING CASES
19. 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
20. Line Listing: example
ID Date
Surg
Sym
onset
age sex C/S Case
defn
OT # Surg
Type
Surg
001 Jan 1 Fev 33 M Await poss/
prob
1 clean Hern
002 Jan 1 Pain 25 M Staph CON
FIRM
2 clean Hern
003 Jan 1 Fev 28 F E coli ?2nd
await
1 clean Biop
004 Jan 1 Red 24 M Staph CON
FIRM
2 clean Hern
Each row represents data for a
subject
Change order by any column and
21.
22. 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
■ Person
■ Place
■ Time
PS: STEPS ARE NOT IN ANY
ORDER
23. 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
26. STEP 5 Continued
Person: who is getting sick ?
■ Age
■ Sex
■ Race/Ethnicity
■ Socio-Economic Status
■ Behavior related
27. STEP 5 Continued
Place: where are the cases
coming from ?
■ Geographic
Distribution
■ Homes
■ Work
■ School
■ Orphanages
■ Hospitals/Clinics
28. STEP 5 Continued
Time: when are they getting
sick?
■ Onset of symptoms
■ Incubation Period
■ Infectious Period
■ Seasonality
■ Baseline vs. epidemic
29. 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
DEVELOP HYPOTHESES
30. 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?
31. 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
32. STEP 7
EVALUATE HYPOTHESES
■ Further analyze existing data
■ Compare ill populations with not ill
populations
■ Compare attack rates
■ Conduct statistical tests
■ Recommend Control Measures
33. 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
34. 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;
35. Basic measure of infectivity
Attack rate =
# of new cases of specific disease in a time
population at risk during the same
period
Define Population
36. Retrospective cohort
Food
Items at
dinner
Ate food item
(exposed)
Did not eat
(unexposed)
RR=
a/b
Ill well AR
% (a)
Ill well AR
% (b)
RR
meat 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 18 14
37. Retrospective cohort
Food
Items at
dinner
Ate food item
(exposed)
Did not eat
(unexposed)
RR=
a/b
Ill well AR
% (a)
Ill well AR
% (b)
RR
meat 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 18 14 5.71
38. Case control
Exposure Case Control Total
Ate at A
restaurant
Yes 30 36 66
No 10 70 80
Total 40 106 146
Calculate the OR
39. Case control
Exposure Case Control Total
Ate at A
restaurant
Yes 30 36 66
No 10 70 80
Total 40 106 146
OR= (30 x 70) / (36 x 10) = 5.83
40. 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
41. RR of illness for the Theatre
Disease→
Exposure↓
SWI
Yes
SWI
No
Attack
Rate
OT 1 9 7
(9/16)
56%
OT 2 7 11
(7/18)
38%
Relative Risk= 56/38=1.4 (95% CI 0.70 – 2.98); (Chi sq) p=0.50
Risk Ratio: Attack rate in OT1 = 56/38 = 1.4
Attack rate in OT2
42. RR of illness for Surgery Duration
Disease→
Exposure↓
SWI
Yes
SWI
No
Attack
Rate
Duration
> 2 hrs 14 6
(14/20)
70%
Duration
<=2 hrs 5 12
(5/12)
29%
Relative Risk= 70/29 = 2.4, 95% CI 1.08-5.25; p value 0.03
Risk Ratio: Attack rate Duration > 2 hrs = 70/29 = 2.4
Attack rate Duration <= 2 hrs
43. 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
PS: STEPS ARE NOT IN ANY
44. 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
46. STEP 10
COMMUNICATE FINDINGS
■ Summarize investigation for
requesting authority
■ Prepare written report
■ Prevention of future similar outbreaks
47. 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.
48. 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.
49. 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
50. 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.
