INVESTIGATION OF AN EPIDEMIC
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Transcript of INVESTIGATION OF AN EPIDEMIC
INVESTIGATION OF AN EPIDEMIC
Dr. Amna Rehana Siddiqui Department of Family and Community MedicineFebruary 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 aThe occurrence of more cases in aplace (or population) and time place (or population) and time than expected than expected (can be 1 case if endemicity was 0)
The terms outbreak and epidemic areused 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. MeaslesPoint-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? 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
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
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?
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 ORDER
Step 3 Continued Possible Exposures
Possible Cause of illnessTry best to know your agent/disease
Clinical picturePathogenesisMode of transmissionNatural ReservoirCommon 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 Definitionfinition
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.
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
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
Step 4 continuedCase 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
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 CONFIRM
2 clean Hern
003 Jan 1 Fev 28 F E coli ?2nd
await
1 clean Biop
004 Jan 1 Red 24 M Staph CONFIRM
2 clean Hern
Each row represents data for a subjectChange 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
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
0
1
2
3
Sep1-7
Sep15-21
Sep29-5
Oct13-19
Oct27-2
Nov10-16
Nov24-30
TIME IN WEEKS
Onset of Gastrointestinal Illness
0
1
2
3
4
5
6
7
2/27/0118:00
2/28/01 0:00 2/28/01 6:00 Noon Feb 28 2/28/0118:00
3/1/01 0:00 3/1/01 6:00 Noon Mar 1 3/1/01 18:00 3/2/01 0:00
Date and Time
Nu
mb
er
of
Ca
se
s
EPIDEMIC CURVE Point Source outbreak
STEP 5 Continued
Person: who is getting sick ?
Age SexRace/EthnicitySocio-Economic StatusBehavior related
STEP 5 Continued Place: where are the cases coming from ?
Geographic Distribution
HomesWorkSchoolOrphanagesHospitals/Clinics
STEP 5 Continued Time: when are they getting sick?
Onset of symptoms Incubation Period Infectious Period Seasonality Baseline vs. epidemic
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 6DEVELOP HYPOTHESES
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 meatAR (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 cohortFood
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
Retrospective cohortFood
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
Case control
Exposure Case Control Total
Ate at A
restaurant
Yes 30 36 66
No 10 70 80
Total 40 106 146
Calculate the ORCalculate the OR
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) = 5.83OR= (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
DiseaseExposure
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
RR of illness for Surgery Duration
DiseaseExposure
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
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 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: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 timeEvaluate 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