Presentation on theme: "INVESTIGATION OF AN EPIDEMIC"— Presentation transcript:
1 INVESTIGATION OF AN EPIDEMIC Dr. Amna Rehana SiddiquiDepartment of Family and Community MedicineFebruary 28, 2011
2 OBJECTIVES List and explain the steps of investigation of an epidemic Draw & interpret the epidemic curveCompare food specific attack rates to identify possible vehiclesList reasons for investigating an epidemic
3 WHAT IS AN EPIDEMIC ? The occurrence of more cases in a place (or population) and timethan expected (can be 1 case if endemicity was 0)The terms outbreak and epidemic areused interchangeablyIt is due to breaks in a system thatneeds to be identified & correctedSome consider the term outbreak to refer to a more localized situation and refer epidemic to a more widespread and perhaps prolonged situation
4 Types of Epidemics/Outbreaks PropagatedIndicative of person to person transmission e.g. MeaslesPoint-sourceIndicative 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 ? ObservationIncrease in disease eventsIncrease in risk factorsSurveillance (ongoing) reportsPredefined Threshold value
7 WHY INVESTIGATE? Identify source of infection/s urgently Detect weakness in existing systemTo prevent mortality and morbidityTo highlight public health needsCould result in policy changesTo improve public healthTo 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 workerwho gets moderately sickExample 2:You are requested to investigate foodpoisoning cases after a gathering at alocal community hall
10 STEP 1. Prepare for fieldwork Identify potential investigation teamResearch the diseaseMake administrative arrangementsClarify your and team members’ roleIdentify & arrange for resourcesDevelop communication strategies for team, hospital employees, and public
11 STEP 2: Establish the existence of epidemic Does the observed number of casesexceed the existing number ?Literature searchConsult the experts / GovernmentSurveillance recordsPrevious reportsIs there a confirmed case ?Are there additional cases?
12 STEP 3 Verify the diagnosis Speak directly with the persons who are affectedClinical criteriaLaboratory methods available ?Arrange to send samples to relevant laboratoriesInitial reports correct ?Possible modes of spreadPS: STEPS ARE NOT IN ANY ORDER
13 Possible Cause of illness Try best to know your agent/disease Step 3 Continued Possible ExposuresPossible Cause of illnessTry best to know your agent/diseaseClinical picturePathogenesisMode of transmissionNatural ReservoirCommon Vehicle or Vector
15 STEP 4 Define and identify cases Develop a case definition, identify and count casesDevelop a line listing for allCreate a working case definition for CCHFFor hospital workers/ContactsAny one with fever &/ sore throatFor CasesAnyone with undiagnosed fever, or has any type of bleedingPS: 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 CASE DEFINITIONTo 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.
18 DEFINING CASESEarly 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.
19 Step 4 continued Case finding and Line listing /Spread SheetEach row represents data for a subject and columns represent followingIdentificationSymptom onset date and timeSymptoms presentSuspected exposure date if knownResidenceAgeGenderLaboratory testsConfirmed case
20 Line Listing: example Each row represents data for a subject IDDate SurgSymonsetagesexC/SCase defnOT #Surg TypeSurg001Jan 1Fev33MAwaitposs/prob1cleanHern002Pain25StaphCONFIRM200328FE coli?2ndawaitBiop004Red24Each row represents data for a subjectChange order by any column and evaluate
22 STEP 5 Descriptive epidemiology Find cases systematicallyEpidemic / Outbreak curveMapIdentify demographic and other characteristics of persons at riskTabulate and orient the data in terms ofPersonPlaceTimePS: 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’ axisIndicates the magnitude of epidemic over timeCan distinguish epidemic from an endemic situationIndicates pattern of spreadOutliers
24 EPIDEMIC CURVE for CCHF 2000 Propagated Outbreak
26 STEP 5 Continued Person: who is getting sick ? AgeSexRace/EthnicitySocio-Economic StatusBehavior related
27 STEP 5 Continued Place: where are the cases coming from ? Geographic DistributionHomesWorkSchoolOrphanagesHospitals/Clinics
28 STEP 5 Continued Time: when are they getting sick? Onset of symptomsIncubation PeriodInfectious PeriodSeasonalityBaseline vs. epidemic
29 STEP 6 DEVELOP HYPOTHESES Following things helpOpen ended and wide ranging interviews/history of travel/foods eatenDescriptive epidemiologyOccupation/residence/raw meat useLine listing of all subjectsEpidemic curveExisting knowledge of disease
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 factorse.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 dataCompare ill populations with not ill populationsCompare attack ratesConduct statistical testsRecommend Control Measures
33 Step 8 Execute additional Studies Refine hypothesisCalculate & 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 butchers2. Which food item caused poisoning? Compare attack rates (e.g. RR) in those who ate meat and in those who did not eat meatAR (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
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 timepopulation at risk during the same periodDefine Population
36 Did not eat (unexposed) Retrospective cohortFoodItems at dinnerAte food item(exposed)Did not eat (unexposed)RR=a/bIllwellAR% (a)% (b)RRmeat2917631259spinach26602062potato2314salad1311542819Ice cream4380318
37 Did not eat (unexposed) Retrospective cohortFoodItems at dinnerAte food item(exposed)Did not eat (unexposed)RR=a/bIllwellAR% (a)% (b)RRmeat29176312591.07spinach266020620.97potato23141.00salad13115428190.90Ice cream43803185.71
38 Case control Exposure Case Control Total Ate at A restaurant Yes 30 36 66No10708040106146Calculate the OR
39 Case control Exposure Case Control Total Ate at A restaurant Yes 30 36 66No10708040106146OR= (30 x 70) / (36 x 10) = 5.83
40 Example of Surgical Wound Infections Investigating Post operative Surgical Wound InfectionsConfirm; case definition, Determine outbreak period, Epidemic curve, Line listing for risk factorsRisk factors; OT, Doctors, Staff, Anesthetists, type of surgery, ASA status, Duration of surgery, aseptic conditions, etc.Calculate attack rate for allEvaluate hypothesis
41 RR of illness for the Theatre DiseaseExposureSWIYesNoAttack RateOT 197(9/16)56%OT 211(7/18)38%Risk Ratio: Attack rate in OT1 = 56/38 = 1.4Attack rate in OT2Relative Risk= 56/38=1.4 (95% CI 0.70 – 2.98); (Chi sq) p=0.50
42 RR of illness for Surgery Duration DiseaseExposureSWIYesNoAttack RateDuration> 2 hrs146(14/20)70%<=2 hrs512(5/12)29%Risk Ratio: Attack rate Duration > 2 hrs = 70/29 = 2.4Attack rate Duration <= 2 hrsRelative Risk= 70/29 = 2.4, 95% CI ; p value 0.03
43 STEP 9: Take Immediate Control Measures Surveillance for hospital employeesSurveillance for casesHealth educationReduce panicHow to avoid exposures/ContactsHow and where to reportAppropriate quarantine and isolation measuresCohortingPS: STEPS ARE NOT IN ANY ORDER
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
45 Control Measuresreducing host susceptibility , - Immunization , - chemoprophylaxis for travelers
46 STEP 10 COMMUNICATE FINDINGS Summarize investigation for requesting authorityPrepare written reportPrevention of future similar outbreaks
47 The report should include: Introducing the identification of epidemicMethods; data collection, analyses, and interpretations for environmental, laboratory, and epidemiological methodsResults; descriptive as well as analyticImplemented preventive and control measuresEffectiveness 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 timeEvaluate interventions and control measuresFormulate 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.
51 Cholera epidemic: evolution of Epidemiology John Snow and the Broad Street Pump, 1854