Assessing Risk from Environmental Exposure to Waterborne Pathogens: Use of Dynamic, Population-Based Analytical Methods and ModelsMay11,2005This lecture is based on lecture material prepared by Prof. Joe Eisenberg, formerly of the University of California-Berkeley and now at the University of MichiganUsed with his permissionOverviewRoleofwaterindiseaseburden– WaterasarouteofdiseasetransmissionMethodsofriskestimation– Direct:interventiontrials– Indirect:riskassessmentPopulation‐levelrisks– Example:theMilwaukeeoutbreakImportance of Waterborne PathogensDomestic:U.S.interestinwaterquality– 1993Cryptosporidium outbreak– Increasingnumberofdiseaseoutbreaksassociatedwithwater– Congressionalmandatesforwaterquality– (SafeDrinkingWaterAct)– EmphasisonriskassessmentandregulationImportance of Waterborne PathogensWorldwide:WHOinterestinwaterquality– EstimatingGBDassociatedwithwater,sanitation,andhygiene– Diarrhealdiseasesareamajorcauseofchildhooddeathindevelopingcountries.– 3millionofthe12.9milliondeathsinchildrenundertheageof5attributabletodiarrhealdisease– EmphasisoninterventionandcontrolThe Disease Transmission ProcessRiskestimationdependsontransmissiondynamicsandexposurepathwaysAnimalsAgriculturalRunoffDrinkingWater2°Trans.RecreationalWatersorWastewaterreuseTransporttootherwatersourcesFoodApproaches to Risk Estimation Directapproach:Theinterventiontrial– Canbeusedtoassessriskfromdrinkingwaterandrecreationalwaterexposures– Problemswithsensitivity(samplesizeissue)– Trialsareexpensive. Indirectapproach:Mathematicalmodels– Mustaccountforpropertiesofinfectiousdiseaseprocesses– Pathogenspecificmodels– Uncertaintyandvariabilitymaymakeinterpretationdifficult.Approaches to Risk Estimation Combiningdirectandindirect approaches– Modelscandefinetheissuesandhelpdesignstudies.– Epidemiologycanconfirmcurrentmodelstructureandprovideinsightintohowtoimprovethemodel.Approaches for Risk Estimation: Direct estimates of waterborne infectious illnesses Surveillance: count waterborne infectious illnesses – How can a waterborne disease outbreak be distinguished from other outbreak causes (food, fomites, etc.)?– What about endemic disease? Observational– Ecologic studies (e.g., serosurvey comparing communities with and without filtration).– Time series (e.g., correlation between turbidity and hospitalization data)Approaches for Risk Estimation:Distinguishing waterborne GI disease from other GI diseases Methodsforaddressingthequestion– Inasinglecommunity:arandomized,blinded,placebo‐controlledtrial– designprovidesanestimateoftheeffectivenessofadrinkingwaterintervention. Basicstudydesign:twogroups “Exposed” group=normaltapwater. “Treated” group=useawatertreatmentdevicetoprovidewateraspathogen‐freeastechnicallypossibleApproaches for Risk Estimation: A Tap Water Intervention Trial Enroll1000subjects 500receiveanactivehomewatertreatmentdevice(andcarrydrinkingwatertowork,etc.whenpractical) 500receivea“placebo” homewaterdrinkingdevice(doesnothingtochangethewater) FollowthesubjectsforoneyearwithdailylogsofGIillness Alternativedesign:Eachhouseholdchangesdevicetypeafter6months.Approaches for Risk Estimation: A Tap Water Intervention TrialPlacebo group (tap water):– 90 illnesses over course of the study– “Rate” = 90 / 500 Rate in placebo group = 0.18 per person per yearTreated group (active device): 60 illnesses in the treated group (active device) “Rate” = 60 / 500Rate in treated group = 0.12 per person per yearApproaches for Risk Estimation: Epidemiologic MeasuresRelative Risk (RR) Incidence in exposed groupIncidence in unexposed groupInterpretation:theriskofdiseaseinthetapwatergroupis1.5timeshigherthanthatofthetreatedgroup5.112.018.0==devicetapwaterIncidenceIncidenceApproaches for Risk Estimation: Epidemiologic MeasuresAttributable Risk (AR) Incidence in exposed – Incidence in unexposedInterpretation: There are 6 excess cases of disease per 100 subjects receiving tap water06.012.018.0=−=−activetapwaterIncidenceIncidenceApproaches for Risk Estimation: Epidemiologic MeasuresAttributable Risk Percent (AR%) Excess cases in exposedIncidence in exposedInterpretation:33%ofthecasesofdiseaseinthetapwatergroupareduetowater33.018.006.0==tapwatertapwaterIncidenceCasesExcessApproaches for Risk Estimation: Epidemiologic MeasuresTogeneralizebeyondthecohort,needanestimateofthecommunityincidence.PAR:populationattributableriskPAR%:populationattributablerisk%ARcomparescompletelyprotectedgroupwithcompletelyunprotectedgroup.PARincorporatesintermediateexposureApproaches for Risk Estimation: Epidemiologic MeasuresPopulation attributable riskIncidence in the community–incidence in the unexposedInterpretation: In the community, 2 excess cases of disease per every 100 subjects in the community02.012.014.0=−=−activeCommIncidenceIncidenceApproaches for Risk Estimation: Epidemiologic MeasuresPopulation attributable risk percentageExcess cases in the communityIncidence in the exposedInterpretation: 14% of the cases of disease in the community are due to tap water14.014.002.0==tapwaterCommIncidenceCasesExcessApproaches for Risk Estimation: Tap Water Intervention TrialsTrials in immunocompetent populations Canada(Payment)‐‐challengedsurfacewater– AR=0.35(Study1),0.14‐0.4(Study2)
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