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System Dynamic Modeling & Decision Tree Analysis to capture uncertainties of intervention choices and weather patterns on West Nile Virus disease outcomes This research was kindly supported by the Research Alliance for the Prevention of Infectious Disease (RAPID) Network created through a grant from the Saskatchewan Heath Research Foundation Introduction • West Nile Virus (WNV) belongs to a group of disease-causing viruses called flaviviruses, which include yellow fever, Japanese encephalitis and dengue Found in both tropic and temperate regions Two genetic lineages: lineage 1 strains are found in North America, Europe, Africa, Asia, and Australia; Can lead to severe inflammation of spinal cord (meningitis) and/or brain (encephalitis). lineage 2 strains have been isolated only in sub-Saharan Africa and Madagascar. Little severe human disease. • • Human Cases of WNV in Canada 2002-2007 Karen 1 Yee , Dr. Nathaniel 2 Osgood , Judith Human Diagnosis and Clinical Manifestation 3 Wright , and Dr. Lisa 1 Lix System Dynamic (SD) Modeling Mosquito and Human WNV Disease Progression PHC 1, 2, 3, 4, 6 • Incubation period of WNV is believed to range from 3 to 14 days • 80% of WNV infections asymptomatic • > 95% symptomatic infections are non-neurological (e.g., West Nile Fever) • < 1% symptomatic infections are neurological (e.g. meningitis and/or encephalitis) …. same tree structure as the “do nothing” branch Current temp = 15oC Number of adult blood meals per day for Given Temperature <InterventionSelected> Death Rate for Larval Female Mosquitoes Source Reduction Death Rate of Immature Female Mosqutioes <Natural Death Rate Of<Death rate for adult Larval Female Mosquitoes> Mosquitoes> Natural Death Rate Of Larval Female Mosquitoes Larvacide Reduction Death Rate of Larval Female Mosquitoes Natural Death Rate of Immature Female Mosquitoes Mean Time to Larval Maturation for Given Temperature Natural Death Rate of Pupal Female Mosquitoes Death from Pupal Female Mosquitoes Source Reduction Death Rate of Pupal Female Mosquitoes Death rate for pupal female mosquitoes Larvacide Reduction Death Rate of Pupal Female Mosquitoes Egg Density Current temp = 20oC Current temp = 20oC CurrentTemperatureInCentigrade Larvaciding = 2 Do Nothing = 0 Birth Coefficient Virus Incubation Rate Mean Time to Pupal Maturation Probability Of Virus Transmission to Mosquito, Per Infectious Bite Susceptible <InterventionSelected> Mosquitoes Density Current temp = 30oC Current temp = 20oC Exposed Female Adult Mosquitoes Density Endogeneously Calculated Infectious Mosquito Density Disease Incubation Larvaciding = 2 Current temp = 30oC Current temp = 30oC Death of Exposed Female Adult Mosquitoes Biting Rate on Infectious Adults Biting Rate on Infectious Juvenile Death of Infectious Mosquitoes Current temp = 20oC Death rate for adult Mosquitoes <Death rate for adult Mosquitoes> <Biting Rate on Adult> <Biting Rate on Juvenile> <Fraction of infectous Adult birds density> <Fraction of infectious juvenile birds density> Adulticiding Death Rate Do Nothing = 0 Current temp = 30oC Pathogen Transmission From Infected Bird to Susceptible Mosquitoes Death of Susceptible Mosquitoes <Death rate for adult Mosquitoes> Current temp = 30oC Current temp = 20oC Adulticiding = 3 Current temp = 30oC InterventionSelected Larvaciding = 2 Current temp = 20oC Larvaciding = 2 Current temp = 30oC <Average lifespan of a human> Current temp = 30oC Recovered and WNV Immune Patients Mean Time to Waning Immunity <Mean Time to Waning Immunity> Vaccinated Humans Rate of Loss Immunity of Vaccinal Humans <Fraction of Exposed Humans that remain asymptomatic> Vaccine Entrants of Humans Recruitment rate of susceptible humans Asymptomactically Infected Humans Asymptomatic Infection Susceptible Humans Newly Infected Pre-symptomatic Human Cases Recovery of Asymptomatically Infected Patients Deaths from Asymptomactically Infected Humans Deaths from Susecptible Humans Force of Infection for Humans <Number of bitings of susceptible humans by infected mosquitoes per day> <Vaccinated Humans> Recovery of WNF Patients Recovery of Neurological WNV Patients Progression to Non-Hospitalized WNF Mean Time in Hospital for WNF Patients Exposed Humans Hospitalized West Nile Patients with Fever (WNF) Mean Time Until WNV Incubates in Humans Hospitalized for WNF Hospitalized for paralysis <Fraction of Exposed Humans present with M and E> Mean Time to Recovery for PostHospital Neurological Cases Recovery from paralysis Hospitalized West Nile Patients with acute flaccid paralysis <Fraction of Exposed Humans present with paralysis> Mean Time in Hospital for Neurological Patients Mean Time in Hospital for Meningitis Patients Hospitalized West Nile Patients with Meningitis Non-Hospitalized Neurological West Nile Patients Under Recovery Recovery from meningitis <Fraction of Exposed Humans present with meningitis> Hospitalized West Nile Patients with Meningitis and Encephalitis Deaths of WNF Patients under Recovery <Average lifespan of a human> Mean Time in Hospital for Patients with Paralysis WNV-induced death rate for humans <Progression to Non-Hospitalized WNF> Non-Hospitalized WNF Patients Under Recovery Discharge of WNF Cases from Hospital <Fraction of Exposed Humans with WNF that are hospitalized> Hospitalized for M and E Mean Time to Recover for PostHospital WNF Patients Average lifespan of a human <Fraction of Exposed Humans with NonHospitalized WNF> Number of Human Cases per Day Completing WNV Incubation Mean Time to Recovery for M and E Patients Recovery from M and E Deaths of Neurological <Average lifespan of WNV Patients a human> under Recovery Deaths due to WNV-induced <Hospitalized for WNF> New Symptomatic Cases <Hospitalized for meningitis> TEMPLATE DESIGN © 2008 www.PosterPresentations.com <Hospitalized for paralysis> Humans <Hospitalized for M and E> NegativeOfCumulativeWNVSymptomaticCases Source: www.azstarnet.com/metro/295104 Hospitalized West Nile Patients with Meningitis and Encephalitis 0.02 Computer simulation showing the proportion of humans hospitalized for meningitis and encephalitis as the infectious mosquito density is raised 3 (red line) and 6 fold (blue line) from baseline (green line). 0.015 0.01 0.005 0 0 5 10 15 20 25 30 35 40 Hospitalized West Nile Patients with Meningitis and Encephalitis : InfectiousMosquitoDensity_0006 Hospitalized West Nile Patients with Meningitis and Encephalitis : InfectiousMosquitoDensity_0003 Hospitalized West Nile Patients with Meningitis and Encephalitis : InfectiousMosquitoDensity_0001 45 Week 2 50 Time (Day) 55 60 • Organizes decisions/actions into a logical tree structure taking into account uncertainties and consequences. • Consequences (outcomes) can be derived from running the SD model on particular scenarios of decisions that are made when faced with uncertainty. • Backward induction will enable the decision maker to identify the optimal decision rules to bring about the most desired outcome. • Takes into consideration present and historical uncertainties (e.g., temperature) Summary • SD modeling is a computer tool used to understand complex issues and problems often associated with many causal forces that create the problem or positively/negatively contribute to it. • Takes into account delays (e.g., time from contact to symptoms of a disease), interactions (e.g., chronic diseases with infectious diseases), feedbacks (e.g., behavioral changes from altered risk perceptions), nonlinear relationships (e.g., risk, costs), and heterogeneities (e.g., differences in infection rates between sexes) • SD models are not about forecasting the future, but rather on how actions in the present can trigger plausible reactions over time • New insights (e.g., on clinical disease progression of WNV) and changing conditions (e.g., weather patterns) or goals (e.g., target host for vaccination) are incorporated into SD models Mosquito Adult Adulticiding = 3 Week 1 Mean Time to Recover for Asymtomatically Infected Patients <Average lifespan of a human> Per Bite Probability of transmission from infected mosquito to human Current temp = 20oC Time (weeks) Deaths of Recovered and WNV Immune Patients Loss of Immunity of Recovered and WNV Immune Patients Vaccine Rate Loss Immunity of Vaccinated Humans WNV Transmission Cycle Source: Penn State University Larvaciding = 2 Mosquitoes WNV Control Planning PHC 3, 4, 6 • WNV control in Saskatchewan: Integrative pest management (IPM) approach larval source reduction surveillance and monitoring of larval vector species public messages about increasing disease transmission, and personal protective equipment (e.g., use of DEET repellent, wearing long sleeves, and avoiding outdoor activities at dusk/dawn). use of ultra-low volume (ULV) malathion spraying for the control of adult mosquitoes (adulticiding) as last resort. • WNV sensitivity registry feasibility study: involved collecting information on sensitivity registries and telephone information lines for SHR to assist them in making informed decisions on the usefulness of each for fulfill the obligation to protect the public’s health in the context of adult mosquito control programs. Current temp = 20oC Do Nothing = 0 Virus Incubation Threshold Temperature Current temp = 20oC Risk communication include weekly average # C. tarsalis, minimum infection rate, maximum likelihood estimate, & risk index provided to SHR weekly from province. Part of this project involved amalgamating 2003-present data relevant for SHR into a single database for easy public messaging & surveillance. Source: www.comosquitocontrol.com/ Mosquito_Biology.html Current temp = 20oC <Total adult female mosquitoes density> Maturation of Pupae Natural death rate for Adult Mosquitoes Do Nothing = 0 Egg Laid per Blood Meal Egg Laying by Adult Female Mosquitoes Density Birth of Mosquito Larvae Pupae Female Mosquitoes Density <Larvacide Reduction Death Rate of Larval Female Mosquitoes> Current temp = 20oC Current temp = 30oC Current temp = 30oC Egg Laying Rate for Adult Female Mosquitoes Maturation of Larvae Mean Time to <CurrentTemperatureInCentigrade> Larval Maturation …. same as below <Mean Time to Larval Maturation> Birth Rate For Larval Female Mosquitoes Mean Time as Egg Larval Female Mosquitoes Density Current temp = 20oC Source Reduction = 1 Number of adult blood meals per day Area Under Study Death From Larval Female Mosquitoes Larvaciding = 2 CumulativeWNVSymptomaticCases Source: unknown Current temp = 20oC Source Reduction Death Rate of Larval Female Mosquitoes Highest risk of transmission to humans when: 1) WNV in birds; 2) WNV in C. tarsalis; 3) increase in average # C. tarsalis / trap night; 4) % C. tarsalis high relative to other mosquito species Mosquito Pupa …. same tree structure as the “do nothing” branch Current temp = 30oC • Birds: Passive - specimens turned in by public to Canadian Cooperative Wildlife Health Centre are tested for WNV • Mosquitoes: Larval testing presence & geographic location of Culex tarsalis mosquito (this species of primary concern in Saskatchewan for transmission of WNV) Adult trapping presence of C. tarsalis in proportion to other mosquito species Pool testing testing batches of Culex mosquitoes for WNV • Environment: Growing degree days the # of days the average nightly temperature above 15°C, ideal for mosquito breeding; 300-350 GDD enough heat accumulation for 4 to 5 generations of mosquitoes over the summer • Horses: Passive - veterinary reports • Humans: Active - physician reporting (reportable disease) Mosquito Larvae Current temp = 15oC Adulticiding = 3 Larvaciding = 2 WNV Surveillance & Risk Communication PHC 2, 3, 4, 6 • Saskatchewan suffered the highest incidence of WNV in the country in 2003 and 2007 • Saskatoon Health Region (SHR) reported 6.5% and 25% of the provincial cases in 2003 and 2007, respectively Decision Tree Analysis PHC 2, 3, 6 Do Nothing = 0 Hospitalized for meningitis Source: PHAC, 2006 School of Public Health, University of Saskatchewan1 Department of Computer Science, University of Saskatchewan2 Saskatoon Public Health Observatory, Public Health Services, Saskatoon Health Region3 65 70 75 80 85 90 95 100 person person person This project provides: • multiple stakeholders with useful information & computer tools for understanding the dynamics of WNV • ability to use current and historic uncertainty to make informed decisions on optimal intervention measures for improved health outcomes. • an in-depth review of sensitivity registries and telephone information lines for informing the public regarding adulticiding should it ever be necessary to control for WNV • amalgamated mosquito trap data from 2003-present in a format suitable for easy communication of health risk to the public Relevant Public Health Competencies (PHC) Key Public Health of Canada Competencies addressed during the WNV portion of this project with Saskatoon Public Health Observatory, Public Health Services SHR & Dr. Nathaniel Osgood include: 1.0 Public Health Sciences; 2.0 Assessment and Analysis 3.0 Policy and Program Planning, Implementation and Evaluation; 4.0 Partnerships, Collaboration and Advocacy 6.0 Communication Contact information Karen Yee, MPH Candidate School of Public Health. Health Sciences Building 107 Wiggins Road. University of Saskatchewan Saskatoon, SK. S7N 5E5; Email: [email protected]