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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]