Modelling the risk of Foot and Mouth Disease transmission ......Population of buffaloes in KNP...
Transcript of Modelling the risk of Foot and Mouth Disease transmission ......Population of buffaloes in KNP...
Modelling the risk of Foot and Mouth Disease transmission at the wildlife/livestock interface of
Kruger National Park
Ferran Jori & Eric Etter UPR 22, CIRAD
FMD context in RSA
• Endemic in KNP
• Efficient control measures: – Fence– FMD zonation– Systematic vaccination– Regular surveillance– Mouvement control
• No Outbreaks of FMD since 1983: Free status by OIE since 1995
• At least 6 declared outbreaks since 2000
• A diversity of factors are considered responsible for this situation
In this context, there is a need • to understand and quantify the pathways leading to FMDV transmission between wildlife and cattle
• to quantify parameters having a major contribution to the risk of transmission
• to identify those areas and scenarios where the risk is higher in order to target surveillance and control efforts
Risk Analysis
A systematic method to deal with risk.
RISK COMMUNICATION
HAZARDIDENTIFICATION RISK ASSESSMENT
Release assessment
Exposure assessment
Consequence assessment
Risk estimation
RISK MANAGEMENT
Risk evaluation
Option evaluation
Implementation
Monitoring and review
Materials and Methods• Risk =annual probability for one
cattle being infected by FMDV due to contacts with wild buffalo at the KNP interface.
• Two events were considered:
1
2
– Event 1: buffaloes escaping from KNP
– Event 2: Cattle entering KNP
Materials and Methods• Quantitative risk assessement was
carried using software package @Risk (Palissade Corporation)
• Inputs are probability distributions calculated according to the information available or produced
• They are combined with each other several thousands of iterations to produce final estimation of risk.
Quantitative estimation of risk
• Monte Carlo simulation• Stochastic & iterative approach
Mathematic modelProb buffalo excreting
Prob buffalo crossing fence
Output (Risk) :Buffalo excreting and crossing the fence
Event pathway considered
Is the buffalo carrying transmissible
virus?
No (1-P1)
P1
R=P1
yes
Event pathway considered
Is the buffalo carrying transmissible
virus?
No (1-P1)
P1
R=P1
Is thisbuffalo crossing
the fence?
yes
Event pathway considered
Is the buffalo carrying transmissible
virus?
No (1-P1)
P1
R=P1xP2
Is thisbuffalo crossing
the fence?
yes
P2 yesNo (1-P2)
Event pathway considered
Is the buffalo carrying transmissible
virus?
No (1-P1)
P1
R=P1xP2xP3
Is thisbuffalo crossing
the fence?
yes
P2 yesNo (1-P2)Is there any
contact with cattleOUTSIDE of the KNP?
yes
No (1-P3)P3
Event pathway considered
Is the buffalo carrying transmissible
virus?
No (1-P1)
P1
R=P1xP2xP3xP4
Is thisbuffalo crossing
the fence?
yes
P2 yesNo (1-P2)Is there any
contact with cattleOUTSIDE of the KNP?
yes
No (1-P3)P3 yes
No (1-P4)
Is this contact responsible of transmission?
P4
Event pathway considered
Is the buffalo carrying transmissible
virus?
No (1-P1)
P1
R=P1xP2xP3xP4
Is thisbuffalo crossing
the fence?
yes
P2 yesNo (1-P2)Is there any
contact with cattleOUTSIDE of the KNP?
yes
No (1-P3)P3 yes
No (1-P4)
Is this contact responsible of transmission?
Risk appearsP4
Event pathway considered
Is the buffalo carrying transmissible
virus?
No (1-P1)
P1
R=(P1xP2xP3xP4)+(P5xP4)
Is thisbuffalo crossing
the fence?
yes
P2 yesNo (1-P2)Is there any
contact with cattleOUTSIDE of the KNP?
yes
No (1-P3)P3 yes
No (1-P4)
Is this contact responsible of transmission?
Risk appears
Is there any contact with cattle
IN the KNP?
No (1-P5)
yesP5
P4yes
No (1-P4)
Is this contact responsible of transmission?
P4
+
Epidemiology of FMD in buffalo.• Seasonal breeders. Most calves born in summer (Dec-April)• Each calf cohort within a herd generally becomes infected from 6
months onwards when calves are weaned.• During acute infection there is considerable excretion of virus in all
body secretions during a period ranging between 1 and 14 days.• At 1 year age, most calves have become infected with FMD virus• After that period, 17 to 70 % of the animals remain carriers
thereafter and this status might persist up to 24 years (Vosloo et al.,2007) or will heal completely.
• Transmission of virus from adult carriers to susceptible animals appears to be a rare event.
Inputs Release AssessmentName Function Mean value Source
Population of buffaloes in KNP (1995-2006) RiskNormal(23377; 5293,1) 23377 KNP records
Fraction of young in KNP herds RiskNormal(0,1171; 0,0264) 0,117 KNP records
Probability for one buffalo to leave KNP RiskNormal(0,0031;0,00106) 0,003105885 KNP/Mpumalaga VS Records
Probability for a young buffalo to leave KNP RiskNormal(0,158;0,092) 0,17 Mpumalaga VS records
Prevalence in young buffalo (< 1 year) 0,89 Thomson, 1984
Prevalence in adult buffalo (> 1 year) 0,92-0,98 Thomson, 1984
Viraemy (in days) RiskPert(1; 6; 14) 6,5 Gainaru et al., 1986
Age of weaning (in months) RiskPert(2;4;6) 4 R. Bengis, pers. comm.
Probability for a young to excrete (Age of weaning-12)/12*days of viraemia/365 0,016 Thomson, 1984Probability of an excreting buffalo becoming a carrier RiskPert(0,17;0,6;0,7) 0,54 Vosloo, 2007
Inputs Exposure AssessmentName Function Mean value Source
Total cattle population in KNP buffer zone Fixed value: 84105 Data from 2007
Vaccination coverage (1996-2006) RiskNormal(0,754545;0,0665) 0,7545157 Mpumalaga VS records
Probability of observing a contact RiskBeta(18+1; 30-18+1) 0,59375 Farmers questionnaire Bushbuckridge
Time of contact (mns) buffalo / cattle RiskDiscrete({8.54.114.1194.2197};{0,33.0,22.0,167.0,167.0,11}) 467,5 Farmers questionnaire
Bushbuckridge
Probability for a cattle to cross the fence RiskBetal(22543;601974;0;1) 0,036038994 Fence questionnaireProbability of contact bw buffalo and cattle IN the park RiskBeta(371+1;8760-371+1) 0,04245606 Farmers questionnaire
BushbuckridgeEstimated time of contact for cattle IN the park (mns) RiskDiscrete({2160.720}; {12.365}) 765,8356 Farmers questionnaire
Bushbuckridge
Consequence Assessment
• Transmission rate for carriers (Trc) 0,0148± 0,014 infs/carrier/month calculated out of 9 experiments buffalo –cattle
• We considered transmission rate for young excreting buffalo to be 100%
• (Pty) = 1 if Tc> 360 mnIf Tc<360 mn then Pty=0.5
Results
Model Outputs
Probabilities on escaped buffalo
On average, It is 13 times more likely to get a carrier buffalo crossing the KNP fence than an acutely excreting buffalo doing so
-10 0 10 20 30 40 50 60
Valu
es x
10^
4
0,0
0,5
1,0
1,5
2,0
2,5
3,0
3,5
4,0
Final risk estimationsEvent 1
Event 2
5,0% 90,0% 5,0%
0,0066 0,1675
-0,0
5
0,00
0,05
0,10
0,15
0,20
0,25
0,30
0,35
0,40
0,45
0,50
0
2
4
6
8
10
12
14
FR1=Probability of one cattle to be infected outside KNP
FR1=Probability of one unvaccinated cattle to be infected by FMDV outside KNP
Minimum 7,244E-005Maximum 0,4895Mean 0,0606Std Dev 0,0557Values 5000
Crude sensitivity analysis FR1
-0,3
-0,2
-0,1 0,0
0,1
0,2
0,3
0,4
0,5
0,6
F
Sensitivity analysis FR2
Results
Scenario playing
Scenario comparison of FR1 depending on numbers of escaped buffaloes
Influence of drop in vaccination coverage in risk of transmission
0,00
0,05
0,10
0,15
0,20
0,25
0,30
0,35
0,40
0,45
ConclusionsAreas of improvement
• Can be improved as new information is producedCf: Vaccination efficiency
• Some inputs still require additional data to reduce uncertainty– Contacts wildlife/cattle difficult
to assess– Number of young animals
escaping
• Model requires peer review & validation before it can be used for management purposes
Positive points
• Development of a tool useful to start modelling transmission at the wildlife/livestock interface
• Provides consistent responses to major FMD control strategies:• escapes of buffaloes,• vaccination coverage
• Highly flexible, integrative and relatively easy to use/ communicate
Possible applications
• Scenario playing / discussion with stakeholders– What if scenarios (cf. Increase
of buffalo populations in KNP)• Compare risk in different areas
or settings– Limpopo vs Mpumalanga
Province– KNP vs LNP or GNP interface
• Identification of high risk areas in absence of efficient reporting systems
• Identify information gaps
Acnowledgements• Bruce Gummow, University of Pretoria (JCU)• Ben Du Plessis, Mpumalanga Veterinary Services• Roy Bengis, KNP• Nada Abu Samra, University of Pretoria • Livio Heath, Wilna Vosloo (ARC‐OVI)• Dipa Brahmbatt, TAMU
Thank you for your attention!