Modelling the risk of Foot and Mouth Disease transmission at the wildlife/livestock interface of Kruger National Park

Transcription

1 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

2 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

3 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

4 Risk Analysis A systematic method to deal with risk. HAZARD IDENTIFICATION RISK ASSESSMENT Release assessment Exposure assessment Consequence assessment RISK MANAGEMENT Risk evaluation Option evaluation Implementation Monitoring and review Risk estimation RISK COMMUNICATION

5 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: Event 1: buffaloes escaping from KNP Event 2: Cattle entering KNP 1 2

6 Materials and Methods Quantitative risk assessement was carried using software (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.

7 Quantitative estimation of risk Monte Carlo simulation Stochastic & iterative approach Prob buffalo excreting Mathematic model Prob buffalo crossing fence Output (Risk) : Buffalo excreting and crossing the fence

8 Event pathway considered Is the buffalo carrying transmissible virus? P1 No (1-P1) R=P1

9 Event pathway considered Is the buffalo carrying transmissible virus? P1 No (1-P1) Is this buffalo crossing the fence? R=P1

10 Event pathway considered Is the buffalo carrying transmissible virus? P1 No (1-P1) P2 Is this buffalo crossing the fence? No (1-P2) R=P1xP2

11 Event pathway considered Is the buffalo carrying transmissible virus? P1 No (1-P1) Is there any contact with cattle OUTSIDE of the KNP? No (1-P3) P2 P3 Is this buffalo crossing the fence? No (1-P2) R=P1xP2xP3

12 Event pathway considered Is the buffalo carrying transmissible virus? P1 No (1-P1) Is there any contact with cattle OUTSIDE of the KNP? No (1-P3) P2 P3 Is this buffalo crossing the fence? Is this contact responsible of transmission? P4 No (1-P2) No (1-P4) R=P1xP2xP3xP4

13 Event pathway considered Is the buffalo carrying transmissible virus? P1 No (1-P1) Is there any contact with cattle OUTSIDE of the KNP? No (1-P3) P2 P3 Is this buffalo crossing the fence? Is this contact responsible of transmission? No (1-P2) P4 Risk appears No (1-P4) R=P1xP2xP3xP4

14 Event pathway considered Is the buffalo carrying transmissible virus? No (1-P1) Is there any contact with cattle OUTSIDE of the KNP? No (1-P3) + P2 P1 P3 Is there any contact with cattle IN the KNP? Is this buffalo crossing the fence? Is this contact responsible of transmission? P5 P4 No (1-P2) No (1-P5) Is this contact responsible of transmission? No (1-P4) P4 Risk appears No (1-P4) R=(P1xP2xP3xP4)+(P5xP4)

15 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.

16 Inputs Release Assessment Population of buffaloes in KNP ( ) Name Function Mean value Source RiskNormal(23377; 5293,1) Fraction of young in KNP herds RiskNormal(0,1171; 0,0264) 0,117 Probability for one buffalo to leave KNP RiskNormal(0,0031;0,00106) 0, Probability for a young buffalo to leave KNP RiskNormal(0,158;0,092) 0,17 Prevalence in young buffalo (< 1 year) 0,89 Prevalence in adult buffalo (> 1 year) 0,92-0,98 Viraemy (in days) RiskPert(1; 6; 14) 6,5 Age of weaning (in months) RiskPert(2;4;6) 4 KNP records KNP records KNP/Mpumalaga VS Records Mpumalaga VS records Thomson, 1984 Thomson, 1984 Gainaru et al., 1986 R. Bengis, pers. comm. Probability for a young to excrete (Age of weaning-12)/12*days of viraemia/365 0,016 Thomson, 1984 Probability of an excreting buffalo becoming a carrier RiskPert(0,17;0,6;0,7) 0,54 Vosloo, 2007

17 Inputs Exposure Assessment Name Function Mean value Source Total cattle population in KNP buffer zone Fixed value: Vaccination coverage ( ) RiskNormal(0,754545;0,0665) 0, Data from 2007 Mpumalaga VS records Probability of observing a contact RiskBeta(18+1; ) 0,59375 Farmers questionnaire Bushbuckridge Time of contact (mns) buffalo / cattle RiskDiscrete({ };{0,33.0,22. 0,167.0,167.0,11}) Probability for a cattle to cross the fence RiskBetal(22543;601974;0;1) 0, Probability of contact bw buffalo and cattle IN the park Estimated time of contact for cattle IN the park (mns) 467,5 Farmers questionnaire Bushbuckridge Fence questionnaire RiskBeta(371+1; ) 0, Farmers questionnaire Bushbuckridge RiskDiscrete({ }; {12.365}) 765,8356 Farmers questionnaire Bushbuckridge

18 Consequence Assessment Transmission rate for carriers (Tr c ) 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% (P ty ) = 1 if Tc> 360 mn If Tc<360 mn then Pty=0.5

19 Results Model Outputs

20 Probabilities on escaped buffalo Values x 10^4 On average, It is 13 times more likely to get a carrier buffalo crossing the KNP fence than an acutely excreting buffalo doing so 0,0 0,5 1,0 1,5 2,0 2,5 3,0 3,5 4,0

21 Event 1 FR1=Probability of one cattle to be infected outside KNP 0,0066 0,1675 5,0% 90,0% 5,0% Final risk estimations FR1=Probability of one unvaccinated cattle to be infected by FMDV outside KNP Minimum 7,244E-005 Maximum 0,4895 Mean 0,0606 Std Dev 0,0557 Values ,05 0,00 0,05 0,10 0,15 0,20 0,25 0,30 0,35 0,40 0,45 0,50 Event 2

22 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

23 Sensitivity analysis FR2

24 Results Scenario playing

25 Scenario comparison of FR1 depending on numbers of escaped buffaloes

26 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

27 Conclusions 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 Areas of improvement Can be improved as new information is produced Cf: 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

28 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

29 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

30 Thank you for your attention!