A diffusion model to predict spatial and temporal population dynamics of Rift valley fever vectors in Northern Senegal

Transcription

1 A diffusion model to predict spatial and temporal population dynamics of Rift valley fever vectors in Northern Senegal Soti V., Tran A., Fontenille D, Lancelot R, Chevalier V., Thiongane Y., Degenne P., Lo Seen D., Bégué A., Guégan J.F. EDEN conference : 12 Mai Montpellier, France

2 Objectives Objective : - Testing the impact of the landscape changes on two mosquito species (Ae. vexans and Cx. poicilipes) diffusion in space and in time in Barkedji area (Ferlo, Senegal) 2

3 Study area Geography and climate - Barkedji village (Ferlo, Senegal) - Sahelian climate ~ 400mm/year - 3 seasons: a dry and cold season from November to March a dry and hot season from April to June a hot and a rainy season from July to Nov. Temporary ponds are favourable areas for RVF transmission 3

4 Mosquito field data 2002/2003 Daily Rainfall 2002/2003 Quickbird imagery 05/08/2005 Quickbird imagery & Field investigation Classification Validation Hydrologic Model (1) Validation DATA Model and Data description Vegetation map Host camp map Water surface (output/input) Mosquito abundance model (2) Roughness index (variable) Mosquito production (output/input) Validation Camp location (variable) Mosquito Diffusion model (3) Mosquito density in space and in time4

5 Mosquito abundance modeling for Furdu pond during the rainy season 2002 Ae. vexans Cx. poicilippes Aedes (relative values) Culex (relative values) Days Days In red, the female mosquito abundance In green, the nulliparous mosquito In black, mosquito captured in Furdu in 2002 In blue, Furdu water surface 5

6 Application to all ponds in the field of study Aedes vexans Culex poicilipes S1 S10 S19 S28 S37 S46 S55 S64 S73 S82 S Ponds 10/08/ /08/ /09/ /09/ /10/ /10/ /11/ /11/ /11/ /12/ /12/ /01/ /01/ /06/ /06/ /07/ /07/ /08/ /08/ /09/ /09/ /10/ /10/ /11/ /11/ /11/ /12/ /12/2002 Days 6 Abundance S1 S6 S11 S16 S21 S26 S31 S36 S41 S46 S51 S56 S61 S66 S71 S76 S81 S86 S91 S

7 Mosquito diffusion model Female mosquitoes spread from the breeding sites (the ponds) to take blood meal on mammals Hypotheses: 1. The spread is not homogeneous in space but depends on the land cover type, mosquitoes prefering sheltered covers 2. The mosquitoes are attracted by the presence of animals or humans The landscape has an influence on the extent and speed of the spread Model used: a diffusion model (Shigesada et al., 1986 ; Okubo & Kareiva, 2001) revised to be adapted to flying insects (Raffy & Tran, 2005) 7

8 Mosquito diffusion model Details of the diffusion model The variation of the mosquito density (ρ) in a pixel P depends on the emergence (α), the number of deaths (β), and the number of mosquitoes arriving to or leaving the pixel [ D( P, t) gradρ ( P, t) ] = α( P, t) β ( P, ) ρ ( P, t) div t t In an homogeneous landscape, the diffusivity tensor is a constant P Taking into account the landscape: D(P,t) = D L (P,t) Adding attractive forces (host density): [ K ( P, t) ( P, t) grad H ( P, t) ] + div H ρ 8

9 Environmental parameters integration Landscape roughness [ Attraction forces (Host Camp) ] [ ] ρ ( P, t ) div DL ρ ( P, t ) grad ρ ( P, t ) + div KH ( P, t ) ρ ( P, t ) grad H ( P, t ) = α ( P, t ) β ( P, t ) t grad H = flux direction to H attractive force H = host density 9

10 Roughness index estimation Legend Dense Forest Savanna Dense Shrub Savanna Dense Grass Savanna Cassia tora Sparse Forest Savanna Sparse Shrub Savanna Dense Grass Savanna Butte Culture Bare soil Vegetation map Landscape influence on mosquito displacement Roughness index Indice 1: Unfavourable Indice 2: Moderate Indice 3: Favourable Landscape roughness map 10

11 Application to Barkedji and Furdu ponds Two different ponds (Barkedji and Furdu) Aedes vexans mosquitoes Different simulated landscapes around the two ponds More or less favorable to mosquito spread No vegetation cover Sparse vegetation Dense vegetation 11

12 Results : Ae vexans spatial diffusion Sparse vegetation Dense vegetation Vegetation in 2005 FURDU BARKEDJI No vegetation cover 12

13 Discussion Methodological aspects Spatial and temporal modelling of a population dynamics Assimilation of environmental variables Contribution to the understanding of RVF epidemiology Previous epidemiologic studies (statistical models) Small ponds Risk factors Ponds surrounded by vegetation (Chevalier et al., 2005) Identification of risk years and risk periods Typology of ponds at risk Mapping in space and time the host-vector contact (Soti et al., 2007) 13

14 Conclusions and perspectives The first results are promising but have to be improved by: - Testing the model to all ponds of the study area - Appling the model to Cx poicilippes mosquito - Testing the effect of the hosts density on host/vector contact - Performing a sensitivity analysis - Performing a model validation with field mosquito data 14

15 Thanks for your attention Valérie Soti 15