- Testing the impact of the landscape changes on two mosquito species (Ae. vexans and Cx.

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

EDEN conference : 12 Mai 2010 - Montpellier, France

Soti V., Tran A., Fontenille D, Lancelot R, Chevalier V., Thiongane Y., Degenne P., Lo Seen D. , Bégué A., Guégan J.F.

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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)

Objectives

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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

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Model and Data description

Vegetation map Host camp map

Roughness index (variable)

Camp location (variable)

Quickbird imagery 05/08/2005

Quickbird imagery

& Field investigation

Classification Validation

Mosquito density in space and in time Mosquito Diffusion

model

Mosquito production (3)

(output/input)

Validation Mosquito field

data 2002/2003

Validation

DATA

Water surface (output/input)

Daily Rainfall 2002/2003

Hydrologic Model (1)

Mosquito abundance

model (2)

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Mosquito abundance modeling for Furdu pond during the rainy season 2002

0 20 40 60 80 100 120 140

0.00.20.40.60.81.0

Days

Aedes(relative values)

0 20 40 60 80 100 120 140

0.00.20.40.60.81.0

Days

Culex (relative values)

Inred, the female mosquito abundance Ingreen, the nulliparous mosquito

Inblack, mosquito captured in Furdu in 2002 Inblue, Furdu water surface

Ae. vexans Cx. poicilippes

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Application to all ponds in the field of study

0 500000 1000000 1500000 2000000 2500000 3000000

Abundance

15/06/2002 29/06/2002

13/07/2002 27/07/2002

10/08/2002 24/08/2002

07/09/2002 21/09/2002

05/10/2002 19/10/2002

02/11/2002 16/11/2002

30/11/2002 14/12/2002

28/12/2002 S1

S10 S19

S28 S37

S46 S55

S64 S73

S82 S91

Days

Ponds

10/08/2002 24/08/2002

07/09/2002 21/09/2002

05/10/2002 19/10/2002

02/11/2002 16/11/2002

30/11/2002 14/12/2002

28/12/2002 11/01/2003

25/01/2003 S1 S6 S11 S16 S21 S26 S31 S36 S41 S46 S51 S56 S61 S66 S71 S76 S81 S86 S91 S96 0

0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

Aedes vexans Culex poicilipes

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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)

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Mosquito diffusion model

P

• 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

[

]

) ,

(P t div D P t grad P t P t P t

t ρ α β

ρ _{− ⋅ = −}

∂

∂

– In an homogeneous landscape, the diffusivity tensor is a constant

– Adding attractive forces (host density):

– Taking into account the landscape: D(P,t) = D_L(P,t)

[

]

div ^H ⋅ ⋅

+ ρ

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Environmental Environmental Environmental

Environmental parameters parameters parameters parameters integration integration integration integration

Attraction forces (Host Camp) Landscape “roughness”

[

] [

]

) ,

(P t div D P t grad P t div K P t P t grad H P t P t P t

t ^L ρ ρ ^H ρ α β

ρ _{− ⋅ + ⋅ ⋅ = −}

∂

∂ ⋅

grad H = flux direction to H attractive force H = host density

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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

Roughness index

Indice 1: Unfavourable Indice 2: Moderate Indice 3: Favourable Landscape influence on mosquito displacement

Vegetation map Landscape roughness map

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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

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Results : Ae vexans spatial diffusion

BARKEDJIFURDU

No vegetation cover Sparse vegetation Dense vegetation Vegetation in 2005

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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

– Ponds surrounded by vegetation

Identification of risk years and risk periods Typology of ponds at risk

Mapping in space and time the host-vector contact

(Chevalier et al., 2005)

Risk factors

(Soti et al., 2007)

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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

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Thanks for your attention

Valérie Soti sv.nev@ntropic.fr

soti@cirad.fr