Mapping of zones potentially occupied by Aedes vexans and Culex poicilipes mosquitoes, the main vectors of Rift Valley fever in Senegal

Abstract

Abstract. A necessary condition for Rift Valley fever (RVF) emergence is the presence of Aedes (Aedimorphus) vexans andCulex (Culex) poicilipes mosquitoes carrying the arbovirus and responsible for the infection. This paper presents a detailed mapping in the Sahelian region of Senegal of zones potentially occupied by these mosquitoes (ZPOMs) whose population density is directly linked to ecozones in the vicinity of small ponds. The vectors habitats and breeding sites have been characterized through an integrated approach combining remote sensing technology, geographical information systems, geographical positioning systems and field observations for proper geo-referencing. From five SPOT-5 images (~10 m spatial resolution) with appropriate channels, a meridional composite transect of 290 x 60 km was first constructed at the height of the summer monsoon. Subsequent ZPOMs covered major ecozones from north to south with different hydrological environments and different patterns pond distributions. It was found that an overall area of 12,817 ha ± 10% (about 0.8% of the transect) is occupied by ponds with an average ZPOM 17 times larger than this (212,813 ha ± 10% or about 14% of the transect). By comparing the very humid year of 2003 with 2006 which had just below normal rainfall, the ZPOMs inter-annual variability was analyzed in a sandy-clayey ecozone with an important hydrofossil riverbed within the Ferlo region of Senegal. Very probably contributing to an increased abundance of vectors by the end of August 2003, it was shown that the aggregate pond area was already about 22 times larger than in August 2006, corresponding to an approximately five times larger total ZPOM. The results show the importance of pin-pointing small ponds (sizes down to 0.1 ha) and their geographical distribution in order to assess animal exposure to the RVF vectors.

The potential for remote sensing and hydrologic modelling to assess the spatio-temporal dynamics of ponds in the Ferlo Region (Senegal)

“…The potential of radar satellite images (Annor et al, 2009;Di Baldassarre et al, 2009;) for water body inventory have also been improved, with the advantage of being independent of cloud cover (Horritt et al, 2001;Herold et al, 2004). Thus, an efficient and simple method to study the spatial dynamics of temporary ponds would consist in mapping the ponds using satellite images acquired at different dates (Lacaux et al, 2007;Tourre et al, 2008). However, a daily follow-up is not possible with this approach, given the strong inverse relationship between spatial resolution and revisit time.…”

TerraSAR-X high-resolution radar remote sensing: an operational warning system for Rift Valley fever risk

“…Its helio-synchronous orbits with X-band, Synthetic Aperture Radar (SAR) of 9.65 GHz, 300 MHz bandwidth with 4-polarization modes and repeat cycles of 11 days allows processing of resolution images on different swath widths. If these new results agree with those obtained from Spot-5 during clear atmospheric conditions, this new tele-epidemiological approach will be integrated into the RVFews as described by Tourre et al (2008).…”

Rift Valley fever dynamics in Senegal: a project for pro-active adaptation and improvement of livestock raising management

“…Rainfall events were to be the primary input, since the distribution and intensity of these events determine pond dynamics with regard to the presence of larvae sites and eggs hatching. The resulting maps were the zone potentially occupied by mosquitoes (ZPOMs) Vignolles et al, 2009), a product of hazards and vulnerability of livestock parked at night (Tourre et al, 2008;Ndione et al, 2009).…”