Highly focused anopheline breeding sites and malaria transmission in Dakar

<p>Abstract</p> <p>Background</p> <p>Urbanization has a great impact on the composition of the vector system and malaria transmission dynamics. In Dakar, some malaria cases are autochthonous but parasite rates and incidences of clinical malaria attacks have been recorded at low levels. Ecological heterogeneity of malaria transmission was investigated in Dakar, in order to characterize the <it>Anopheles </it>breeding sites in the city and to study the dynamics of larval density and adult aggressiveness in ten characteristically different urban areas.</p> <p>Methods</p> <p>Ten study areas were sampled in Dakar and Pikine. Mosquitoes were collected by human landing collection during four nights in each area (120 person-nights). The <it>Plasmodium falciparum </it>circumsporozoite (CSP) index was measured by ELISA and the entomological inoculation rates (EIR) were calculated. Open water collections in the study areas were monitored weekly for physico-chemical characterization and the presence of anopheline larvae. Adult mosquitoes and hatched larvae were identified morphologically and by molecular methods.</p> <p>Results</p> <p>In September-October 2007, 19,451 adult mosquitoes were caught among which, 1,101 were <it>Anopheles gambiae s.l</it>. The Human Biting Rate ranged from 0.1 bites per person per night in Yoff Village to 43.7 in Almadies. Seven out of 1,101 <it>An. gambiae s.l</it>. were found to be positive for <it>P. falciparum </it>(CSP index = 0.64%). EIR ranged from 0 infected bites per person per year in Yoff Village to 16.8 in Almadies. The <it>An</it>. <it>gambiae </it>complex population was composed of <it>Anopheles arabiensis </it>(94.8%) and <it>Anopheles melas </it>(5.2%). None of the <it>An. melas </it>were infected with <it>P. falciparum</it>. Of the 54 water collection sites monitored, 33 (61.1%) served as anopheline breeding sites on at least one observation. No <it>An</it>. <it>melas </it>was identified among the larval samples. Some physico-chemical characteristics of water bodies were associated with the presence/absence of anopheline larvae and with larval density. A very close parallel between larval and adult densities was found in six of the ten study areas.</p> <p>Conclusion</p> <p>The results provide evidence of malaria transmission in downtown Dakar and its surrounding suburbs. Spatial heterogeneity of human biting rates was very marked and malaria transmission was highly focal. In Dakar, mean figures for transmission would not provide a comprehensive picture of the entomological situation; risk evaluation should therefore be undertaken on a small scale.</p

Rift Valley fever in a zone potentially occupied by Aedes vexans in Senegal: dynamics and risk mapping

This paper presents an analysis of the interaction between the various variables associated with Rift Valley fever (RVF) such as the mosquito vector, available hosts and rainfall distribution. To that end, the varying zones potentially occupied by mosquitoes (ZPOM), rainfall events and pond dynamics, and the associated exposure of hosts to the RVF virus by Aedes vexans, were analyzed in the Barkedji area of the Ferlo, Senegal, during the 2003 rainy season. Ponds were identified by remote sensing using a high-resolution SPOT-5 satellite image. Additional data on ponds and rainfall events from the Tropical Rainfall Measuring Mission were combined with in-situ entomological and limnimetric measurements, and the localization of vulnerable ruminant hosts (data derived from QuickBird satellite). Since “Ae. vexans productive events” are dependent on the timing of rainfall for their embryogenesis (six days without rain are necessary to trigger hatching), the dynamic spatio-temporal distribution of Ae. vexans density was based on the total rainfall amount and pond dynamics. Detailed ZPOM mapping was obtained on a daily basis and combined with aggressiveness temporal profiles. Risks zones, i.e. zones where hazards and vulnerability are combined, are expressed by the percentages of parks where animals are potentially exposed to mosquito bites. This new approach, simply relying upon rainfall distribution evaluated from space, is meant to contribute to the implementation of a new, operational early warning system for RVF based on environmental risks linked to climatic and environmental conditions

Modeling the dynamics of mosquito breeding sites vs rainfall in Barkedji area, Senegal

International audienceMosquito-borne diseases like Rift Valley Fever and Malaria that cause serious health threat to human and livestock populations are known to correlate with the tremendous increase of associated mosquito vectors following periods of widespread and heavy rainfall. In the Barkedji area, Senegal, rainfall occur only during the July–October wet season, and mosquito breeding sites are provided by relatively small temporary ponds, which account for the vast majority of the water surfaces during the rainy season. Given that rain fed ponds play a key role in the epidemiology of the mosquito-borne diseases, we have developed an approach allowing to model the flooding/drying dynamics of rain fed ponds in the Barkedji area by combining the detection of ponds using optical remote sensing techniques, field data on a small set of monitored ponds and modeling of both the pond profile, shapes and the flooding/drying dynamics at the single-pond level for each pond in the entire region.As a result, we have computed on output the daily flooding/drying dynamics for each of the 1345 rain fed ponds detected in Barkedji area as a function of the daily rainfall in input. As dry and fluctuating ponds are less productive in mosquitoes, the ponds are characterizes in terms of flooding-drying fluctuations and of resistance to dryness by computing the total number of time per year a pond dries out and the pond lifetime, respectively. And, clusters of ponds with identical behavior, i.e., clusters of temporary, semi-permanent and permanent ponds, were subsequently identified.As a perspective, we show how this work can be used for modelling mosquito population dynamics and addressing the issues of associated impacts of climate change

Climate impact on malaria in northern Burkina Faso

The Paluclim project managed by the French Centre National d’Etudes Spatiales (CNES) found that total rainfall for a 3-month period is a confounding factor for the density of malaria vectors in the region of Nouna in the Sahel administrative territory of northern Burkina Faso. Following the models introduced in 1999 by Craig et al. and in 2003 by Tanser et al., a climate impact model for malaria risk (using different climate indices) was created. Several predictions of this risk at different temporal scales (i.e. seasonal, inter-annual and low-frequency) were assessed using this climate model. The main result of this investigation was the discovery of a significant link between malaria risk and low-frequency rainfall variability related to the Atlantic Multi-decadal Oscillation (AMO). This result is critical for the health information systems in this region. Knowledge of the AMO phases would help local authorities to organise preparedness and prevention of malaria, which is of particular importance in the climate change context

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

Abstract. In the vicinity of the Barkedji village (in the Ferlo region of Senegal), the abundance and aggressiveness of the vector mosquitoes for Rift Valley fever (RVF) are strongly linked to rainfall events and associated ponds dynamics. Initially, these results were obtained from spectral analysis of high-resolution (~10 m) Spot-5 images, but, as a part of the French AdaptFVR project, identification of the free water dynamics within ponds was made with the new high-resolution (down to 3-meter pixels), Synthetic Aperture Radar satellite (TerraSAR-X) produced by Infoterra GmbH, Friedrichshafen/Potsdam, Germany. During summer 2008, within a 30 x 50 km radar image, it was found that identified free water fell well within the footprints of ponds localized by optical data (i.e. Spot-5 images), which increased the confidence in this new and complementary remote sensing technique. Moreover, by using near real-time rainfall data from the Tropical Rainfall Measuring Mission (TRMM), NASA/JAXA joint mission, the filling-up and flushingout rates of the ponds can be accurately determined. The latter allows for a precise, spatio-temporal mapping of the zones potentially occupied by mosquitoes capable of revealing the variability of pond surfaces. The risk for RVF infection of gathered bovines and small ruminants (~1 park/km 2) can thus be assessed. This new operational approach (which is independent of weather conditions) is an important development in the mapping of risk components (i.e. hazards plus vulnerability) related to RVF transmission during the summer monsoon, thus contributing to a RVF early warning system