Highly focused anopheline breeding sites and malaria transmission in Dakar

Abstract

BackgroundUrbanization 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 Anopheles breeding sites in the city and to study the dynamics of larval density and adult aggressiveness in ten characteristically different urban areas.MethodsTen 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 Plasmodium falciparum 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.ResultsIn September-October 2007, 19,451 adult mosquitoes were caught among which, 1,101 were Anopheles gambiae s.l. 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 An. gambiae s.l. were found to be positive for P. falciparum (CSP index = 0.64%). EIR ranged from 0 infected bites per person per year in Yoff Village to 16.8 in Almadies. The An. gambiae complex population was composed of Anopheles arabiensis (94.8%) and Anopheles melas (5.2%). None of the An. melas were infected with P. falciparum. Of the 54 water collection sites monitored, 33 (61.1%) served as anopheline breeding sites on at least one observation. No An. melas 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.ConclusionThe 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.

Patterns and determinants of malaria risk in urban and peri-urban areas of Blantyre, Malawi

“…In our study, attempts to capture by aspiration indoor-resting mosquitoes were successful for Culex species in both urban and peri-urban settings, but the only Anopheles adults captured were from households of participants attending peri-urban HFs. The absence of Anopheles mosquitoes in samples from households within urban Blantyre has previously been reported [17] and is in contrast to entomological findings from other cities in Africa where Anopheles mosquitoes have been identified [18–20]. It is not clear whether the scarcity of Anopheles sampled mosquitoes within Blantyre city limits is due to the genuine paucity of vectors in the city or the method of collection (the Prokopack aspirator).…”

Spatial Patterns of Plasmodium falciparum Clinical Incidence, Asymptomatic Parasite Carriage and Anopheles Density in Two Villages in Mali

“…6 Those who are bitten most have the highest chance of being infected and amplify transmission by transmitting malaria parasites to a large proportion of mosquitoes. 10 The variation in mosquito exposure can partly be explained by the variation in the presence of water bodies, and thus potential mosquito breeding sites, which have been related to malaria morbidity in various urban 5,11,12 and rural 8,13 settings. It has been hypothesized that by targeting malaria control efforts to areas of intense malaria transmission the community impact of interventions can be maximized.…”

The use of remotely sensed environmental data in the study of malaria

“…Blood meal sources are abundant in cities so the dispersion of the vectors is low resulting in urban transmission dynamics being primarily driven by proximity of breeding sites (Trape et al, 1992;Staedke et al, 2003). The malaria risk is heterogeneous over small distances, and transmission can vary among different districts of the same city, as shown in Brazzaville (Trape and Zoulani, 1987) and Dakar (Machault et al, 2009). The consequence of this situation is that effective control can be expected from environmental management, including vegetation clearance, modification of river boundaries, draining swamps and insecticide treatment of open water bodies.…”

“…In Dakar, the non-populated areas, mapped using SPOT images (2.5 m and 20 m resolution) were masked when predicting malaria risk (Machault et al, 2010). Indeed, as it is known that the peak of anopheline aggressiveness occurs in the middle of the night in Dakar (Machault et al, 2009) and because evening and night activities are expected to take place mainly in or around dwellings, areas without infrastructure were excluded from predictions. On a worldwide scale, a recent study provided a map of the global spatial extent of P. vivax malaria, together with estimates of the human population at risk of any level of transmission (Guerra et al, 2010).…”