Low and seasonal malaria transmission in the middle Senegal River basin: identification and characteristics of Anopheles vectors

Abstract

BackgroundDuring the last decades two dams were constructed along the Senegal River. These intensified the practice of agriculture along the river valley basin. We conducted a study to assess malaria vector diversity, dynamics and malaria transmission in the area.MethodsA cross-sectional entomological study was performed in September 2008 in 20 villages of the middle Senegal River valley to evaluate the variations of Anopheles density according to local environment. A longitudinal study was performed, from October 2008 to January 2010, in 5 selected villages, to study seasonal variations of malaria transmission.ResultsAmong malaria vectors, 72.34% of specimens collected were An. arabiensis, 5.28% An. gambiae of the S molecular form, 3.26% M form, 12.90% An. pharoensis, 4.70% An. ziemanni, 1.48% An. funestus and 0.04% An. wellcomei. Anopheles density varied according to village location. It ranged from 0 to 21.4 Anopheles/room/day and was significantly correlated with the distance to the nearest ditch water but not to the river.Seasonal variations of Anopheles density and variety were observed with higher human biting rates during the rainy season (8.28 and 7.55 Anopheles bite/man/night in October 2008 and 2009 respectively). Transmission was low and limited to the rainy season (0.05 and 0.06 infected bite/man/night in October 2008 and 2009 respectively). During the rainy season, the endophagous rate was lower, the anthropophagic rate higher and L1014F kdr frequency higher.ConclusionsMalaria vectors are present at low-moderate density in the middle Senegal River basin with An. arabiensis as the predominant species. Other potential vectors are An. gambiae M and S form and An. funestus. Nonetheless, malaria transmission was extremely low and seasonal.

Insecticide Resistance Allele Frequencies in Anopheles gambiae before and after Anti-Vector Interventions in Continental Equatorial Guinea

“…It has been reported before that kdr allele frequencies can vary seasonally, and in our data, samples were pooled for an entire year. 19,62,63 We also have shown that selection has acted strongly on this locus after the application of pyrethroid insecticides.…”

Desiccation tolerance as a function of age, sex, humidity and temperature in adults of the African malaria vectorsAnopheles arabiensisPatton andAnopheles funestusGiles

“…Alternatively, pockets of individuals displaying a greater tolerance for desiccation may also be able to persist through the dry season; and indeed, this seems to be the case, especially given the seasonality of malaria in some areas (e.g. Patz et al, 1998;Charlwood et al, 2000;Tanser et al, 2003;Ndiath et al, 2012).…”

Plasmodium falciparum infection rates for some Anopheles spp. from Guinea-Bissau, West Africa

“…This is consistent with the earlier study in Senegal ( Ndiath et al , 2011) but differs from a recent survey conducted in Mali ( Fryxell et al , 2012). The study site in Senegal located in the village of Dielmo (13°43'N, 16°24'W) ( Ndiath et al , 2012)) was geographically closer (200km) than Mali sites (>800km) to our collection sites in Guinea-Bissau. The Senegal study site at Dielmo and nine of our study sites were proximal (<50km) to the Atlantic Ocean, while Mali is a land-locked country at least 500km away from the Atlantic Ocean.…”

“…Very low infection rates and absence of malarial parasites, traditionally found in West and Central African populations of A. pharoensis has always led to the conclusion that this mosquito plays little role in malaria transmission despite its anthropophilic habits and that it can be easily experimentally infected ( DeMeillon, 1947; Ndiath et al , 2012; Tchouassi et al , 2012). In drier Sahel regions of Africa where the major vectors of malaria are absent or very rare and irrigated rice and other crop lands are increasing, A. pharoensis is considered more important at maintaining low levels of malaria ( Kerah-Hinzoumbe et al , 2009; Kibret et al , 2010).…”