Results from our recent study showed that variation in vector species and insecticide resistance in The Gambia is associated with malaria endemicity; with a notably higher prevalence of infection and insecticide resistance in the east of the country.
Malaria hotspots, areas with consistently higher than average transmission, may become increasingly common as malaria declines. This phenomenon, currently observed in The Gambia, may be caused by several factors, including some related to the local vectors, whose contribution is poorly understood. Therefore, this study looked at variation in mosquitoes to see if resistance to insecticides could explain hotspots of malaria transmission in The Gambia. In The Gambia, malaria transmission has decreased substantially over the last few years and has become increasingly heterogeneous. Malaria transmission follows rainfall pattern, beginning after the onset of the rains and peaking between October and November. Malaria prevalence in children under the age of 5 years is nationally 4–5%, though in some areas between 2 and 15%. In eastern Gambia, a cross-sectional survey across all ages in 2012 estimated malaria prevalence at above 30%.Vector control activities carried out by (GNMCP) have probably played a major role in reducing transmission. Through the introduction of long-lasting insecticide-treated bed nets (LLINs) and indoor residual spraying (IRS) with DDT since 1998. However, these gains may be reversed by insecticide resistance that has been recently observed in The Gambia.

Vector control activities carried out by (GNMCP) have probably played a major role in reducing transmission. Through the introduction of long-lasting insecticide-treated bed nets (LLINs) and indoor residual spraying (IRS) with DDT since 1998. However, these gains may be reversed by insecticide resistance that has been recently observed in The Gambia.

As part of a larger study investigating malaria transmission dynamics in The Gambia, the distribution and patterns of phenotypic resistance and mechanisms in An. gambiae s.l. populations was characterized. Specifically, the hypothesis that variation in the intensity of malaria transmission may be linked with variation in insecticide resistance, mediated by differences in species composition and resistance-related mutations was examined. Using WHO susceptibility bioassays, insecticide resistance status was determined in vector populations sampled from six pairs of villages across the country, each pair contained a low and high prevalence village based on malaria prevalence determined by a nationwide cross-sectional survey.

Lead by Kevin Ochieng’ Opondo, MRCG PhD Candidate registered at Liverpool School of Tropical Medicine, the study was jointly funded by the UK Medical Research Council (MRC) and the UK Department for International Development (DFID) under the MRC/DFID Concordat agreement and EDCTP2 programme supported by the European Union. The study was conducted by MRC Unit The Gambia in collaboration Gambia National Malaria Control Programme (GNMCP).

Results from the study indicate that three vector species were observed (23.5% Anopheles arabiensis, 31.2% Anopheles gambiae, 43.3%Anopheles coluzzii and 2.0% An. coluzzii × An. gambiae hybrids). Even at a fine scale, significant differences in species composition were detected within village pairs. Resistance to both DDT and deltamethrin was more common in An. gambiae, most markedly in the eastern part of The Gambia and partly attributable to differing frequencies of resistance mutations. The Vgsc-1014F target site mutation was strongly associated with both DDT (OR = 256.7, (95% CI 48.6–6374.3, p < 0.001) and deltamethrin survival (OR = 9.14, (95% CI 4.24–21.4, p < 0.001). A second target site mutation, Vgsc-1575Y, which co-occurs with Vgsc-1014F, and a metabolic marker of resistance, Gste2-114T, conferred additional survival benefits to both insecticides. DDT resistance occurred significantly more frequently in villages with high malaria prevalence (p = 0.025) though this did not apply to deltamethrin resistance.

According to Kevin, lead investigator, “Malaria mosquitoes, just like other organisms in the universe are fighting to survive no matter the cost. As they get smart to out maneuver the current arsenal put against them, we must aim to be smarter. The ability of malaria mosquitoes to withstand insecticides is just but one way that they employ to survive in the ever changing environment. Linking insecticide resistance and differences in malaria transmission is difficult to study let alone measure its impact on malaria transmission.”

Acknowledgements
We thank the leaders and members of six villages across The Gambia, who willingly allowed us to collect the data including the National Malaria Control Programme and MRCG staff.

Authors
Kevin Ochieng’ Opondo, David Weetman, Musa Jawara, Mathurin Diatta, Amfaal Fofana, Florence Crombe, Julia Mwesigwa, Umberto D’Alessandro and Martin James Donnelly

Read more about the study on the Biomedcentral website and blog
http://malariajournal.biomedcentral.com/articles/10.1186/s12936-016-120…
http://malariajournal.biomedcentral.com/articles/10.1186/s12936-016-120…