Abstract
BackgroundVector control is an effective way of reducing malaria transmission. The main vector control methods include the use of insecticide-treated bed nets and indoor residual spraying (IRS). Both interventions rely on the continuing susceptibility of Anopheles to a limited number of insecticides. However, insecticide resistance, in particular pyrethroid-DDT cross-resistance, is a challenge facing malaria vector control in Africa because pyrethroids represent the only class of insecticides approved for treating bed nets and DDT is commonly used for IRS. Here baseline data are presented on the insecticide susceptibility levels of malaria vectors prior to The Gambian indoor residual spraying intervention programme.MethodsAnopheles larvae were collected from six malaria surveillance sites (Brikama, Essau, Farafenni, Mansakonko, Kuntaur and Basse) established by the National Malaria Control Programme and the UK Medical Research Council Laboratories in The Gambia. The mosquitoes were reared to adulthood and identified using morphological keys and a species-specific polymerase chain reaction assay. Two- to three-day old adult female mosquitoes were tested for susceptibility to permethrin, deltamethrin and DDT using standard WHO protocols, insecticide susceptibility test kits and treated papers.ResultsAll Anopheles mosquitoes tested belonged to the Anopheles gambiae complex. Anopheles arabiensis was predominant (54.1%), followed by An. gambiae s.s. (26.1%) and Anopheles melas (19.8%). Anopheles gambiae s.s. and An. arabiensis were found at all six sites. Anopheles melas was recorded only at Brikama. Mosquitoes from two of the six sites (Brikama and Basse) were fully susceptible to all three insecticides tested. However, DDT resistance was found in An. gambiae from Essau where the 24 hours post-exposure mortality was <80% but 88% for permethrin and 92% for deltamethrin.ConclusionThis current survey of insecticide resistance in Anopheles provides baseline information for monitoring resistance in The Gambia and highlights the need for routine resistance surveillance as an integral part of the proposed nation wide IRS intervention using DDT.
Highlights
IntroductionThe main vector control methods include the use of insecticide-treated bed nets and indoor residual spraying (IRS)
Vector control is an effective way of reducing malaria transmission
This study presents baseline data in support of the National Malaria Control Programme (NMCP) prior to The Gambian indoor residual spraying (IRS) intervention
Summary
The main vector control methods include the use of insecticide-treated bed nets and indoor residual spraying (IRS) Both interventions rely on the continuing susceptibility of Anopheles to a limited number of insecticides. Knockdown resistance (kdr) associated with a single point mutation in the gene encoding the voltage-gated sodium channel is a common mechanism of resistance to both pyrethroids and DDT This mutation results in a leucine to phenylalanine substitution found predominantly in West Africa (kdr-w) or a leucine to serine substitution (kdr-e), which was originally identified in Kenya but is found in several other African countries including Cameroon, Equatorial Guinea, Gabon, Angola and Uganda [15,16,17,18]. Both forms of kdr presumably function through reducing the affinity of DDT and pyrethroids for their target site on the sodium channel [15]
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