Susceptibility and BehaVioural Responses of the Genus Anopheles (Diptera: Culicidae).To bednets . Impregnated With a Synergized Pyrethrum Insecticide ,Formulation and Potential for Malaria Control
KEFA, Stanley Orwa Sum
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The use of insecticide-treated nets (ITNs) has become a key cost-effective intervention in malaria control. However, there is need to develop new insecticide formulations to sustain the technology and manage the threat of Anopheles mosqu ito resistance to pyrethroid insecticides solely used in ITNs. Susceptibility and behavioural responses of Anopheles gambiae s.1. to different bednet fabrics treated with synergized natural pyrethrum-formulation was assessed using WHO cone and tunnel bioassay procedures. Effect on indoor resting and feeding behaviour was evaluated under field situation in 24 selected village huts in Ahero area. Anopheles siblings were identified using polymerase chain reaction (PCR) while real time PCR (RT-PCR) was used to genotype kdr gene mutation on species obtained from Kisian, Ahero and Kipsitet areas. Data were transformed into logarithms or arcsine and subjected to analysis of variance (ANaYA). Means were compared using least significant difference (LSD, P=O.OS) while logit or regression analysis was used to model dose-response relationships. Results showed no significant additional effect on knockdown at durations longer than IS minutes post-initial exposure to treated nets regardless of the dose (KD-IS min and KD-30, P= 0.6312; KD-IS and KD-60min, P=0.IS90). After 1-3 washes, nets impregnated with natural pyrethrum-formulation at 500 mg/m ' still achieved more than 8S% knockdown and mortality. Unwashed nets had 90% feeding inhibition and over 90% knockdown and kill up to six months. There was a significant (P=O.OOO I) fabric-dose interaction where nylon achieved standard 80% mortality on mosquitoes at 200 mg/rn ' as compared to 37S mg/m ' with polyester. It was however, not possible to achieve 80% kill with cotton at the test doses. No kdr genes were detected in An. arabiensis but there was 100% frequency of the L 10 14S kdr mutation in the An. gambiae s.s. Natural pyrethrum formulation achieved significantly (P=O.OOO I) greater kill than pyrethroids against An. gambiae s.s. with kdr genes. There were significantly (P=O.OOO I) higher numbers of indoor resting and fed An. gambiae s.1. collected in houses with untreated nets than houses with bed nets treated with natural pyrethrum-formulation. Overall, the results· demonstrate that knockdown (KD) tests on Anopheles mosquitoes could be standardized at IS minutes post-initial exposure and not 60 minutes, as previously. reported, to save on time and resources. The results give empirical evidence and parameters pertinent to effective use of natural pyrethrum formulation in ITNs. Susceptibility of An. gambiae s.s. with kdr alleles and wild forms to the formulation provides crucial lead to management of resistance in malaria vectors.