Residual Effects of Insecticide-Based Malaria Control Interventions on Malaria Vectors and the Status of Insecticide Resistance in Western Kenya
Wanjala, Christine Ludwin
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Malaria is a human disease caused by a sporozoan from the genus Plasmodium, transmitted by a bite of Anopheles mosquitoes. Insecticides remain the major tool for control of malaria vectors in Kenya and therefore the potential of such programs to be compromised by the reported insecticide resistance is a major concern. Studies in western Kenya have reported reemergence of morbidity and malaria attributed child mortality which has been linked to reported spread of insecticide resistance in anophiline mosquitoes. The aim of this study was to evaluate the residual effect of insecticides used for indoor residual spraying and impregnated on long lasting insecticide nets, and also determine the status of insecticide resistance in malaria vectors from western Kenya. Wall bioassays were performed on mud slabs and filter papers sprayed with lambdacyhalothrin and deltamethrin using mosquitoes collected from Ahero, Kisian, Chulaimbo, Emutete, Emakakha, Iguhu and Kabula. Net bioassays were performed on long lasting insecticide nets (LLINs) collected from the field using wild caught mosquitoes from Emutete and Kabula. Kisumu strain, a susceptible reference strain was used as a control. Chemical analysis of the netting material was performed using gas chromatography. World Health Organization tube bioassays was conducted using standard diagnostic dosages of Lambdacyhalothrin, Deltamethrin, Permethrin, DDT, Bendiocarb and Malathion tested on Anopheles mosquitoes collected from Ahero, Kisian, Chulaimbo, Emutete, Emakakha, Iguhu and Kabula. Biochemical assays, where the enzymatic activity of three enzymes (monooxygenases, esterases and Glutathione S-trasferases) were performed on susceptible and resistant mosquitoes preserved after WHO tube bioassays. Mosquitoes were identified to species level using Polymerase Chain Reaction. Genotyping was done on the susceptible and resistant mosquitoes after the WHO tube bioassays using Real-Time Polymerase Chain Reaction. Pyrethroid susceptible An. gambiae and An. arabiensis colonies from Bungoma and Ahero was raised and their genetic and biochemical changes monitored from generation to generation. The mortality of mosquitoes from all sites decreased significantly with time after spraying (75% mortality after six months) and with the age of the LLINs (60% mortality after 24 months). Insecticide concentration decreased significantly from 0.14 μg/ml in new LLINs to 0.077 μg/ml in LLINs older than 18 months. WHO susceptibility tests indicated that An. gambiae has developed high level of resistance to pyrethroids and DDT in Ahero, Kisian, Chulaimbo, Emutete, Emakakha, Iguhu and Bungoma. Resistant to bendiocarb in Iguhu and Kabula and susceptible to Malathion (100% mortality) in Ahero, Kisian, Chulaimbo, Emutete, Emakakha, Iguhu and Bungoma. There was an elevation of monooxygenases and esterases enzymatic activities in resistant An. gambiae mosquito populations exposed to Lambdacyhalothrin, Permethrin, Deltamethrin and DDT but no elevation in glutathione s- transferases. A high frequency of L1014S allele was detected in xviii An. gambiae s.s. population but there was no kdr allele found in An. arabiensis mosquitoes. Successive selection for deltamethrin resistance showed a steady increase in the mosquito mortality with 100% mortality in fifth and third generations for Kabula and Ahero mosquitoes respectively. The frequency of the homozygous ss (L1014s) allele was high in the first generation of Kabula mosquito populations but reduced in the subsequent generations. Long lasting insecticide nets should be replaced with new nets every three years and not five years as recommended and also high levels of ITN coverage and usage should be maintained. There is also an urgent need for development and deployment of non-pyrethroid based vector control tools.