An Assessment of the Heterogeneity, Age Modulation and Protective Effect of Insecticide Resistance and Evaluation of Insecticide Treated Net Condition among Children under Five Years Of Age Infected With Malaria In Western Kenya.

Abstract

Vector control remains central to the fight against malaria with insecticide-treated bednets (ITNs) and indoor residual spraying (IRS) being a central component of the World Health Organization (WHO) global strategy. The recent upsurge of insecticide resistance in various parts of the world has led to fears that control efforts may be compromised. The frequency of insecticide resistance in western Kenya has not been mapped in a large scale and the effect it is likely to have on ability of vectors to survive exposure to ITNs is unknown. With malaria control programs embarking universal coverage every 3 years, it is important to determine whether ITNs are able to protect person using them once they have holes. The effect massive vector control have on vector age structure needs to be evaluated if successful transmission reduction is to be achieved, hence the need to evaluate age grading tools. Anopheles mosquitoes were sampled from clusters (sub-locations) then susceptibility to permethrin or deltamethrin determined. A subset of An. arabiensis samples from Bondo and Nyando underwent gene-expression profiling using whole genome microarray. Cross-sectional surveys of ITNs were conducted in houses in Gem and Bungoma in May 2013 where ITN condition, number and species of mosquitoes resting inside, and insecticidal activity against susceptible and resistant mosquitoes were quantified. Children under 5 years of age were recruited into a case control study at the local health facility and followed back to their homes and their ITNs taken for evaluation. A questionnaire detailing household characteristics and net use was administered. Ages of wild mosquitoes collected at different times were estimated using the Near Infra-Red Spectrometry (NIRS). Mortality to deltamethrin ranged from 45-100%, and to permethrin from 30-100% and varied between An. arabiensis and An. gambiae s.s. although this was not statistically significant. There was no correlation between mortality to permethrin and deltamethrin in An. arabiensis (Z=2.9505, P=0.2483). Microarray results revealed that CYP6M2 (the mutant allele) was consistently expressed at higher levels in Bondo. A total of 552 An. gambiae s.l., 5 An. funestus s.l. and 137 Culex spp. were found resting inside nets in Bungoma. The ITNs from Bungoma retained strong activity against a susceptible laboratory (>90% mortality) but not against first filial generation (Fl) offspring of field-collected An. gambiae s.s. «60% mortality). There was no difference in the number and size of holes in ITN s used by cases versus controls thus there was no association of the presence of malaria infection with sleeping under a holed ITN. Roof type and number of people that slept inside the households was associated with malaria infection (P=O.013; P=O.006). Higher resistance was observed in older mosquitoes compared to the younger ones. Insecticide resistance is heterogeneous within small geosraphical areas. The CYP6M2, identified in this study is associated with resistance to -multiple insecticide classes. In areas with pyrethroid resistant vectors, mosquitoes are able to enter ITNs and survive. In an area of high insecticide resistance, ITN s may not offer protection against malaria once they have holes. Age was observed to select for resistance in mosquitoes such that there was an increase in insecticide resistance with increasing mosquito age. Results from this study will inform resistance monitoring programs on the need for multiple sampling within a district. The finding of mosquitoes inside ITN s indicates the need for integrated vector management for example IRS with a non-pyrethroid. There is a need for prompt replacement of ITNs especially those used by children once they start acquiring holes. The observation of older mosquitoes having higher insecticide resistance means these mosquitoes may survive insecticide exposure in ITNs and thus the need for insecticide resistance mitigation strategies.