Distribution, molecular characterization and insecticide resistance of Malaria vectors in Kakamega county, western Kenya

Abstract

Despite a significant global decline in malaria cases and mortality over the past two decades, recent years have seen resurgence in Western highlands of Kenya. This study aimed to assess the distribution, genetic diversity, and insecticide resistance of malaria vectors in Kakamega County located in the highlands of Western Kenya. Ecological distributions, productivity, species composition of anopheline mosquito in various aquatic habitats were cross-sectionally surveyed. Furthermore, the genetic analysis of Cytochrome Oxidase gene 1 (Cox1) gene from mitochondrial deoxyribonucleic acid (mtDNA) was performed. Additionally, phenotypic and mechanisms of resistance was conducted using World Health Organization (WHO) tube bioassays tests and genotyping of alleles associated with resistance to An. gambiae s.l to evaluate mortality rates against deltamethrin, permethrin, Dichlorodiphenyltrichloroethane (DDT) and pirimiphos-methyl was conducted respectively. Data analysis, which is Analysis of Variance (ANOVA), was done to compare mosquito abundance, distribution and species composition among study sites at 0.05 significance level. Genetic analysis was performed by Clustal 1.81 and genetic distances calculated using Kimura’s 2-parameter model in MEGA6. For resistance mechanisms of allele’s analysis, a Hardy-Weinberg equilibrium test for kdr and Ace-1R genotypes was applied. A total of 1,566 habitats were sampled; mean aquatic densities varied by species and habitat type. Highest densities of An. gambiae s.l (20.4) were observed in farmland, while artificial forests favored An. funestus s.l (8.2) and An. coustani s.l (4.0). A total of 1,566 aquatic habitats were sampled in various land use types. The mean aquatic densities of An. gambiae s.l. (46.2 ± 5.54 SD), An. funestus s.l. (5.3), An. coustani s.l. (1.7), An. implexus (0.13) and An. squamosus (2.0) were observed in fish ponds, burrow pits, drainage ditches, and tire tracks, respectively. A high mean density of An. gambiae s.l. was reported in farmland (20.4) while high mean abundance of An. funestus s.l. (8.2) and An. coustani s.l. (4.0) were observed in artificial forests. The isolates had minimal genetic diversity with 28 (90%), 2 (7%) and 1 (3%) clustering with An. gambiae, An. macupalpis and An. Rufipes, respectively, on a phylogenetic tree. The study revealed that, Anopheles arabiensis was predominant species in all exposed tests, Mumias east (62%), Malava (68%), Ikolomani (77%) and Lurambi (82%). Results showed mortality was low in Mumias east (80.6%) and high in Lurambi (89.0%). Resistance to pyrethroids (Deltamethrin and Permethrin) and organochlorines (DDT) were observed in all mosquito populations with < 90% mortality. Pirimiphos-methyl resistance in An. arabiensis and An. gambiae s.s were observed in mosquitoes collected from Mumias East and Malava subcounties. Kdr L1014 mutations observed were heterozygous in both An. arabiensis and An. gambiae s.s with all populations being consistent with Hardy-Weinberg equilibrium (P>0.05). A low frequency of G119S mutation was observed in resistance population. Populations tested for