Browsing by Author "Onyango, Shirley A."

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    Genetic diversity and population structure of the human malaria parasite Plasmodium falciparum surface protein Pfs47 in isolates from the lowlands in Western Kenya
    (PLOS ONE, 2021-11-29) Onyango, Shirley A.; Ochwedo, Kevin O.; Machani, Maxwell G.; Omondi, Collince J.; Debrah, Isaiah; Ogolla, Sidney O.; Ming-Chieh, Lee; Guofa, Zhou; Kokwaro, Elizabeth; Kazura, James W.; Afrane, Yaw A.; Githeko, Andrew K.; Daibin, Zhong; Guiyun, Yan
    Plasmodium falciparum parasites have evolved genetic adaptations to overcome immune responses mounted by diverse Anopheles vectors hindering malaria control efforts. Plasmo dium falciparum surface protein Pfs47 is critical in the parasite’s survival by manipulating the vector’s immune system hence a promising target for blocking transmission in the mos quito. This study aimed to examine the genetic diversity, haplotype distribution, and popula tion structure of Pfs47 and its implications on malaria infections in endemic lowlands in Western Kenya. Cross-sectional mass blood screening was conducted in malaria endemic regions in the lowlands of Western Kenya: Homa Bay, Kombewa, and Chulaimbo. Dried blood spots and slide smears were simultaneously collected in 2018 and 2019. DNA was extracted using Chelex method from microscopic Plasmodium falciparum positive samples and used to genotype Pfs47 using polymerase chain reaction (PCR) and DNA sequencing. Thirteen observed haplotypes of the Pfs47 gene were circulating in Western Kenya. Popula tion-wise, haplotype diversity ranged from 0.69 to 0.77 and the nucleotide diversity 0.10 to 0.12 across all sites. All the study sites displayed negative Tajima’s D values although not significant. However, the negative and significant Fu’s Fs statistical values were observed across all the study sites, suggesting population expansion or positive selection. Overall genetic differentiation index was not significant (FST = -0.00891, P > 0.05) among parasite populations. All Nm values revealed a considerable gene flow in these populations. These results could have important implications for the persistence of high levels of malaria trans mission and should be considered when designing potential targeted control interventions.
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    Insecticide Resistance and Its Intensity in Urban Anopheles Arabiensis in Kisumu City, Western Kenya: Implications for Malaria Control in Urban Areas
    (PLoS ONE, 2024-11) MachaniI, Maxwell G.; Nzioki, Irene; Onyango, Shirley A.; Onyango, Brenda; Githure, John; Atieli, Harrysone; Wang, Chloe; Lee, Ming-Chieh; Githeko, Andrew K.
    Background The rise of insecticide resistance poses a growing challenge to the effectiveness of vector control tools, particularly in rural areas. However, the urban setting has received comparatively less focus despite its significance in attracting rural to urban migration. Unplanned urbanization, often overlooked, exacerbates insecticide resistance as Anopheles mosquitoes adapt to the polluted environments of rapidly expanding cities. This study aimed to assess the insecticide susceptibility status of malaria vectors and identify potential underlying mechanisms across three distinct ecological settings characterized by differing levels of urbanization in Kisumu County, Kenya. Methods The study was conducted in 2022–2023 in Kisumu County, western Kenya. Field-derived An. gambiae (s.l.) larvae collected from a long stretch of urban-to-rural continuum were phenotyped as either resistant or susceptible to six different insecticides using the World Health Organization (WHO) susceptibility test. Polymerase chain reaction (PCR) techniques were used to identify the species of the An. gambiae complex and screened for mutations at voltage-gated sodium channels (Vgsc-1014F, Vgsc-1014S, Vgsc-1575Y) and acetylcholinesterase (Ace1) target site mutation 119S. Metabolic enzyme activities (non-specific β-esterases and monooxygenases) were evaluated in mosquitoes not exposed to insecticides using microplate assays. Additionally, during larval sampling, a retrospective questionnaire survey was conducted to determine pesticide usage by the local inhabitants. Results Anopheles arabiensis dominated in urban (96.2%) and peri-urban (96.8%) areas, while An. gambiae (s.s.) was abundant in rural settings (82.7%). Urban mosquito populations showed high resistance intensity to deltamethrin (Mortality rate: 85.2% at 10x) and suspected resistance to Pirimiphos-methyl and bendiocarb while peri-urban and rural populations exhibited moderate resistance intensity to deltamethrin (mortality rate >98% at 10x). Preexposure of mosquitoes to a synergist piperonyl butoxide (PBO) significantly increased mortality rates: from 40.7% to 88.5% in urban, 51.9% to 90.3% in peri-urban, and 55.4% to 87.6% in rural populations for deltamethrin, and from 41.4% to 78.8% in urban, 43.7% to 90.7% in peri-urban, and 35% to 84.2% in rural populations for permethrin. In contrast, 100% mortality to chlorfenapyr and clothianidin was observed in all the populations tested. The prevalence of L1014F mutation was notably higher in urban An. arabiensis (0.22) unlike the peri-urban (0.11) and rural (0.14) populations while the L1014S mutation was more prevalent in rural An. gambiae (0.93). Additionally, urban An. arabiensis exhibited elevated levels of mixed function oxidases (0.8/mg protein) and non-specific esterases (2.12/mg protein) compared to peri-urban (0.57/mg protein and 1.5/mg protein, respectively) and rural populations (0.6/mg protein and 1.8/mg protein, respectively). Pyrethroids, apart from their use in public health through LLINs, were being highly used for agricultural purposes across all ecological settings (urban 38%, peri-urban 36% and rural 37%) followed by amidine group, with organophosphates, neonicotinoids and carbamates being of secondary importance. Conclusion These findings show high resistance of An. arabiensis to insecticides commonly used for vector control, linked with increased levels of detoxification enzymes. The observed intensity of resistance underscores the pressing issue of insecticide resistance in urban areas, potentially compromising the effectiveness of vector control measures, especially pyrethroid-treated LLINs. Given the species’ unique behavior and ecology compared to An. gambiae, tailored vector control strategies are needed to address this concern in urban settings.
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    Molecular characterization and genotype distribution of thioester-containing protein 1 gene in Anopheles gambiae mosquitoes in western Kenya
    (Malaria Journal, 2022) Onyango, Shirley A.; Ochwedo, Kevin O.; Machani, Maxwell G.; Olumeh, Julius O.; Debrah, Isaiah; Omondi, Collince J.; Ogolla, Sidney O.; Ming Chieh, Lee; Guofa, Zhou; Kokwaro, Elizabeth; Kazura, James W.; Afrane, Yaw A.; Githeko, Andrew K.; Zhong, Daibin; Yan, Guiyun
    Background: Evolutionary pressures lead to the selection of efcient malaria vectors either resistant or susceptible to Plasmodium parasites. These forces may favour the introduction of species genotypes that adapt to new breeding habitats, potentially having an impact on malaria transmission. Thioester-containing protein 1 (TEP1) of Anopheles gambiae complex plays an important role in innate immune defenses against parasites. This study aims to character ize the distribution pattern of TEP1 polymorphisms among populations of An. gambiae sensu lato (s.l.) in western Kenya. Methods: Anopheles gambiae adult and larvae were collected using pyrethrum spray catches (PSC) and plastic dippers respectively from Homa Bay, Kakamega, Bungoma, and Kisumu counties between 2017 and 2020. Collected adults and larvae reared to the adult stage were morphologically identifed and then identifed to sibling species by PCR. TEP1 alleles were determined in 627 anopheles mosquitoes using restriction fragment length polymorphisms polymerase chain reaction (RFLP-PCR) and to validate the TEP1 genotyping results, a representative sample of the alleles was sequenced. Results: Two TEP1 alleles (TEP1*S1 and TEP1*R2) and three corresponding genotypes (*S1/S1, *R2/S1, and *R2/R2) were identifed. TEP1*S1 and TEP1*R2 with their corresponding genotypes, homozygous *S1/S1 and heterozygous *R2/S1 were widely distributed across all sites with allele frequencies of approximately 80% and 20%, respectively both in Anopheles gambiae and Anopheles arabiensis. There was no signifcant diference detected among the popula tions and between the two mosquito species in TEP1 allele frequency and genotype frequency. The overall low levels in population structure (FST=0.019) across all sites corresponded to an efective migration index (Nm=12.571) and low Nei’s genetic distance values (<0.500) among the subpopulation. The comparative fxation index values revealed minimal genetic diferentiation between species and high levels of gene fow among populations. Conclusion: Genotyping TEP1 has identifed two common TEP1 alleles (TEP1*S1 and TEP1*R2) and three corre sponding genotypes (*S1/S1, *R2/S1, and *R2/R2) in An. gambiae s.l. The TEP1 allele genetic diversity and population structure are low in western Kenya.