Dengue virus, transmitted primarily by Aedes aegypti, is endemic along the Kenyan coast, including Lamu County. With no broadly applicable vaccine or specific antiviral treatment available for dengue, vector control remains the principal strategy to limit transmission. Widespread or prolonged insecticide use can select for resistance in vector populations, potentially compromising control measures. Permethrin, a pyrethroid insecticide commonly used for vector control, is the focus of this study, which aimed to determine permethrin resistance levels and compare DENV-2 vector competence among Ae. aegypti sub‑populations from selected sites in Lamu County.
Adult Ae. aegypti were collected from Shela, Hindi, and Mokowe in Lamu County using traps. Field-collected females were reared to an F2 generation for experimental assays. WHO insecticide resistance assays were performed to classify populations as susceptible or resistant to permethrin based on mortality.
For vector competence testing, F2 female mosquitoes were offered an infectious bloodmeal containing dengue‑2 virus. Fully engorged females were selected and held for up to 21 days. At scheduled time points, mosquitoes were processed and tested for midgut infection and systemic dissemination by plaque assay.
For genetic analysis, legs were dissected from 31 field (F0) female mosquitoes for genomic DNA isolation. Polymerase Chain Reaction amplified a mitochondrial DNA target using primers HCO2198 and LCO1490 (COI region). Amplicons were Sanger sequenced to assess genetic identity and diversity among sampled mosquitoes.
WHO assays revealed clear differences among the three sub‑populations. The Shela sub‑population was classified as susceptible, showing >98% mortality following permethrin exposure. In contrast, the Hindi and Mokowe sub‑populations displayed permethrin resistance with mortality rates of 47% and 37.6%, respectively. These data indicate heterogeneous insecticide susceptibility across collection sites within Lamu County.
Following exposure to dengue‑2 infectious bloodmeals and incubation, the study reported midgut infection and disseminated infection (virus detected beyond the midgut) across all three sub‑populations. Among the tested mosquitoes, midgut infection counts and proportions were: Hindi 14/60 (23%), Mokowe 24/90 (27%), and Shela 40/130 (31%). Dissemination was detected in Hindi 12/60 (20%), Mokowe 11/90 (12%), and Shela 21/130 (16%).
Overall, the results indicate substantial infection and dissemination rates for DENV‑2 across sub‑populations regardless of observed permethrin susceptibility status, although absolute rates varied by site.
All sequenced specimens were identified as Aedes aegypti formosus. Genetic diversity metrics reported high haplotype diversity (1.0) but low nucleotide diversity (0.01184). The combination of many distinct haplotypes with low nucleotide divergence suggests a population structure in which multiple closely related mitochondrial lineages co‑exist in Lamu County. The genetic analysis used standard COI primers (HCO2198 and LCO1490) and Sanger sequencing; details of individual haplotypes and sequence alignments are reported in the study supporting information.
The co‑occurrence of high DENV‑2 infection and dissemination rates with permethrin resistance in some Lamu sub‑populations raises concerns for reliance on pyrethroid‑based interventions alone. The Shela population, susceptible to permethrin, nevertheless demonstrated DENV‑2 infection and dissemination, indicating that susceptibility status does not eliminate transmission potential.
Identification of the Ae. aegypti formosus sub‑species and the observed genetic pattern (high haplotype but low nucleotide diversity) provide baseline information for regional entomologic surveillance and may inform investigations into gene flow, population connectivity, and the evolutionary dynamics of resistance.
From a public health perspective, the findings support continued insecticide resistance monitoring and consideration of integrated vector management approaches that do not rely solely on pyrethroids. The study emphasizes the need for local susceptibility data to guide insecticide choice and for ongoing surveillance of vector competence alongside resistance profiling.
This investigation in Lamu County found heterogeneous permethrin susceptibility across Ae. aegypti sub‑populations, with Shela susceptible (>98% mortality) and Hindi and Mokowe resistant (47% and 37.6% mortality). All tested sub‑populations exhibited measurable DENV‑2 midgut infection and dissemination following experimental exposure. Genetic analysis identified the sampled mosquitoes as Aedes aegypti formosus, with high haplotype diversity (1.0) and low nucleotide diversity (0.01184). These results highlight the persistence of transmission potential across insecticide‑susceptible and -resistant populations and support targeted resistance monitoring and diversified vector control strategies in the region.