---
title: "Permethrin resistance and dengue‑2 vector competence of Aedes aegypti sub‑populations in Lamu Coun"
id: "plos-one-16-vector-competence-for-dengue-2-virus-of-permethrin-resistant-susceptible-aedes"
canonical_url: "https://medichelpline.com/clinical-feed/plos-one-16-vector-competence-for-dengue-2-virus-of-permethrin-resistant-susceptible-aedes"
content_type: "clinical_feed_article"
specialty: "Infectious Disease"
source_name: "PLOS ONE (Medicine)"
source_url: "https://journals.plos.org/plosone/article?id=10.1371/journal.pone.0359810"
published_at: "2026-10-07T14:00:00.000Z"
evidence_level: "Journal Feed"
license: "CC-BY-NC-4.0 / Informational Use"
---
# Permethrin resistance and dengue‑2 vector competence of Aedes aegypti sub‑populations in Lamu Coun
## Provenance & Clinical Metadata
- **Canonical URL:** https://medichelpline.com/clinical-feed/plos-one-16-vector-competence-for-dengue-2-virus-of-permethrin-resistant-susceptible-aedes
- **Specialty:** [Infectious Disease](https://medichelpline.com/clinical-feed/infectious-disease.md)
- **Primary Source:** PLOS ONE (Medicine)
- **Source URL:** [Original Journal Publication](https://journals.plos.org/plosone/article?id=10.1371/journal.pone.0359810)
- **Published At:** 2026-10-07T14:00:00.000Z
- **Evidence Rating:** Journal Feed
## Executive GIST (TL;DR)
- Dengue virus in Lamu County is primarily transmitted by **Aedes aegypti**; vector control remains the main prevention method because specific treatments and widely applicable vaccines are lacking. - The study assessed permethrin resistance and compared vector competence for **DENV-2** among three Lamu sub‑populations: Shela, Hindi, and Mokowe. - Wild mosquitoes were collected, reared to F2, and exposed to infectious DENV-2 bloodmeals; infection and dissemination were evaluated by plaque assay up to 21 days post‑feed. - WHO insecticide resistance assays showed Shela was susceptible to **permethrin** (>98% mortality), while Hindi and Mokowe were resistant (47% and 37.6% mortality, respectively). - Midgut infection rates were: Shela 31% (40/130), Hindi 23% (14/60), Mokowe 27% (24/90). Dissemination rates were: Shela 16% (21/130), Hindi 20% (12/60), Mokowe 12% (11/90). - Mitochondrial DNA (COI) was amplified using HCO2198 and LCO1490 primers, Sanger sequenced; all analyzed specimens were identified as **Aedes aegypti formosus**. - Genetic diversity: haplotype diversity was high (1.0) while nucleotide diversity was low (0.01184), indicating many distinct haplotypes with low sequence divergence. - The study reports substantial infection and dissemination of DENV-2 across sub‑populations and documents coexisting permethrin resistance in some sites, with implications for insecticide‑based control strategies in the region.
## Clinical Analysis & Structured Key Points
Vector competence for dengue-2 virus of permethrin resistant/ susceptible Aedes aegypti sub-populations from selected sites in Lamu County, Kenya | PLOS One Browse Subject Areas ? Click through the PLOS taxonomy to find articles in your field. For more information about PLOS Subject Areas, click here . Article Authors Metrics Comments Media Coverage Reader Comments Figures Figures Abstract Dengue virus, primarily transmitted by Aedes aegypti mosquitoes, is endemic in Lamu County. There is currently no definitive treatment for dengue; therefore, vector control remains the most effective method to curb transmission. Prolonged exposure of Ae. aegypti to insecticides may confer resistance. This study aimed to determine the levels of resistance to permethrin, a commonly used insecticide, and to compare the vectorial competence and genetic differences of resistant versus susceptible populations for dengue-2 virus. Ae. aegypti mosquitoes were collected from Lamu (Shela, Hindi, and Mokowe) using traps and assayed for permethrin resistance/ susceptibility. WHO insecticide resistance assays were conducted. The mosquitoes (F 2 ) were exposed to dengue-2 infectious bloodmeals and fully engorged mosquitoes selected, incubated for up to 21 days, and tested for infection and dissemination by plaque assay. Individual legs were dissected from 31 female (F 0 ) mosquitoes for isolation of genomic DNA. Polymerase Chain Reaction was used to amplify the target mitochondrial DNA using HCO2198 and LCO1490 primers. The amplicons were Sanger sequenced to determine genetic diversity. The Shela sub-population displayed susceptibility (>98% mortality), whereas the Hindi and Mokowe sub-populations exhibited resistance, with mortality rates of 47% and 37.6%, respectively. Of the mosquitoes from Hindi (n = 60); Mokowe (n = 90); and Shela (n = 130), n = 14 (23%), n = 24 (27%), and n = 40 (31%) respectively, had midgut infections with dengue-2; while n = 12 (20%), n = 11 (12%), and n = 21 (16%) respectively had disseminated infection. Genetic analysis showed that all the samples were Ae. aegypti formosus sub-species. Haplotype diversity was high at 1.0 while nucleotide diversity was low at 0.01184. These findings suggest Ae. aegypti formosus sub-species is prevalent in Lamu County. The study also demonstrated high infection and dissemination rates among the Ae. aegypti sub- populations. Citation: Qureish KS, Mulwa F, Odinga R, Yalwala S, Lutomiah J, Bargul JL, et al. (2026) Vector competence for dengue-2 virus of permethrin resistant/ susceptible Aedes aegypti sub-populations from selected sites in Lamu County, Kenya. PLoS One 21(10): e0359810. https://doi.org/10.1371/journal.pone.0359810 Editor: Gaurav Kumar, ICMR-National Institute of Malaria Research, INDIA Received: January 15, 2026; Accepted: September 16, 2026; Published: October 7, 2026 This is an open access article, free of all copyright, and may be freely reproduced, distributed, transmitted, modified, built upon, or otherwise used by anyone for any lawful purpose. The work is made available under the Creative Commons CC0 public domain dedication. Data Availability: All relevant data are within the manuscript and its Supporting Information files. The minimal data set and accompanying code is available at Harvard Dataverse repository via: https://doi.org/10.7910/DVN/YMK2LK . Funding: FE received funding from the Armed Forces Health Surveillance Branch (AFHSB) and its Global Emerging Infections Surveillance (GEIS) Section, FY2022 ProMIS ID: P0116_22_KY and FY2023 ProMIS ID P0094_23_KY. The funders had a role in validation of the project, administration and decision to publish. Competing interests: The authors have declared that no competing interests exist. Introduction Aedes aegypti mosquito is the primary vector for dengue virus [ 1 ]. It is mainly found in tropical and sub-tropical regions such as sub-Saharan Africa, the South American continent and South Asian countries [ 2 ]. It has also spread and managed to establish populations in temperate regions such as parts of Europe and East Asia [ 3 ]. Dengue fever is caused by four dengue virus (DENV) serotypes: DENV-1, DENV-2, DENV-3 and DENV-4 [ 4 ]. Dengue virus serotype 2 (DENV-2) is the most prevalent in Kenya, particularly along the coastal region [ 5 ]. Globally, an estimated 390 million dengue virus infections occur annually, of which 500,000 infected are hospitalized and more than 20,000 result in death [ 6 ]. In 2023, 6.5 million cases and over 6,800 deaths attributed to DENV were recorded globally, majority of cases being from South America, and South and Southeast Asian regions [ 7 ]. In 2024, the number of reported cases increased to 14.1 million globally, with the number of reported dengue-related deaths being more than 9,508 [ 8 ]. More than 3.6 million dengue cases and close to 2000 deaths were reported in 2025 [ 9 ]. In Kenya, the first dengue outbreak, which was caused by DENV-2, was reported in 1982 in the coastal towns of Malindi and Kilifi [ 10 ]. Since then, there have been subsequent dengue outbreaks in Kenya. There was a DENV-3 outbreak in 2011 in Mandera, which is in northern Kenya, with 381 suspected cases [ 11 ]; and a multiple serotypes (DENV-1, DENV-2, and DENV-3) outbreak in Mombasa city in the coastal region between 2013 and 2014 with 267 suspected cases; 101 confirmed and 1 death [ 12 ]. In 2017, there was another DENV-2 outbreak along the Kenyan coast which resulted in 1,117 reported cases and one death [ 13 ]. In 2019, the Kenyan south coast experienced a DENV-3 outbreak [ 14 ] with 286 confirmed cases. A reemergence of dengue occurred in Lamu and Mombasa counties in 2021 with 867 suspected cases; 36 of which were confirmed and 2 deaths [ 15 ]. In 2024, there was a multi-country outbreak of dengue which affected several countries in Africa including 13 countries in West Africa and two countries in Eastern Africa: Ethiopia and Kenya. There was a total of 88 cases reported from Kenya by April, 2024 [ 16 ]. These high infection rates cause a strain on health systems by overwhelming hospital capacity and healthcare providers. Dengue fever has a negative impact on the economy as well through high medical costs and lost productivity of the infected people. Currently, there is neither a specific drug treatment for dengue nor a suitable approved vaccine for the general population [ 17 ] leaving Aedes vector control as the only way to combat this disease. Vector control is achieved by various means, for instance by using insecticide treated nets and screens, and by spraying with insecticides [ 18 ]. While nets and screens have proven effective, their impact is increasingly constrained by the rise of insecticide resistance among target vectors, which is caused by a combination of genetic, biochemical, and environmental factors [ 19 ]. This resistance reduces their insecticidal efficacy, compounded by sustainability challenges such as wear and tear of the deployed screens over time [ 20 ]. The screens/ nets are highly effective against Anopheles mosquitoes but less so against other species such as Culex and Aedes mosquitoes [ 21 ]. Aedes mosquitoes are dawn and dusk feeders, therefore nets/ screens may not be very effective against them [ 22 ]. They may also be too expensive or unavailable to deploy in needy households [ 23 ]. This positions insecticides as the primary approach to vector control. Different types of insecticides are used for vector control including organophosphates such as malathion and P-methyl which inhibit the enzyme acetylcholinesterase (AChE), resulting in overabundance of the neurotransmitter acetylcholine. It accumulates at nerve synapses leading to nervous system overstimulation which causes hyperactivity, paralysis then death [ 24 ]. Organochlorines such as dieldrin and dichloro-diphenyl-trichloroethane (DDT) interact with the neuron membrane and alter the transmission of the nerve impulses by opening their sodium ion channels which causes them to fire rapidly leading to convulsive seizures and death [ 25 ]. Carbamates like bendiocarb interfere with acetylcholinesterase therefore disrupting the nervous system leading to overstimulation of receptors, paralysis, and death [ 26 ]. These groups of insecticides are gradually being replaced by pyrethrins due to their significant and persistent toxic effects on humans and the environment [ 27 ]. One such pyrethrin is permethrin, which the present study focuses on. Extensive use of insecticides and agricultural pesticides has led to development of resistance in many insect species [ 19 ]. Permethrin resistance has been observed in some Ae. aegypti populations to varying degrees [ 28 , 29 ] probably due to the differences in their genetic makeup [ 30 ]. In Lamu County, located on the Kenyan coast, there have been outbreaks of arboviral diseases such as dengue and chikungunya [ 31 ], as recently as 2025. It is believed that this is due to ineffective vector control due to insecticide resistance among vector populations [ 32 ]. Knockdown resistance (kdr), also known as altered target – site resistance [ 33 ] due to the mutation that alters the target- site, reduces or removes the toxic effect of the insecticide [ 34 ]. These kdr mutations are passed down through generations. The mode of inheritance of insecticide resistance shows homozygosity of the resistance and susceptible genes involved [ 35 ]. Some studies have shown that insecticide resistance may affect the vector competence of Ae. aegypti mosquitoes by increasing dissemination rates and altering their gut microbiota [ 36 ]. Vector competence is the intrinsic ability of a vector to be infected by a pathogen, amplify, disseminate, and transmit it [ 37 ]. This is affected by extrinsic and intrinsic factors [ 38 ]. Extrinsic factors include temperature and gut microbiota; higher temperatures increase susceptibility of mosquitoes to viruses [ 39 ]. The development of many pathogens is temperature sensitive with higher temperatures decreasing the extrinsic incubation period for the viruses [ 40 ]. Microbiota found in the gut of insect vectors also decrease viral and parasitic infections in mosquito vectors by activating immune responses or directly inhibiting pathogen development [ 41 ]. Internal factors include the immune responses and barriers such as: midgut infection barrier (MIB) which prevents the virus from attaching to the receptors present on midgut epithelial cells; midgut escape barrier (MEB) which prevents the release of viral particles from midgut epithelial cells; salivary gland infection barrier (SIB) which prevents the virus from infecting acinar cells; and salivary gland escape barrier (SEB) which prevents releasing of viral particles from salivary gland acinar cells [ 42 ]. To determine the genetic differences between sub-populations, the Cytochrome c oxidase subunit 1 (CO1) gene has proven very useful [ 43 ]. The CO1 gene is considered ideal for barcoding due to its presence in most living eukaryotes [ 44 ]. There have been recurring outbreaks of dengue, mostly associated with DENV-2, in most of coastal Kenya since 2011, with limited data on insecticide resistance of Ae. aegypti in Kenya, and how this impacts arbovirus transmission. Therefore, this study aimed to investigate the insecticide resistance among Ae. aegypti sub-populations in Lamu County and its effect on vector ability to acquire, amplify and transmit DENV-2, as well as entomological factors that contribute to dengue transmission. The findings of this study will help to understand the epidemiology of dengue in the county. Materials and methods Ethics approval This study was approved by the Scientific Ethics Review Unit (SERU) of the Kenya Medical Research Institute (KEMRI) under approval number, SERU 4768. Approval of the protocol and use of mice in the study was granted via the Animal Care and Use Committee (ACUC) (KEMRI/ACUC/02.09.23). The mice were anesthetized during feeding of Ae. aegypti mosquitoes using Pentobarbital sodium injection at a dosage of 75 mg/kg, and all efforts were made to minimize suffering. They were sacrificed by injecting them intraperitoneally with Ketamine Hydrochloride at a dosage of 150 mg/kg. The mice were monitored daily over a period of six weeks through the mosquito feedings until they were euthanized. Informed consent was not required owing to this study not involving human participants. Study sites This study was conducted in Lamu County, along the Kenyan Coast ( Fig 1 ) where three sites were selected for sample collection: Shela, Hindi, and Mokowe. Lamu County (latitude 2.2696° S, longitude 40.9006° E) generally experiences a warm, humid tropical climate with temperatures ranging from 24°C to 30°C throughout the year making it conducive for mosquito breeding. The county has a bimodal pattern of rainfall with the long rains occurring between April and July, with the highest rainfall occurring in the month of May; and short rains in November and December. Download: PNG larger image TIFF original image Fig 1. The map of Kenya and, inset, Lamu County showing the sampling sites. Base maps, boundaries and shape files of Kenyan map and administrative boundaries of the County and Sub-county were derived from GADM data version 4.1 ( https://gadm.org ) and the maps were generated using ArcGIS Version 10.2.2 ( http://desktop.arcgis.com/en/arcmap ) advanced license courtesy of Samuel Owaka. https://doi.org/10.1371/journal.pone.0359810.g001 Collection and identification of mosquito samples Adult mosquitoes were sampled using BG-Sentinel 2 Traps and CDC light traps baited with carbon dioxide (CO 2 ) [ 45 , 46 ]. The mosquitoes were collected during periods of higher population densities which usually coincide with rainy seasons, that is, April – June of 2023 ( S1 Table , S1 and S2 Files ). The traps were set outdoors close to potential breeding grounds like stagnant water and vegetation, between 6.30 pm and 7:00 pm in the evening and collected the following morning between 6:00am and 6.30am. The trapped mosquitoes were knocked down using 50% trimethylamine [ 47 ] and transported in mesh cages to the KEMRI laboratories in Kisumu for morphological identification to species level using taxonomic keys [ 48 , 49 ]. Genetic diversity determination DNA extraction. Field collected (n = 31) female mosquitoes (F 0 ) were sampled from various collection sites in Shela (n = 11), Hindi (n = 10), and Mokowe (n = 10). Each mosquito was from a different site to avoid analyzing siblings. Individual mosquito legs [ 50 ] were dissected using sterile forceps and placed in a sterile RNase/DNase-free Eppendorf tube containing a sterile copper bead, 200 ml of Phosphate buffered saline was added into each Eppendorf tube, and the sample homogenized in an Omni Bead Rapture 24 (OMNI International) set to run at the speed of 2.10 mm for 20 seconds. Qiagen™ DNeasy Blood and Tissue Extraction Kit [ 51 ] was then used to extract DNA from the homogenates following the manufacturers directives. Polymerase chain reaction. Polymerase Chain Reaction (PCR) was used to amplify the target mitochondrial DNA (COI) in mosquito samples using the universal reverse primer HCO2198 and forward primer LCO1490. The PCR master mix was prepared in a Biosafety level II molecular laboratory and contained 12.5 µl Amplitaq solution, 1 µl forward primer, 1 µl reverse primer, and 7.5 µl sterile nuclease free water. Next, 22µl of the master mix was transferred into well-labelled RNase-free, 0.2 ml (8-strip format, Invitrogen) PCR tubes and 3 µl of sample DNA added into each tube. The tubes were tightly sealed, vortexed, and placed in an Eppendorf Thermocycler set to run 35 reaction cycles of 94°C for 30 secs, 55°C for 30s, and 72°C for 60s. The amplicons were visualized in a 2% agarose gel with an expected band size of approximately 650 bp, stained with SYBR Green. The amplicons underwent clean up to remove leftover primers and deoxyribonucleotide triphosphates (dNTPs) from PCR. This was done using the Exo-CIP™ Rapid PCR Cleanup protocol [ 52 ]. The products were then quantified to determine the concentration of DNA using a nanodrop spectrophotometer, with an expected threshold of 50ng [ 53 ], and Sanger sequenced [ 54 ]. Bioassays for determination of susceptibility and resistance of Ae. aegypti to permethrin Aedes aegypti mosquitoes were subjected to World Health Organization (WHO) tube bioassays for susceptibility to permethrin [ 55 ]. Twenty-five mosquitoes were aspirated at a time from a total of one hundred from each of the three sites (n = 300) and transferred into a holding tube to ensure that damaged specimens were not included in the test. The holding tubes were set upright, mesh-screen end uppermost for approximately 30 minutes after which, any mosquitoes that may have been damaged during aspiration were removed. A sheet of paper impregnated with 0.75% permethrin insecticide was introduced into each of the exposure tubes then rolled into a cylinder and fastened into position with a copper clip. This dose is not the recommended one for evaluating the susceptibility of Aedes mosquitoes, however, it is a commonly used one [ 56 – 58 ]. The recommended WHO permethrin dose for Aedes mosquitoes is 0.4% [ 59 ]. The mosquitoes were introduced into the exposure tube by attaching it to a vacant screw-top in the slide. The mosquitoes were gently blown down into the exposure tube. The slide was closed and the holding tube detached. The exposure tubes were left standing upright with mesh-screen end uppermost for 1 hour at 27 °C ± 2 °C and 75% ± 10% relative humidity. The behavior of the mosquitoes was monitored and the number of those knocked down recorded after every 10 minutes for 1 hour. At the end of the 1 hour exposure period, the mosquitoes were transferred to the holding tubes set on the slide and a pad of cotton wool soaked in 10% sugar solution placed on the mesh-screen. The holding tubes were kept for 24 hours (recovery period) in a secluded, shady place where the temperatures did not exceed 30 °C. At the end of recovery period, (24-hour post-exposure), the number of knocked down or dead mosquitoes was counted and recorded. An adult mosquito was considered to be alive if it was able to fly regardless of the number of legs remaining. Affected specimens that were unable to walk or fly in a coordinated manner were considered knocked down. Calculation and interpretation of mortality rates. After 24-hour post-exposure, the number of dead mosquitoes in the holding tube was counted. The mortality is calculated by summing the number of dead mosquitoes per site (i.e., out of a total of 100 mosquitoes from four replicates of 25 mosquitoes each per site) expressed as a percentage of the number of exposed mosquitoes using the formula: The World Health Organization (WHO, 2022) interprets the insecticide bioassay results as follows: a vector population is considered to be resistant to an insecticide if mortality within the treatment group is 20%, the test is invalid and must be discarded and repeated. For sample size, WHO (2022) recommends testing at least 100 mosquitoes per insecticide (typically four replicates of 20–30 mosquitoes each). Interpretation of resistance status is considered reliable only when this minimum sa
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