Transmission Dynamics of Arboviruses along the Kenyan Coast

Abstract

Arboviruses are arthropod-borne viruses transmitted to humans and animals primarily through the bites of infected insects, mainly mosquitoes. While most arboviral infections are self-limiting, some can be life-threatening. Recent outbreaks have been reported along Coastal and northeastern Kenya. Understanding the factors influencing the occurrence and spread of these viruses including vector population density, mosquito feeding behavior, transovarial transmission, and the link between sylvatic and urban transmission cycles is crucial for developing effective control measures to prevent future outbreaks. This study employed a cross-sectional design to investigate the transmission dynamics of arboviruses in four urban areas, two forested areas, and two peri-urban areas along coastal Kenya. It also aimed establish the link between sylvatic and urban transmission cycles to better understand the factors contributing to arboviral outbreaks in urban settings. Mosquito eggs, larvae, and adult were collected across the study sites namely Mombasa Islands, Changamwe, Likoni and Nyari (urban areas), Haller Park and Arabuko Sokoke forest (forest areas) and Bamburi and Gede (peri-urban areas). The eggs were collected using ovipositor cups targeting aedes mosquitoes and dispensed in larval trays for hatching and together with larvae collected using dipper and pipetting methods reared to adults. Adult mosquitoes were collected using Center for Disease Control and Prevention (CDC) resting traps, Carbon dioxide (CO2)-baited CDC light traps, and Biogents’ mosquito traps (BG sentinel traps). All mosquitoes were identified morphologically to species level using taxonomic keys. Identified mosquitoes were pooled into pools of 25 based on species, sex and collection date and screened for arboviruses by both cell culture and Reverse Transcription-Polymerase Chain reaction (RT-PCR). Blood meal analysis from engorged females was conducted by amplifying the 12S mitochondrial ribosomal gene, followed by sequencing and BLAST analysis in GenBank to identify blood meal source. A total of 13,009 larvae were collected from 17 different habitats along coastal Kenya with discarded tires being a major breeding site that contributed to higher larval populations. Approximately 4,735 adult mosquitoes, representing 19 species, were collected from peri-urban and forested sites. Culex quinquefasciatus (50.17%) and Ae. aegypti (38.73%) were the most predominant species in urban areas while Ae. vittatus (89%) dominated the forested area among the larval collection. Ae. tricholabis (45%) and Ae. aegypti (20.0%) were predominant adult species in forested and urban areas with Haller Park recording the highest adult population (87.2%). Out of 638 pools screened for flaviviruses, bunyavirus and alphavirus 23(3.6%) pools were positive for flavivirus and 6(0.9%) for alphavirus by RT-PCR. Haller Park reported highest number of arboviruses with 16(55.2%) flaviviruses and 6(20.7%) alphaviruses followed by Bamburi with 5(17.2%) flaviviruses and Gede with 2(6.9%) flaviviruses. Ae. aegypti recorded the highest number of infections rate, with 6(20.7%) in Haller Park, 3(10.3%) in Bamburi and 2(6.9%) in Gede. The two positive pools in Gede were from larval collection. Other positive species in Haller Park included Ae. tricholabis (4), An. Funestus (3), Ae. vittatus (2), Cx. pipiens (2), Cx. vansomereni (2) and Ae. hirsutus and Fi. Circumtesta 1 positive sample each. Additionally, flaviviruses were detected in 2 pools of Ae. vittatus from Bamburi. Blood fed mosquitoes included Ae. tricholabis (10), Ae. Aegypti (1) and Cx. watti (1). Blood mel analysis identified various animal hosts, including white tailed mongoose (1), bushbuck (2), beisa oryx (1), Somali giraffe (4), and four unidentified sources. The high abundance of Ae. aegypti in urban, peri-urban and forested areas as well as Ae. tricholabis in forested areas, poses great risk of arboviruses outbreak in the region due to their role in dengue virus (DENV) transmission. The study confirms active transmission of flaviviruses and alphaviruses in Haller Park, Gede and Bamburi. The high mosquito diversity and population across the study sites combined with their ability to feed on both animals and humans, suggests a potential link between the sylvatic and urban transmission cycles. Further studies, including serological surveys in both humans and animals are recommended to determine the extent of active and past infections. Well-funded longitudinal surveys would be recommended to inform on breeding cycles, seasonal peaks and variations in vector density over time. The key findings of this study will add into the existing knowledge and help establish reliable control and management strategies of arboviral infection to avert future outbreaks.