Vector Competence of an Indian Ocean Coastal Population of Aedes aegypti for Dengue Virus 2 and 3 Serotypes in Kenya

Abstract

Dengue is a public health concern because it is a viral hemorrhagic disease spread by the Aedes aegypti mosquito vector. The disease is caused by the dengue virus (DENV) which has four different serotypes (DENV 1-4). Dengue outbreaks were previously believed to be caused by the DENV-2 serotype solely; however, recent outbreaks in Kenya have led to the discovery of all four serotypes. The differential distribution of these serotypes in certain regions suggests that the virus may be transmitted differently among local mosquito vector populations. This study aimed to evaluate and compare the competence of an Indian Ocean Coastal population of Aedes aegypti as a vector for the transmission of DENV-2 (predominant) and DENV-3 (less dominant) serotypes in Kenya. In total, 590 female mosquitoes were orally exposed to either DENV-2 or 3, out of which only 489 (DENV-2; n=240 and DENV-3; n=249) were tested for susceptibility to the virus serotypes. Cytochrome c oxidase I gene (cox-I) sequences were analyzed to examine the proportions of domestic Ae. aegypti aegypti and sylvatic Ae. aegypti formosus among DENV-susceptible and non-susceptible cohorts. The possible contribution of different genetic forms, domestic Ae. aegypti aegypti and sylvatic Ae. aegypti formosus was examined to determine the potential for transmission of these viruses. Female mosquitoes (3-10-day-old F1) derived from field-collected eggs from Rabai, Kilifi County, were orally exposed in the laboratory to either DENV-2 (105.30 pfu/ml) or DENV-3 (105.13 pfu/ml). The body, legs and saliva of mosquitoes individually exposed to the serotypes were analysed to see if there was evidence of infection, dissemination or transmission at 7-, 14- and 21 days post-infection (DPI), respectively. The comparison of rates between the two serotypes revealed the following: the infection rate was 16% (38 out of 240) for DENV-2 and 10% (26 out of 249) for DENV-3; the dissemination rate was 47% (18 out of 38) for DENV-2 and 35% (9 out of 26) for DENV-3; and the transmission rate was 39% (7 out of 18) for DENV2 and 11% (1 out of 9) for DENV-3. These rates were consistently higher for DENV-2 than DENV-3 but the differences were not statistically significant. DENV-2 had a shorter extrinsic incubation period (EIP) for disseminated infection (7-DPI vs 14-DPI) and transmission in saliva (14-DPI vs 21-DPI) compared to DENV-3, but there were no significant differences observed. Two cox-I lineages were recovered among DENVsusceptible and non-susceptible mosquitoes, a dominant one that clustered with referenced domestic Ae. aegypti aegypti and a minor lineage that grouped with the sylvatic Ae. aegypti formosus. Infected mosquitoes and disseminated infection were represented in both lineages, while those that transmitted the viruses were limited to the lineage that grouped with Ae. aegypti aegypti. The data suggests that the coastal Kenya population of Ae. aegypti is capable of transmitting both DENV-2 and DENV-3 viruses, with the domestic strain likely playing a key role in transmission given its predominance. The shorter EIP to attain dissemination and transmission for DENV-2 could make it more likely for this virus to be transmitted than DENV-3. However, further studies are needed to confirm this.