The drivers and patterns of zoonotic virus emergence in the human population are poorly understood. The mosquito Aedes aegypti is a major arbovirus vector native to Africa that invaded most of the world’s tropical belt over the past four centuries, after the evolution of a “domestic” form that specialized in biting humans and breeding in water storage containers. Here, we show that human specialization and subsequent spread of A. aegypti out of Africa were accompanied by an increase in its intrinsic ability to acquire and transmit the emerging human pathogen Zika virus. Thus, the recent evolution and global expansion of A. aegypti promoted arbovirus emergence not solely through increased vector–host contact but also as a result of enhanced vector susceptibility.

Aedes aegypti is the primary vector of several arboviruses, including dengue virus (DENV), chikungunya virus (CHIKV), yellow fever virus (YFV), and Zika virus (ZIKV). This vector is widespread globally in tropical and subtropical areas, but also found in temperate areas. Kenya experienced its first chikungunya outbreaks in Lamu County in 2004 and later in Mandera: 2016, and Mombasa: 2017. While there is yet to be a report of Zika outbreaks in Kenya, sero-surveillance studies indicate low-level transmission of this virus in coastal and northern parts of the country. Despite the presence of Ae. aegypti in Kisumu and Busia counties in sufficient densities, and free movement of people between the coast and the two western Kenya counties, no outbreaks of either disease have been reported in these regions. To investigate this phenomenon, we collected Ae. aegypti mosquitoes from county headquarter towns near railway stations connecting the coast and western Kenya and reared them under controlled laboratory conditions. The mosquitoes were then assessed for genetic variability using CO1 genes as well as their efficiency to transmit viruses using Laboratory colonies (F1) of the field mosquitoes challenged with an infectious blood meal containing CHIKV and ZIKV. Genetic analysis revealed the presence of both Ae. aegypti subspecies, (Ae. aegypti aegypti [Aaa] and Ae. aegyptiformosus [Aaf]) in the two western Kenya counties, with Aaf being dominant (19:8 for Kisumu samples and 25:6 for Busia samples). Additionally, pairwise comparison revealed minimal genetic differentiation (0.62%) between the study populations, with a high genetic variation (99.38%) observed within each population, indicating significant diversity within individual populations. Ae. aegypti populations from Kisumu and Busia counties exhibited competence for CHIKV, with infection, dissemination, and transmission rates of 55.2%, 85.5%, and 27.1% for Kisumu; and 57.8%, 71.8%, and 25% for Busia populations, respectively. There was no significant difference in vector competence between these two populations. Interestingly, neither population was competent for ZIKV. In conclusion, the data shows that the Ae. aegypti populations in the two cities were homogeneous. This could explain the observed similarity in vector competence for CHIKV and ZIKV.