Dengue fever has been endemic to Sri Lanka for several decades. Due to the unavailability of an established prophylactic medicine, dengue prevention depends largely on vector control, where vector surveillance plays a key role. The present study aimed to assess the Aedes mosquito abundance and the risk of disease outbreak using ovitrap surveillance in 14 areas in Sri Lanka, covering four districts with high dengue incidence during 2014 -2016. A total of 1537 ovitraps were placed in Colombo (Kirulapone, Dematagoda, Grandpass and Thummulla), Gampaha (Kurana and Imbulgoda), Kalutara (Horana, Keselwaththa and Kalamulla) and Kandy (Nawalapitiya, Peradeniya, Edanduwawa, Hanthana Road and Thalwaththa) districts in both indoor and outdoor sites and were collected after five days. The larval counts were used to calculate the Container Index (CI) and Ovitrap Index (OI). Our results revealed significantly higher CI for Aedes species for outdoor compared to indoor sites, indicating a tendency of Ae. aegypti and Ae. albopictus to breed more in outdoor habitats (p < 0.05). In most co-bred ovitraps, the number of Ae. aegypti larvae were higher than those of Ae. albopictus. The abundance of Ae. aegypti was higher in urban areas compared to rural areas (p < 0.05) whereas no such difference was observed for Ae. albopictus. This suggests that Ae. albopictus has been successful in invading habitats with different levels of urbanization. Further, all studied areas showed an OI > 10 % for either or both Aedes species reflecting a possible risk of dengue outbreaks as per the guidelines. Nevertheless, only the abundance of Ae. aegypti (in terms of OI) showed a positive correlation with the number of dengue cases (r = 0.96, p < 0.05) indicating its substantial contribution towards dengue incidences in the studied areas.

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Aedes aegypti is the major epidemic vector of dengue outbreaks in the world. Here, a 359 bp region of the mitochondrial NADH hydrogenase subunit 4 (ND4) gene was taken as the candidate locus for study purposes from 27 Aedes aegypti mosquito larvae collected from an inland mountain area (Badulla), and two distantly located coastal areas (Hambantota and Trincomalee) in Sri Lanka. Eleven parsimonious informative sites were observed, and 7 haplotypes were identified from the 27 samples. The coastal populations displayed higher genetic diversity compared to those from the inland mountain-contained region. The phylogenetic analyses of Ae. aegypti from Badulla, Hambantota and Trincomalee including published Sri Lankan and global samples (a total of 106 sequences) revealed two separate clades: a basal clade and a derived clade. Eighty percent of the samples found from Badulla, were of the basal haplotype (H1) clustered together with West African (Senegal) samples in close proximity to the outgroup. On the contrary, Hambantota and Trincomalee samples were found dispersed in both the basal and derived clades clustering closely with samples from Mexico, Brazil, Kenya, USA and Myanmar indicating their close genetic relationship to many New World, East African and Asian varieties. Taken together, our study proposes that colonization of Ae aegypti in Sri Lanka occurred through multiple founder events (ancient multiple invasions in coastal sites and a more recent West African invasion to the inland mountain-demarcated Badulla region) and continue to exist in mutational drift equilibrium following the settlement event.