A preliminary virological surveillance of field-caught adult Aedes aegypti (L.) and Aedes albopictus (Skuse) mosquitoes in selected sites in the District of Colombo and Gampaha of Sri Lanka

Abstract

Aedes aegypti (L.) is widely recognized as the principal dengue vector. Aedes albopictus (Skuse) has also reported to mediate dengue outbreak situations in some countries. In Sri Lanka, Ae. albopictus predominates over Ae. aegypti in some areas of high dengue prevalence. As such, current study was aimed to conduct a virological surveillance of field caught Ae. aegypti and Ae. albopictus mosquitoes in two dengue endemic areas in Sri Lanka to assess their relative roles in dengue transmission. Adult mosquitos were collected weekly for 2-5 weeks from the Districts of Colombo (urban) and Gampaha (suburban) using Improved Prokopack Aspirator during 8.00–11.00 am and were identified, sexed and counted. The number of adult mosquitoes per house for each species was calculated and the statistical significance was determined by the Student’s t test. RNA was extracted using pooled heads and thoraxes of 1-5 female Aedes mosquitoes and the presence of dengue virus was tested with nested reverse transcriptase PCR. A total of 4154 mosquitoes belonging to four species; Culex quinquefasciatus (3151; 75.9%), Anophelus culcifaices (204; 4.9%), Ae. aegypti (487;11.7%), Ae. albopictus (312;7.5%) was collected from 19 sampling sites in the two districts. In the District of Colombo, there were more (p< 0.05) Ae. aegypti per house (male: 1.8, female: 1.36) compared to Ae. albopictus (male: 0.21, female: 0.23). In contrast, District of Gampaha had more Ae. albopictus per house (male: 0.9, female: 0.67) compared to Ae. aegypti (male: 0.32, female: 0.16). In both districts, the number of males were significantly higher than the females for the dominating Aedes species (P<0.05) in the district. None of the adult female Aedes mosquitoes collected were positive for dengue virus. Our results confirm the distorted sex ratio characteristic of Culicine mosquitoes and the preference of Ae. aegypti for more urban habitats compared to Ae albopictus. The absence of dengue infection among the captured mosquitos, irrespective of the reported dengue incidences in the studied areas, might point at the ability of an infected Aedes mosquito to transmit the virus to many healthy individuals. Our results suggest that dengue epidemics are possible even with extremely low mosquito infection rates. However, further analysis is necessary to confirm this observation.