Adult Survivorship of the Dengue Mosquito Aedes aegypti Varies Seasonally in Central Vietnam

The survival characteristics of the mosquito Aedes aegypti affect transmission rates of dengue because transmission requires infected mosquitoes to survive long enough for the virus to infect the salivary glands. Mosquito survival is assumed to be high in tropical, dengue endemic, countries like Vietnam. However, the survival rates of wild populations of mosquitoes are seldom measured due the difficulty of predicting mosquito age. Hon Mieu Island in central Vietnam is the site of a pilot release of Ae. aegypti infected with a strain of Wolbachia pipientis bacteria (wMelPop) that induces virus interference and mosquito life-shortening. We used the most accurate mosquito age grading approach, transcriptional profiling, to establish the survival patterns of the mosquito population from the population age structure. Furthermore, estimations were validated on mosquitoes released into a large semi-field environment consisting of an enclosed house, garden and yard to incorporate natural environmental variability. Mosquito survival was highest during the dry/cool (January-April) and dry/hot (May-August) seasons, when 92 and 64% of Hon Mieu mosquitoes had survived to an age that they were able to transmit dengue (12 d), respectively. This was reduced to 29% during the wet/cool season from September to December. The presence of Ae. aegypti older than 12 d during each season is likely to facilitate the observed continuity of dengue transmission in the region. We provide season specific Ae. aegypti survival models for improved dengue epidemiology and evaluation of mosquito control strategies that aim to reduce mosquito survival to break the dengue transmission cycle. Author Summary Prolonged survival of adult mosquitoes is critical to dengue transmission cycles, particularly because it takes time for the dengue viruses to infect the mosquito midgut, replicate and infect the salivary glands before transmission can occur. Reducing mosquito lifespan is one avenue to fight dengue, and may be achievable using mosquito parasites such as a life-shortening strain of Wolbachia bacteria. We measured the survival rates of Aedes aegypti mosquitoes in a village in central Vietnam during the region's three prevailing seasons. In order to predict the ages of wild-caught mosquitoes, we used a method of predicting mosquito age based on the activity of age-responsive genes. We first tested our estimations on mosquitoes that were released into a large field cage that enclosed a house and yard. The survival patterns of mosquitoes in the village differed between seasons, both in terms of the death rate over time and longevity. We estimate that approximately 92 and 64% of the mosquito population survived long enough to transmit dengue during the dry/cool season (January-April) and dry/hot season (May-August), respectively; however, this was reduced to 29% during the wet/cool season (September-December). The relatively high survival of Aedes aegypti year round is likely to be a major contributor to the continuity of dengue transmission in the region and provides rationale for reducing mosquito survival to break the transmission cycle.