Estimating the age of the dengue vector Aedes aegypti under lab, semi-field, and field conditions

Dengue is a worldwide threat that could potentially infect millions of people annually. The primary mosquito vector, Aedes aegypti, is well established throughout urban areas of the southwestern US, yet intriguingly the disease is not present. Although many factors may influence dengue’s distribution, we hypothesize that the lifespan of female Ae. aegypti mosquitoes in the southwestern US is too short for the virus to complete development. We utilized two age grading techniques to determine the age of adult mosquitoes within three brackets, nonvectors (1-7 days), potential vectors (8-24 days), and excellent vectors (25+ days). We first determined parity by analyzing ovary tracheation. The absence of coiled trachea indicated that females previously completed a gonotrophic cycle. Intriguingly only ~50% of females were parous, while similar studies in dengue endemic regions showed up to 90% parity. The second technique examined transcript levels of two age associated genes, SCP-1 and 1615. SCP-1 expression consistently decreased as mosquitoes age regardless of blood-feeding. Using colony mosquitoes under lab or semi-field conditions, we accurately grouped mosquitoes of known age into three age brackets using only SCP-1. In addition, SCP-1 expression in wild collected mosquitoes closely correlated with the parity status of those mosquitoes. 1615 can be used to identify the oldest mosquitoes (25+ days). Developing tools to accurately quantify the age structure of mosquito populations would greatly aid our understanding of mosquito-borne disease transmission and could help explain why dengue is not endemic in the southwestern US even though the mosquito vector is present.