Long term impacts of combined sewer overflow remediation on water quality and population dynamics of Culex quinquefasciatus, the main urban West Nile virus vector in Atlanta, GA

Background: Combined sewers are a significant source of urban water pollution due to periodic discharges into natural streams. Such events (called combined sewer overflows, or CSOs) contribute to the impairment of natural waterways and are associated with increased mosquito productivity and elevated risk of West Nile virus transmission. Objectives: We investigated the impact of CSOs on water quality and immature mosquito productivity in the city of Atlanta, Georgia, one year before and four years after CSO facility remediation. Methods: Water quality (ammonia, phosphate, nitrate and dissolved oxygen concentrations), immature mosquitoes (larvae and pupae), water temperature and rainfall were quantified biweekly between June–October at two urban creeks during 2008–2012. A before–after control–intervention design tested the impact of remediation on mosquito productivity and water quality, whereas generalized linear mixed-effect models quantified the factors explaining the long term impacts of remediation on mosquito productivity. Results: Ammonia and phosphate concentrations and late immature (fourth-instar and pupae) mosquito populations were significantly higher in CSO than in non-CSO creeks, while dissolved oxygen concentrations were lower. Remediation significantly improved water quality estimates (particularly ammonia and dissolved oxygen) and reduced the number of overflows, mosquito productivity and the overall contribution of CSO-affected streams as sources of vectors of West Nile virus. Conclusions: The quality of water in CSOs provided a suitable habitat for immature mosquitoes. Remediation of the CSO facility through the construction of a deep storage tunnel improved water quality indices and reduced the productivity of mosquito species that can serve as vectors of West Nile virus