Urban stream pollution increases mosquito fitness and disease vector potential

Background/Question/Methods

Anthropogenic disturbance of natural cycles in organic elements (Nitrogen and Phosphorus) is one of the major causes of the disruption of species interactions across ecosystems.  In urban landscapes the crowding of humans and their waste products in freshwater systems is likely to exacerbate distressing patterns of intra and inter-specific interactions, especially those relevant for disease emergence. Here, we present the results of field observations and two semi-natural experiments addressing the effects of ecosystem level changes on the density dependent fitness of the most common tropical and subtropical urban mosquito, Culex quinquefasciatus Say (Diptera: Culicidae).   The semi-natural experiments were designed to quantify both the oviposition preference for water influenced by sewage overflows, and the relative importance of density, weather variability and water quality on larval mortality, sex ratio and size at adult emergence. These phenotypic traits are important in determining vectorial capacity for the transmission of several pathogens.  

Results/Conclusions

The field observations showed this mosquito species to be present in streams where N and P levels were significantly higher than in streams where it was absent, the difference mediated by the effects of combined sewage overflows. The oviposition experiment showed water from these systems to be more attractive for oviposition by this mosquito species than water uninfluenced by the sewage overflows. The density-dependence experiment revealed that mortality hazards were independent of larval density, decreased in sewage overflow water and increased with raising minimum temperatures. Under all rearing conditions adult mosquito size decreased with density.  Mosquitoes from sewage overflow water emerged faster, were bigger and had an increased ratio of females to males. All these traits could determine the density dependent regulation of mosquito populations and the ability to transmit pathogens through size mediated fecundity and mating induced feeding behavior. Finally, our results show the importance of urban stream quality as a factor for the urban emergence of arboviral diseases, calling for the implementation of environmentally sound strategies for water management, given the potential to diminish the risk of some vector-borne diseases and other health hazards, while conserving biodiversity in cities.