Dry Season Ecology of Malaria Vectors

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Despite extensive research on malaria for over a century, critical gaps remain in our understanding of the vectors; gaps that also limit our success in malaria control. We address some of the most critical gaps, namely the strategies used by African malaria vectors to persist through the long dry season without surface waters for several months. Combining field and laboratory studies, our results provide compelling evidence that malaria vectors persist through the dry season by a form of dormancy (aestivation) and also engage in wind-assisted long-distance migration probably over hundreds of kilometers. These fundamental facets of vector biology have been controversial and, until now, were ignored due to insufficient evidence. Conventional and novel malaria and vector control strategies cannot afford to ignore aestivation and long-distance migration as processes that may hinder or aid the ultimate outcome. This year, Nature has published our investigation of this problem based on the vector population dynamics. Using time series analysis, we extracted and compared the seasonal components of these dynamics between the vector species. Strong species-specific pattern provided signatures for the vector strategy of persistence. For example, density peaks during the dry season, coupled with a rapid population growth closely following the first rain provided evidence for aestivation in A. coluzzii, whereas total absence throughout the dry season coupled with with 6-8 weeks delay in timing of normal population growth of A. gambiae s.s. (after than of A. coluzzii) was consistent with long-distance migration. These results extend other studies we have carried out based on mosquito mark-release-recapture experiments, variation in spatial distribution, aerial mosquito sampling (100-200 m above ground), seasonal changes in reproductive activity, flight activity, cuticular hydrocarbons, and responses to changes in photoperiod and temperature. Based on these findings, experiments designed to (i) find the hidden shelters used by aestivating A. coluzzii mosquitoes during the dry season, (ii) Sample mosquitoes flying in high altitutdes (>100 m above ground) using traps tethered to helium filled balloons, (iii) determine the weather conditions promoting long-distance mosquito flights 10--200 m above ground), among others. Over the past year, we have been able to collect dozens of mosquitoes flying in the lower jet stream among them at least 14 A. gambiae s.l. More than 20 tanks of (balloon grade) helium have been used as part of this work. These results and ongoing studies using novel approaches provide fresh insights in malariology and vector biology. The findings that Anopheline vector of malaria and other disease vectors engage in long-distance flights and some are capable of dry-season dormancy open new frontiers of basic and applied research.