Abstract
BackgroundHow anopheline mosquitoes persist through the long dry season in Africa remains a gap in our understanding of these malaria vectors. To span this period in locations such as the Sahelian zone of Mali, mosquitoes must either migrate to areas of permanent water, recolonize areas as they again become favorable, or survive in harsh conditions including high temperatures, low humidity, and an absence of surface water (required for breeding). Adult mosquitoes surviving through this season must dramatically extend their typical lifespan (averaging 2–3 weeks) to 7 months. Previous work has found evidence that the malaria mosquito An. coluzzii, survives over 200 days in the wild between rainy seasons in a presumed state of aestivation (hibernation), but this state has so far not been replicated in laboratory conditions. The inability to recapitulate aestivation in the lab hinders addressing key questions such as how this state is induced, how it affects malaria vector competence, and its impact on disease transmission.MethodsIn effort to induce aestivation, we held laboratory mosquitoes in climate-controlled incubators with a range of conditions that adjusted humidity (40–85% RH), temperature (18–27 °C), and light conditions (8–12 h of light) and evaluated their survivorship. These conditions were chosen to mimic the late rainy and dry seasons as well as relevant extremes these mosquitoes may experience during aestivation.ResultsWe found that by priming mosquitoes in conditions simulating the late wet season in Mali, and maintaining mosquitoes in reduced light/temperature, mean mosquito survival increased from 18.34 ± 0.65 to 48.02 ± 2.87 days, median survival increased from 19 (95% CI 17–21) to 50 days (95% CI 40–58), and the maximum longevity increased from 38 to 109 days (P-adj < 0.001). While this increase falls short of the 200 + day survival seen in field mosquitoes, this extension is substantially higher than previously found through environmental or dietary modulation and is hard to reconcile with states other than aestivation. This finding will provide a platform for future characterization of this state, and allow for comparison to field collected samples.
Highlights
How anopheline mosquitoes persist through the long dry season in Africa remains a gap in our understanding of these malaria vectors
As in the first experiments, we found significant extension of lifespan with both M and S form mosquitoes when maintained in cooler, darker environments than general insectary conditions with median survival ranging from 18 days in 27 °C / 12:12 h L:D / unprimed / Anopheles gambiae (S-form) mosquitoes to a maximum median survival of 63 days in 20 °C / 8:16 h L:D / primed / S-form mosquitoes (Table 1)
Photoperiod, priming, and species significantly increased survival (P-values of < 0.001, < 0.001, < 0.001 and 0.009, event time ratios of 2.02, 1.24, 1.19, 1.12, respectively, Table 2), indicating that storage at 20 °C vs 27 °C doubles survival, 8 h of light a day vs 12 increased survival by 24%, end of wet season priming conditions over standard insectary increased survival by 19%, and Anopheles coluzzii (M-form) overall had slightly higher survival across all conditions by 12%
Summary
How anopheline mosquitoes persist through the long dry season in Africa remains a gap in our understanding of these malaria vectors. While Mali has a strong seasonal transmission cycle, with ~ 7 months of the year largely unconducive to mosquito presence, the rates of malaria have remained consistent [3], even with increasing intervention in the form of bednets [4], insecticide residual spraying [5], and other interventions The reasons for this disease persistence are multifactored, but it is likely that the persistent nature of the vectors themselves play a role. The An. coluzzii mosquitoes that do appear during the late dry season peak (a yearly, high density, 1–2-week event), host-seek normally, but exhibit reproductive depression compared with the wet season mosquitoes [14] While this phenotype has been repeatedly described over the last century, the methodology to induce such a long-lived state with laboratory, colony mosquitoes has remained beyond reach
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