Evaluating the contribution of aestivation to the persistence of malaria mosquitoes through the Sahelian dry season using stable isotopes

Data suggests the malaria vector Anopheles coluzzii persists in the Sahel by dry-season aestivation though its specific contribution compared with alternative strategies, i.e., migration is unknown. We have marked Anopheles mosquitoes using deuterium ( 2 H) to assess the contribution of aestivation to persistence of mosquitoes through the seven-month dry season. If local aestivation is the only way An. coluzzii persists, the frequency of marked mosquitoes should remain stable throughout, whereas finding no marked mosquitoes would be evidence against aestivation. Larval sites were enriched with 2 H at the end of the 2017 wet season (September) in two Sahelian villages in Mali. We monitored 2 H-enriched populations until the onset of rains (June 2018). By the end of the enrichment period (November 2017), 33% of An. coluzzii mosquitoes were strongly marked. Expectedly, 2 H levels in marked mosquitoes degraded over time. We utilized three methods to estimate the fraction of marked mosquitoes in the population. Seven months following enrichment all three methods supported the presence of marked mosquitoes: 7% of the population had 2 H values above the highest pre-enrichment value. An excess of 21% exceeded the expected 25% in the 3rd quartile of the pre-enrichment population. Finally, a finite mixed population model showed 2.5% represented a subpopulation of marked mosquitoes with elevated 2 H, compatible with our predictions. We provide evidence that aestivation is a major persistence mechanism of An. coluzzii in the Sahel, contributing at least 20% of the adults at the onset of rains, suggesting either mixture among local populations or that An. coluzzii utilizes several persistence strategies. The implications for mosquito control and malaria elimination campaigns are highlighted.