Dry Season Ecology of Malaria Vectors

National Institute Of Allergy And Infectious Diseases

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Abstract

With a billion cases and one million deaths annually, vector borne diseases remain important. After two decades of intense control efforts, malaria is prevalent worldwide with an especially heavy burden in Africa. Outstanding gaps remain in our understanding of vector biology. Such gaps include the strategies used by African malaria mosquitoes to persist through the long dry season. Our work has produced compelling evidence that Sahelian malaria mosqitoes persist locally through the dry season by dormancy (aestivation) and also engage in wind-assisted migration over hundreds of kilometers. 1. Using high-altitude aerial sampling of insects 40-290 m above ground, we have demonstrated regular and numerically massive migration of many African mosquitoes in a paper published in Nature (Huestis et al. 2019). Importantly, >80% of the migrants were gravid females, suggesting many probably carry pathogens such as Plasmodium they might transmit after landing. The ongoing malaria elimination efforts should consider that infected mosquitoes flying from distant populations could reintroduce parasites into areas where elimination has been achieved. To assess viability of mosquitoes after high altitude flight and the epidemiological consequences, we subjected wild Anopheles to a high-altitude survival assay, followed by oviposition (egg laying) and blood feeding assays. Similar to other high-altitude migrating insects, our results showed that mosquitoes withstand >11 hours high-altitude flight and subsequently reproduce and blood-feed on new hosts, thus potentially transmitting pathogens after migration (Sanogo et al. 2021). 2) We measured flight aptitude of wild mosquitoes in Mali and found that the population includes a minority exhibiting high-flight activity. The median total flight was 586 s and the maximum value was 4.5 h. In accordance with aerial sampling results, flight aptitude peaked in the wet season and was higher in gravid females than in non-blood-fed females. The results suggest a degree of discrimination between appetitive flyers and long-distance migrants although identifying migrants requires further data (Faiman et al. 2020). 3) To compare windborne flight activity of mosquitoes with that of other insects captured at altitude, sub-samples of aerial-collected insects were identified. Migration of all species exhibited high temporal and spatial regularity and frequent flights on southerly winds, accounting for the recolonization of the Sahel from southern source populations. The annual number of migrants per species crossing Mali at latitude 14N were in the trillions. The magnitude and diversity of windborne insect migration highlight its impact on food security and public health (Florio et al. 2020). 4) To determine if mosquitoes engage in high-altitude flights beyond the Sahel, we have initiated sampling in the dry and wet savanna ecozones (Mali) as well in dry and equatorial forests (Ghana) as part of our recently awarded Gates Foundation Grand Challenge Exploration project. Following the training, monthly operations were carried out. Initial work on the specimens collected has begun as well. 5) To understand the scope of mosquito windborne migration, a literature survey of African mosquito borne diseases has been undertaken to generate a unique database of the African mosquito borne diseases from a One Health perspective. Ongoing analyses of the database allow for novel inquiries into the African mosquito borne diseases as a biological system, with new implications for disease surveillance. 6) Dormancy (aestivation) in Anopheline mosquitoes through the long dry season was demonstrated in An. coluzzii by a singled marked female that survived >200 days between rainy seasons in the wild, but this state has not been replicated in laboratory conditions, hindering progress on key questions such as how this state affects malaria vector competence. We compared survivorship of mosquitoes in incubators that adjusted humidity, temperature, and light conditions. Conditions were chosen to mimic the late rainy and dry seasons. By priming mosquitoes in conditions simulating the late wet season in Mali, and maintaining them in reduced light/temperature, mean mosquito survival increased from 18.3 to 48.0 d (max: 38 to 109 d (Krajacich et al. 2020). 7) A follow up study on the laboratory simulation of aestivation has began, focusing on changes in nutritional reserves (lipids, carbohydrates, and protein) across age in mosquitoes raised under aestivation simulating conditions versus typical rainy season conditions. 8) This extension of lifespan is substantially higher than previously found in laboratories and is hard to reconcile with states other than aestivation. A transcriptomic analysis of mosquitoes representing different seasons from the Sahel and perennial wet area in the savanna has been carried out. Whole-body transcriptomes of mosquitoes collected during the early and late wet season as well as early and late dry season. We found unique expression patterns in stress (heat shock 70 and alpha-crystallin beta chain), immunity (e.g., TEP1, Defensin), and others affecting longevity, and metabolic pathways. These wild caught, mosquitoes clustered within sampling periods, indicating differences in mosquito transcriptional state between seasons. Analysis is ongoing. 9) We evaluated the contribution of aestivation to the persistence of mosquitoes in the Sahel by tracking mosquitoes marked by deuterium from the end of the wet season until the beginning of the subsequent wet season. If local aestivation is the only way An. coluzzii persists, the frequency of marked mosquitoes should remain stable, whereas finding no marked mosquitoes would be evidence against aestivation. By the end of the enrichment period, 33% of A. coluzzii were marked. Seven months after the end of enrichment (onset of rains), 7% of the population had 2H values above the highest pre-enrichment value, an excess of 21% (over the 25% expected) were above the 3rd quartile of the pre-enrichment population, and a finite mixed population model showed a mixed population of which 2.5% represented marked mosquitoes, compatible with our predictions. Our data provide compelling evidence that aestivation is a major mechanism facilitating persistence of A. coluzzii in the Sahel, contributing at least 20% of the adults at the onset of rains. Our results suggest that A. coluzzii utilizes multiple persistence strategies which enable its populations to build up rapidly. 10. Mark release recapture methods are key to address certain problems of vector biology. We have developed a new method using a fluorescent dye combined with synthetic DNA tags to mark unlimited groups of mosquitoes. Handheld UV flashlights enable quick screening in the field. A simple, PCR on individual mosquito legs provides highly informative mark content. Marking did not affect survival, oviposition, or Plasmodium competence of An. gambiae. Fluorescence and DNA tags had 100% retention up to 3 weeks (longest period tested, Faiman et al. 2021). 11. In collaboration with LMIV, we tracked movement of mosquitoes between clusters of houses, which were assigned to transmission-blocking-vaccine (TBV) or placebo. This mark-release-recapture experiment was tailored to determine if the movement of mosquitoes can undermine the measurement of the vaccine efficacy. Experiment was also conducted in a smaller Sahelian village. We marked a total of 33,922 Anopheles spp., recapturing 253 (0.42 to 1.65% depending on village). Dispersal across the village, spanning over 700 m was common even within 3-7 d after release, which was the maximum distance between release and capture points. These results reveal that mosquito movement in the village where the TBV was tested undermine the TBV evaluation.