Geographically extensive larval surveys reveal an unexpected scarcity of primary vector mosquitoes in a region of persistent malaria transmission in western Zambia

Abstract

Background The Barotse floodplains of the upper Zambezi River and its tributaries are a highly dynamic environment, with seasonal flooding and transhumance presenting a shifting mosaic of potential larval habitat and human and livestock blood meals for malaria vector mosquitoes. However, limited entomological surveillance has been undertaken to characterize the vector community in these floodplains and their environs. Such information is necessary as, despite substantial deployment of insecticide-treated nets (ITNs) and indoor residual spraying (IRS) against Anopheles vectors, malaria transmission persists across Barotseland in Zambia’s Western Province. Methods Geographically extensive larval surveys were undertaken in two health districts along 102 km of transects, at fine spatial resolution, during a dry season and following the peak of the successive wet season. Larvae were sampled within typical Anopheles flight range of human settlements and identified through genetic sequencing of cytochrome c oxidase I and internal transcribed spacer two regions of mitochondrial and nuclear DNA. This facilitated detailed comparison of taxon-specific abundance patterns between ecological zones differentiated by hydrological controls. Results An unexpected paucity of primary vectors was revealed, with An. gambiae s.l. and An. funestus representing < 2% of 995 sequenced anophelines. Potential secondary vectors predominated in the vector community, primarily An. coustani group species and An. squamosus. While the distribution of An. gambiae s.l. in the study area was highly clustered, secondary vector species were ubiquitous across the landscape in both dry and wet seasons, with some taxon-specific relationships between abundance and ecological zones by season. Conclusions The diversity of candidate vector species and their high relative abundance observed across diverse hydro-ecosystems indicate a highly adaptable transmission system, resilient to environmental variation and, potentially, interventions that target only part of the vector community. Larval survey results imply that residual transmission of malaria in Barotseland is being mediated predominantly by secondary vector species, whose known tendencies for crepuscular and outdoor biting renders them largely insensitive to prevalent vector control methods.

Temporally consistent predominance and distribution of secondary malaria vectors in the Anopheles community of the upper Zambezi floodplain

“…species O/15 larvae were comparatively fewer in 2019, they occurred in Zambezi floodplain habitats comparable to those surveyed by adjacent transects in 2018; several 2019 occurrences (Fig. 5 e) were within 600 m of where the species was encountered in the preceding wet season 36 . In the Luena flats, locations positive for An.…”

“…4 b). Comparisons between anopheline species distributions in the 2017 dry season (larvae 36 ), after the peak of the 2018 wet season (larvae 36 ), and after the peak of the 2019 wet season (adults and larvae) revealed considerable inter-annual consistency in the spatial distribution of some anopheline species where sampling effort coincided. For example, An.…”

“…gambiae s.l. larvae in 2017 (2017 sample identities were resolved to species complex level only, as ITS2 sequences were not then available for all specimens 36 ). In the floodplain edge ecological zone, adult An.…”

“…5 e and Cross et al . 36 ).

Figure 8 Distribution of Anopheles larvae across ecological zones in western Zambia.

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“…Cross et al . 36 employed DNA barcoding of anopheline larvae sampled in an extensive spatial survey to reveal an unexpectedly high prevalence of secondary vector species in a region of persistent residual malaria transmission in western Zambia, with partitioning of species across ecological zones and stability of patterns observed over dry and wet seasons. The present study aims to build on that work by combining geographically extensive surveys of larval habitats and DNA barcoding of larvae sampled in the same region in the following year to assess inter-annual consistency of anopheline vector community composition, ecological zonation and seasonal patterns.…”