Abstract
BackgroundEffective malaria surveillance requires detailed assessments of mosquitoes biting indoors, where interventions such as insecticide-treated nets work best, and outdoors, where other interventions may be required. Such assessments often involve volunteers exposing their legs to attract mosquitoes [i.e., human landing catches (HLC)], a procedure with significant safety and ethical concerns. Here, an exposure-free, miniaturized, double-net trap (DN-Mini) is used to assess relationships between indoor–outdoor biting preferences of malaria vectors, Anopheles arabiensis and Anopheles funestus, and their physiological ages (approximated by parity and insemination states).MethodsThe DN-Mini is made of UV-resistant netting on a wooden frame and PVC base. At 100 cm × 60 cm × 180 cm, it fits indoors and outdoors. It has a protective inner chamber where a volunteer sits and collects host-seeking mosquitoes entrapped in an outer chamber. Experiments were conducted in eight Tanzanian villages using DN-Mini to: (a) estimate nightly biting and hourly biting proportions of mosquitoes indoors and outdoors; (b) compare these proportions to previous estimates by HLC in same villages; and, (c) compare distribution of parous (proxy for potentially infectious) and inseminated mosquitoes indoors and outdoors.ResultsMore than twice as many An. arabiensis were caught outdoors as indoors (p < 0.001), while An. funestus catches were marginally higher indoors than outdoors (p = 0.201). Anopheles arabiensis caught outdoors also had higher parity and insemination proportions than those indoors (p < 0.001), while An. funestus indoors had higher parity and insemination than those outdoors (p = 0.04). Observations of indoor-biting and outdoor-biting proportions, hourly biting patterns and overall species diversities as measured by DN-Mini, matched previous HLC estimates.ConclusionsMalaria vectors that are behaviourally adapted to bite humans outdoors also have their older, potentially infectious sub-populations concentrated outdoors, while those adapted to bite indoors have their older sub-populations concentrated indoors. Here, potentially infectious An. arabiensis more likely bite outdoors than indoors, while potentially infectious An. funestus more likely bite indoors. These observations validate previous evidence that even outdoor-biting mosquitoes regularly enter houses when young. They also demonstrate efficacy of DN-Mini for measuring indoor–outdoor biting behaviours of mosquitoes, their hourly biting patterns and epidemiologically relevant parameters, e.g., parity and insemination status, without exposure to volunteers. The trap is easy-to-use, easy-to-manufacture and affordable (prototypes cost ~ 100 US$/unit).
Highlights
Effective malaria surveillance requires detailed assessments of mosquitoes biting indoors, where interventions such as insecticide-treated nets work best, and outdoors, where other interventions may be required
Evidence suggests that vector control tools, mainly insecticide-treated nets (ITNs) and indoor residual spraying (IRS), have played a major role in the observed declines of malaria mortality and morbidity [2]
To accelerate malaria elimination efforts and sustain the gains made, these important aspects of persistent malaria transmission must be measured in adequate detail, and effective interventions deployed to complement ITNs and IRS
Summary
Effective malaria surveillance requires detailed assessments of mosquitoes biting indoors, where interventions such as insecticide-treated nets work best, and outdoors, where other interventions may be required. Evidence suggests that vector control tools, mainly insecticide-treated nets (ITNs) and indoor residual spraying (IRS), have played a major role in the observed declines of malaria mortality and morbidity [2] These approaches, which effectively target indoor-biting and indoor-resting mosquitoes are increasingly compromised by factors such shifts in mosquito behaviours (e.g., early-evening biting and outdoor biting), which overlap with human activities outdoors [3, 4] and the spread of mosquito resistance to common public health pesticides [5, 6]. The World Health Organization (WHO) recently outlined key indicators that should be assessed to comprehensively understand the prevailing malaria transmission and its vectors in any given area [7] These include when and where biting exposure occurs and whether available interventions are effective. With the growing attention on residual malaria transmission, such tools should to measure transmission and monitor interventions, but more importantly compare proportions of exposure in different locations, indoors versus outdoors
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