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      Longevity of the insecticidal effect of three pyrethroid formulations applied to outdoor vegetation on a laboratory-adapted colony of the Southeast Asian malaria vector Anopheles dirus

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          Abstract

          Outdoor residual spraying is proposed for the control of exophilic mosquitoes. However, the residual effect of insecticide mists applied to outdoor resting habitats of mosquitoes is not well characterized. The objective of this study was to assess the longevity of the residual insecticidal effect of three pyrethroid formulations applied to outdoor vegetation against the Southeast Asian malaria vector Anopheles dirus. Lambda-cyhalothrin capsule suspension, deltamethrin emulsifiable concentrate and bifenthrin wettable powder were sprayed on dense bamboo bushes on the Thailand-Myanmar border during the dry season 2018. The duration and magnitude of the residual insecticidal effect were assessed weekly with a standard cone assay, using freshly collected insecticide-treated bamboo leaves and a laboratory-adapted colony of Anopheles dirus sensu stricto susceptible to pyrethroids. The experiment was repeated during the rainy season to assess the persistence of the lambda-cyhalothrin formulation after natural rains and artificial washings. During the dry season (cumulative rainfall = 28 mm in 111 days), mortality and knockdown (KD) rates were >80% for 60 days with bifenthrin and 90 days with lambda-cyhalothrin and deltamethrin. The 50% knockdown time (TKD50) was <15 min with lambda-cyhalothrin and deltamethrin, and <30 min with bifenthrin. During the rainy season (cumulative rainfall = 465 mm in 51 days), mortality and KD rates were >80% for 42 days and TKD50 was <15 min with lambda-cyhalothrin. Additional artificial washing of the testing material with 10L of tap water before performing the cone tests had no significant effect on the residual insecticidal effect of this formulation. Long-lasting residual insecticidal effect can be obtained when spraying pyrethroid insecticides on the outdoor resting habitats of malaria vectors.

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          The dominant Anopheles vectors of human malaria in the Asia-Pacific region: occurrence data, distribution maps and bionomic précis

          Background The final article in a series of three publications examining the global distribution of 41 dominant vector species (DVS) of malaria is presented here. The first publication examined the DVS from the Americas, with the second covering those species present in Africa, Europe and the Middle East. Here we discuss the 19 DVS of the Asian-Pacific region. This region experiences a high diversity of vector species, many occurring sympatrically, which, combined with the occurrence of a high number of species complexes and suspected species complexes, and behavioural plasticity of many of these major vectors, adds a level of entomological complexity not comparable elsewhere globally. To try and untangle the intricacy of the vectors of this region and to increase the effectiveness of vector control interventions, an understanding of the contemporary distribution of each species, combined with a synthesis of the current knowledge of their behaviour and ecology is needed. Results Expert opinion (EO) range maps, created with the most up-to-date expert knowledge of each DVS distribution, were combined with a contemporary database of occurrence data and a suite of open access, environmental and climatic variables. Using the Boosted Regression Tree (BRT) modelling method, distribution maps of each DVS were produced. The occurrence data were abstracted from the formal, published literature, plus other relevant sources, resulting in the collation of DVS occurrence at 10116 locations across 31 countries, of which 8853 were successfully geo-referenced and 7430 were resolved to spatial areas that could be included in the BRT model. A detailed summary of the information on the bionomics of each species and species complex is also presented. Conclusions This article concludes a project aimed to establish the contemporary global distribution of the DVS of malaria. The three articles produced are intended as a detailed reference for scientists continuing research into the aspects of taxonomy, biology and ecology relevant to species-specific vector control. This research is particularly relevant to help unravel the complicated taxonomic status, ecology and epidemiology of the vectors of the Asia-Pacific region. All the occurrence data, predictive maps and EO-shape files generated during the production of these publications will be made available in the public domain. We hope that this will encourage data sharing to improve future iterations of the distribution maps.
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            Characterizing, controlling and eliminating residual malaria transmission

            Long-lasting insecticidal nets (LLINs) and indoor residual spraying (IRS) interventions can reduce malaria transmission by targeting mosquitoes when they feed upon sleeping humans and/or rest inside houses, livestock shelters or other man-made structures. However, many malaria vector species can maintain robust transmission, despite high coverage of LLINs/IRS containing insecticides to which they are physiologically fully susceptible, because they exhibit one or more behaviours that define the biological limits of achievable impact with these interventions: (1) Natural or insecticide-induced avoidance of contact with treated surfaces within houses and early exit from them, thus minimizing exposure hazard of vectors which feed indoors upon humans; (2) Feeding upon humans when they are active and unprotected outdoors, thereby attenuating personal protection and any consequent community-wide suppression of transmission; (3) Feeding upon animals, thus minimizing contact with insecticides targeted at humans or houses; (4) Resting outdoors, away from insecticide-treated surfaces of nets, walls and roofs. Residual malaria transmission is, therefore, defined as all forms of transmission that can persist after achieving full universal coverage with effective LLINs and/or IRS containing active ingredients to which local vector populations are fully susceptible. Residual transmission is sufficiently intense across most of the tropics to render malaria elimination infeasible without new or improved vector control methods. Many novel or improved vector control strategies to address residual transmission are emerging that either: (1) Enhance control of adult vectors that enter houses to feed and/or rest by killing, repelling or excluding them; (2) Kill or repel adult mosquitoes when they attack people outdoors; (3) Kill adult mosquitoes when they attack livestock; (4) Kill adult mosquitoes when they feed upon sugar or; (5) Kill immature mosquitoes in aquatic habitats. To date, none of these options has sufficient supporting evidence to justify full-scale programmatic implementation. Concerted investment in their rigorous selection, development and evaluation is required over the coming decade to enable control and, ultimately, elimination of residual malaria transmission. In the meantime, national programmes may assess options for addressing residual transmission under programmatic conditions through pilot studies with strong monitoring, evaluation and operational research components, similar to the Onchocerciasis Control Programme.
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              A global index representing the stability of malaria transmission.

              To relate stability of malaria transmission to biologic characteristics of vector mosquitoes throughout the world, we derived an index representing the contribution of regionally dominant vector mosquitoes to the force of transmission. This construct incorporated published estimates describing the proportion of blood meals taken from human hosts, daily survival of the vector, and duration of the transmission season and of extrinsic incubation. The result of the calculation was displayed globally on a 0.5 degrees grid. We found that these biologic characteristics of diverse vector mosquitoes interact with climate to explain much of the regional variation in the intensity of transmission. Due to the superior capacity of many tropical mosquitoes as vectors of malaria, particularly those in sub-Saharan Africa, antimalaria interventions conducted in the tropics face greater challenges than were faced by formerly endemic nations in more temperate climes.
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                Author and article information

                Contributors
                Role: ConceptualizationRole: Data curationRole: Formal analysisRole: Funding acquisitionRole: InvestigationRole: MethodologyRole: Project administrationRole: ResourcesRole: SupervisionRole: ValidationRole: VisualizationRole: Writing – original draftRole: Writing – review & editing
                Role: Investigation
                Role: Data curationRole: InvestigationRole: Supervision
                Role: Investigation
                Role: Investigation
                Role: Investigation
                Role: Investigation
                Role: Investigation
                Role: ConceptualizationRole: Funding acquisitionRole: Project administration
                Role: ConceptualizationRole: Funding acquisitionRole: Project administrationRole: ResourcesRole: Writing – original draftRole: Writing – review & editing
                Role: Editor
                Journal
                PLoS One
                PLoS ONE
                plos
                plosone
                PLoS ONE
                Public Library of Science (San Francisco, CA USA )
                1932-6203
                14 April 2020
                2020
                : 15
                : 4
                Affiliations
                [1 ] Shoklo Malaria Research Unit, Mahidol-Oxford Tropical Medicine Research Unit, Faculty of Tropical Medicine, Mahidol University, Mae Sot, Thailand
                [2 ] Centre for Tropical Medicine and Global Health, Nuffield Department of Medicine, University of Oxford, Oxford, England, United Kingdom
                University of Saskatchewan College of Agriculture and Bioresources, CANADA
                Author notes

                Competing Interests: The authors have declared that no competing interests exist.

                Article
                PONE-D-19-22372
                10.1371/journal.pone.0231251
                7156039
                32287300
                © 2020 Chaumeau et al

                This is an open access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.

                Page count
                Figures: 4, Tables: 1, Pages: 12
                Product
                Funding
                Funded by: funder-id http://dx.doi.org/10.13039/100004440, Wellcome Trust;
                Award ID: 101148
                Award Recipient :
                Funded by: funder-id http://dx.doi.org/10.13039/100000865, Bill and Melinda Gates Foundation;
                Award ID: GH OPP 1177406
                Award Recipient :
                FN recieved grants from the Wellcome Trust [101148] ( https://wellcome.ac.uk/) and the Bill and Melinda Gates Foundation [GH OPP 1177406] ( https://www.gatesfoundation.org/). The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.
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