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      Residual malaria transmission dynamics varies across The Gambia despite high coverage of control interventions

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          Abstract

          Over the last decades, malaria has declined substantially in The Gambia but its transmission has not been interrupted. In order to better target control interventions, it is essential to understand the dynamics of residual transmission. This prospective cohort study was conducted between June 2013 and April 2014 in six pairs of villages across The Gambia. Blood samples were collected monthly during the transmission season (June-December) from all residents aged ≥6 months (4,194 individuals) and then in April (dry season). Entomological data were collected monthly throughout the malaria transmission season. Ownership of Long-Lasting Insecticidal Nets was 71.5% (2766/3869). Incidence of malaria infection and clinical disease varied significantly across the country, with the highest values in eastern (1.7/PYAR) than in central (0.2 /PYAR) and western (0.1/PYAR) Gambia. Malaria infection at the beginning of the transmission season was significantly higher in individuals who slept outdoors (HR = 1.51, 95% CI: 1.02–2.23, p = 0.04) and in those who had travelled outside the village (HR = 2.47, 95% CI: 1.83–3.34, p <0.01). Sub-patent infections were more common in older children (HR = 1.35, 95% CI: 1.04–1.6, p <0.01) and adults (HR = 1.53, 95% CI: 1.23–1.89, p<0.01) than in younger children. The risk of clinical malaria was significantly higher in households with at least one infected individual at the beginning of the transmission season (HR = 1.76, p<0.01). Vector parity was significantly higher in the eastern part of the country, both in the south (90.7%, 117/129, p<0.01) and the north bank (81.1%, 227/280, p<0.01), than in the western region (41.2%, 341/826), indicating higher vector survival. There is still significant residual malaria transmission across The Gambia, particularly in the eastern region. Additional interventions able to target vectors escaping Long-Lasting Insecticidal Nets and indoor residual spraying are needed to achieve malaria elimination.

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          High sensitivity of detection of human malaria parasites by the use of nested polymerase chain reaction.

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            Epidemiology and infectivity of Plasmodium falciparum and Plasmodium vivax gametocytes in relation to malaria control and elimination.

            Malaria remains a major cause of morbidity and mortality in the tropics, with Plasmodium falciparum responsible for the majority of the disease burden and P. vivax being the geographically most widely distributed cause of malaria. Gametocytes are the sexual-stage parasites that infect Anopheles mosquitoes and mediate the onward transmission of the disease. Gametocytes are poorly studied despite this crucial role, but with a recent resurgence of interest in malaria elimination, the study of gametocytes is in vogue. This review highlights the current state of knowledge with regard to the development and longevity of P. falciparum and P. vivax gametocytes in the human host and the factors influencing their distribution within endemic populations. The evidence for immune responses, antimalarial drugs, and drug resistance influencing infectiousness to mosquitoes is reviewed. We discuss how the application of molecular techniques has led to the identification of submicroscopic gametocyte carriage and to a reassessment of the human infectious reservoir. These components are drawn together to show how control measures that aim to reduce malaria transmission, such as mass drug administration and a transmission-blocking vaccine, might better be deployed.
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              The silent threat: asymptomatic parasitemia and malaria transmission.

              Scale-up of malaria control interventions has resulted in a substantial decline in global malaria morbidity and mortality. Despite this achievement, there is evidence that current interventions alone will not lead to malaria elimination in most malaria-endemic areas and additional strategies need to be considered. Use of antimalarial drugs to target the reservoir of malaria infection is an option to reduce the transmission of malaria between humans and mosquito vectors. However, a large proportion of human malaria infections are asymptomatic, requiring treatment that is not triggered by care-seeking for clinical illness. This article reviews the evidence that asymptomatic malaria infection plays an important role in malaria transmission and that interventions to target this parasite reservoir may be needed to achieve malaria elimination in both low- and high-transmission areas.
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                Author and article information

                Contributors
                Role: Data curationRole: Formal analysisRole: InvestigationRole: MethodologyRole: Project administrationRole: SupervisionRole: ValidationRole: VisualizationRole: Writing – original draftRole: Writing – review & editing
                Role: Writing – review & editing
                Role: Formal analysisRole: ValidationRole: Writing – original draftRole: Writing – review & editing
                Role: MethodologyRole: SupervisionRole: ValidationRole: Writing – review & editing
                Role: MethodologyRole: SupervisionRole: Writing – review & editing
                Role: MethodologyRole: SoftwareRole: ValidationRole: Writing – review & editing
                Role: Formal analysis
                Role: MethodologyRole: Writing – review & editing
                Role: MethodologyRole: ValidationRole: Writing – review & editing
                Role: ConceptualizationRole: Writing – review & editing
                Role: ConceptualizationRole: Writing – review & editing
                Role: Writing – review & editing
                Role: Writing – review & editing
                Role: SupervisionRole: Writing – review & editing
                Role: Funding acquisitionRole: Project administrationRole: ResourcesRole: SupervisionRole: ValidationRole: VisualizationRole: Writing – review & editing
                Role: Editor
                Journal
                PLoS One
                PLoS ONE
                plos
                plosone
                PLoS ONE
                Public Library of Science (San Francisco, CA USA )
                1932-6203
                2 November 2017
                2017
                : 12
                : 11
                : e0187059
                Affiliations
                [1 ] Medical Research Council Unit The Gambia, Banjul, The Gambia
                [2 ] Department of Global Health, Faculty of Medicine and Health Sciences, University of Antwerp, Antwerp, Belgium
                [3 ] Centre for Primary Care and Public Health, Queen Mary University of London, London, United Kingdom
                [4 ] West African Centre for Cell Biology of Infectious Pathogens, Department of Biochemistry, Cell & Molecular Biology University of Ghana, Accra, Ghana
                [5 ] Department of Medical Microbiology, Radboud University Medical Center, Nijmegen, The Netherlands
                [6 ] Faculty of Infectious and Tropical Diseases, London School of Hygiene and Tropical Medicine, London, United Kingdom
                [7 ] Department of Public Health, Institute of Tropical Medicine, Antwerp, Belgium
                [8 ] Amsterdam Institute for Social Science Research, University of Amsterdam, Amsterdam, The Netherlands
                [9 ] School of Tropical Medicine and Global Health, Nagasaki University, Nagasaki, Japan
                [10 ] School of Biological & Biomedical Sciences, Durham University, Durham, United Kingdom
                Centro de Pesquisas Rene Rachou, BRAZIL
                Author notes

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

                Author information
                http://orcid.org/0000-0003-2452-3677
                Article
                PONE-D-17-10620
                10.1371/journal.pone.0187059
                5667860
                29095834
                0edf71aa-03fe-43c1-92a3-793c43141c33
                © 2017 Mwesigwa 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.

                History
                : 17 March 2017
                : 12 October 2017
                Page count
                Figures: 8, Tables: 11, Pages: 24
                Funding
                Funded by: MRC/DFID
                Award ID: MC_EX_MR/J002364/1
                Award Recipient :
                This study is funded The UK Medical Research Council (MRC) and the UK Department for International Development (DFID) under the MRC/DFID Concordat agreement. Grant number MC_EX_MR/J002364/1. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.
                Categories
                Research Article
                Medicine and Health Sciences
                Parasitic Diseases
                Malaria
                Medicine and Health Sciences
                Tropical Diseases
                Malaria
                Biology and Life Sciences
                Parasitology
                Parasite Groups
                Apicomplexa
                Plasmodium
                People and Places
                Geographical Locations
                Africa
                Gambia
                Medicine and Health Sciences
                Infectious Diseases
                Infectious Disease Control
                Biology and Life Sciences
                Cell Biology
                Cellular Types
                Animal Cells
                Germ Cells
                Gametocytes
                Medicine and Health Sciences
                Infectious Diseases
                Vector-Borne Diseases
                Earth Sciences
                Seasons
                Medicine and Health Sciences
                Infectious Diseases
                Disease Vectors
                Insect Vectors
                Mosquitoes
                Biology and Life Sciences
                Species Interactions
                Disease Vectors
                Insect Vectors
                Mosquitoes
                Biology and Life Sciences
                Organisms
                Eukaryota
                Animals
                Invertebrates
                Arthropoda
                Insects
                Mosquitoes
                Custom metadata
                The participant data has been collected following provision of informed consent under the prerequisite of strict participant confidentiality (SCC reference 1318). Access to participant data can be accessed following review of the request by the Gambia Government/MRC Joint Ethics Committee to protect the rights and interests of the study participants. The review process and release of data will be facilitated by MRC Unit The Gambia (MRCG) and access will not be unduly restricted. MRCG is however committed to data sharing and open access via its respective policies. Contact information for the Gambia Government/MRC Joint Ethics Committee: Ms Naffie Jobe, Secretary Gambia Government/MRC Joint Ethics Committee, njobe@ 123456mrc.gm .

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