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      Nationwide Investigation of the Pyrethroid Susceptibility of Mosquito Larvae Collected from Used Tires in Vietnam

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          Abstract

          Pyrethroid resistance is envisioned to be a major problem for the vector control program since, at present, there are no suitable chemical substitutes for pyrethroids. Cross-resistance to knockdown agents, which are mainly used in mosquito coils and related products as spatial repellents, is the most serious concern. Since cross-resistance is a global phenomenon, we have started to monitor the distribution of mosquito resistance to pyrethroids. The first pilot study was carried out in Vietnam. We periodically drove along the national road from the north end to the Mekong Delta in Vietnam and collected mosquito larvae from used tires. Simplified susceptibility tests were performed using the fourth instar larvae of Aedes aegypti, Aedes albopictus, and Culex quinquefasciatus. Compared with the other species, Ae. aegypti demonstrated the most prominent reduction in susceptibility. For Ae. aegypti, significant increases in the susceptibility indices with a decrease in the latitude of collection points were observed, indicating that the susceptibility of Ae. aegypti against d-allethrin was lower in the southern part, including mountainous areas, as compared to that in the northern part of Vietnam. There was a significant correlation between the susceptibility indices in Ae. aegypti and the sum of annual pyrethroid use for malaria control (1998–2002). This might explain that the use of pyrethroids as residual treatment inside houses and pyrethroid-impregnated bed nets for malaria control is attributable to low pyrethroid susceptibility in Ae. aegypti. Such insecticide treatment appeared to have been intensively administered in the interior and along the periphery of human habitation areas where, incidentally, the breeding and resting sites of Ae. aegypti are located. This might account for the strong selection pressure toward Ae. aegypti and not Ae. albopictus.

          Author Summary

          Pyrethroid is one of the most successful insecticidal chemicals for controlling insect pests and vectors of household and public health importance. However, extensive treatment of photo-stable and highly effective pyrethroids sometimes causes resistance to insect populations. Resistance to pyrethroids belonging to the knockdown agent groups that have been used as the spatial repellents, such as mosquito coils and related products, is the most serious concern. Since this is a global phenomenon, we have started to monitor the distribution of mosquito resistance to pyrethroids and the first pilot study was carried out in Vietnam. Our study concluded that Aedes aegypti, the most important vector of dengue and dengue haemorrhagic fever, demonstrated the prominent reduction in susceptibility against d-allethrin, one of the most popular pyrethroids for spatial repellents. The extensive treatment of photo-stable pyrethroids for malaria control seemed to be one of the attributable factors. In order to effectively manage pyrethroid resistance, the establishment of a feasible insecticide management system and a regular monitoring system of pyrethroid susceptibility will be essential.

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          Most cited references39

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          Insecticide susceptibility of Aedes aegypti and Aedes albopictus across Thailand.

          Aedes aegypti (L.) and Aedes albopictus (Skuse), two important vectors of dengue fever and dengue hemorrhagic fever, were collected from Mae Sot, Nakhon Sawan, Nakhon Ratchasima, Surat Thani, and Phatthalung, Thailand, from July 2003 to April 2004. The patterns of insecticide susceptibility to temephos, malathion, and permethrin of both Ae. aegypti and Ae. albopictus larvae were determined. Ae. aegypti from all study sites were resistant to permethrin, they but were susceptible to malathion. Resistance to temephos was detected in all strains of Ae. aegypti, except those from Nakhon Ratchasima. Ae. albopictus larvae had low levels of resistance to all three insecticides, except Mae Sot and Phatthalung strains, which were resistant to permethrin.
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            Different spatial distribution of Aedes aegypti and Aedes albopictus along an urban-rural gradient and the relating environmental factors examined in three villages in northern Thailand.

            A larval survey of dengue vectors was conducted from July to November 1966 and from May to November 1997 in Chiangmai Province, Thailand. Three villages in urban, transition, and rural areas were selected for the survey to clarify the spatial distribution of Ae. aegypti and Ae. albopictus along an urban-rural ecological gradient. The average number of Ae. aegypti larvae in larvitraps was higher in the urban area than in the rural area, as we expected, whereas the opposite was found for Ae. albopictus, rural area > urban area. A house survey of larvae-inhabiting containers showed significant differences in the number and composition of these containers among the study areas. Significant differences were also found in the average distance between houses, average tree height, and average percentage of vegetation cover for each house. The seasonal pattern of rainfall recorded in each study area did not show great differences among the study areas. The response of Ae. aegypti and Ae. albopictus to the urban-rural gradient is discussed in relation to the possibility of applying geographic information system techniques to plan the control strategy and surveillance of dengue vectors.
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              Malaria in central Vietnam: analysis of risk factors by multivariate analysis and classification tree models

              Background In Central Vietnam, forest malaria remains difficult to control due to the complex interactions between human, vector and environmental factors. Methods Prior to a community-based intervention to assess the efficacy of long-lasting insecticidal hammocks, a complete census (18,646 individuals) and a baseline cross-sectional survey for determining malaria prevalence and related risk factors were carried out. Multivariate analysis using survey logistic regression was combined to a classification tree model (CART) to better define the relative importance and inter-relations between the different risk factors. Results The study population was mostly from the Ra-glai ethnic group (88%), with both low education and socio-economic status and engaged mainly in forest activities (58%). The multivariate analysis confirmed forest activity, bed net use, ethnicity, age and education as risk factors for malaria infections, but could not handle multiple interactions. The CART analysis showed that the most important risk factor for malaria was the wealth category, the wealthiest group being much less infected (8.9%) than the lower and medium wealth category (16.6%). In the former, forest activity and bed net use were the most determinant risk factors for malaria, while in the lower and medium wealth category, insecticide treated nets were most important, although the latter were less protective among Ra-glai people. Conclusion The combination of CART and multivariate analysis constitute a novel analytical approach, providing an accurate and dynamic picture of the main risk factors for malaria infection. Results show that the control of forest malaria remains an extremely complex task that has to address poverty-related risk factors such as education, ethnicity and housing conditions.
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                Author and article information

                Contributors
                Role: Editor
                Journal
                PLoS Negl Trop Dis
                plos
                plosntds
                PLoS Neglected Tropical Diseases
                Public Library of Science (San Francisco, USA )
                1935-2727
                1935-2735
                March 2009
                10 March 2009
                : 3
                : 3
                : e391
                Affiliations
                [1 ]Department of Vector Ecology & Environment, Institute of Tropical Medicine, Nagasaki University, Nagasaki, Japan
                [2 ]National Institute of Hygiene and Epidemiology, Hanoi, Vietnam
                George Washington University, United States of America
                Author notes

                Conceived and designed the experiments: HK YH YTN HTN MT. Performed the experiments: HK YH YTN SHT. Analyzed the data: HK. Contributed reagents/materials/analysis tools: HK MT. Wrote the paper: HK.

                Article
                08-PNTD-RA-0224R2
                10.1371/journal.pntd.0000391
                2647736
                19274072
                2f9d3b99-a94c-4692-99c0-5f32bfa2834a
                Kawada 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
                : 7 July 2008
                : 9 February 2009
                Page count
                Pages: 8
                Categories
                Research Article
                Public Health and Epidemiology

                Infectious disease & Microbiology
                Infectious disease & Microbiology

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