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      Efficacy of bifenthrin-impregnated bednets against Anopheles funestus and pyrethroid-resistant Anopheles gambiae in North Cameroon

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          Abstract

          Background

          Recent field studies indicated that insecticide-treated bednets (ITNs) maintain their efficacy despite a high frequency of the knock-down resistance ( kdr) gene in Anopheles gambiae populations. It was essential to evaluate ITNs efficacy in areas with metabolic-based resistance.

          Methods

          Bifenthrin was used in this experiment because it is considered a promising candidate for bednets impregnation. Nets were treated at 50 mg/m 2, a dose that has high insecticidal activity on kdr mosquitoes and at 5 mg/m 2, a dose that kills 95% of susceptible mosquitoes under laboratory conditions with 3 minutes exposure. Bednets were holed to mimic physical damage. The trial was conducted in three experimental huts from Pitoa, North-Cameroon where Anopheles gambiae displays metabolic resistance and cohabits with An. funestus.

          Results

          Bifenthrin at 50 mg/m 2 significantly reduced anophelines' entry rate (>80%). This was not observed at 5 mg/m 2. Both treatments increased exophily in An. gambiae, and to a lesser extent in An. funestus. With bifenthrin at high dosage, over 60% reduction in blood feeding and 75–90% mortality rates were observed in both vectors. Despite presence of holes, only a single An. gambiae and two An. funestus females were collected inside the treated net, and all were found dead. The same trends were observed with low dosage bifenthrin though in most cases, no significant difference was found with the untreated control net.

          Conclusion

          Bifenthrin-impregnated bednets at 50 mg/m 2 were efficient in the reduction of human-vector contact in Pitoa. Considerable personal protection was gained against An. funestus and metabolic pyrethroid resistant An. gambiae populations.

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          Most cited references 38

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          Simultaneous identification of species and molecular forms of the Anopheles gambiae complex by PCR-RFLP.

          For differential identification of sibling species in the Anopheles gambiae Giles complex (Diptera: Culicidae), including simultaneous separation of M and S molecular forms within An. gambiae Giles sensu stricto, we describe a PCR-RFLP method. This procedure is more efficient, faster and cheaper than those used before, so is recommended for large-scale processing of field-collected larval and adult specimens to be identified in malaria vector studies.
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            Anopheles funestus resistant to pyrethroid insecticides in South Africa.

            Northern Kwazulu/Natal (KZN) Province of South Africa borders on southern Mozambique, between Swaziland and the Indian Ocean. To control malaria vectors in KZN, houses were sprayed annually with residual DDT 2 g/ m2 until 1996 when the treatment changed to deltamethrin 20-25 mg/m2. At Ndumu (27 degrees 02'S, 32 degrees 19'E) the recorded malaria incidence increased more than six-fold between 1995 and 1999. Entomological surveys during late 1999 found mosquitoes of the Anopheles funestus group (Diptera: Culicidae) resting in sprayed houses in some sectors of Ndumu area. This very endophilic-vector of malaria had been eliminated from South Africa by DDT spraying in the 1950s, leaving the less endophilic An. arabiensis Patton as the only vector of known importance in KZN. Deltamethrin-sprayed houses at Ndumu were checked for insecticide efficacy by bioassay using susceptible An. arabiensis (laboratory-reared) that demonstrated 100% mortality. Members of the An. funestus group from Ndumu houses (29 males, 116 females) were identified by the rDNA PCR method and four species were found: 74 An. funestus Giles sensu stricto, 34 An. parensis Gillies, seven An. rivulorum Leeson and one An. leesoni Evans. Among An. funestus s.s. females, 5.4% (4/74) were positive for Plasmodium falciparum by ELISA and PCR tests. To test for pyrethroid resistance, mosquito adults were exposed to permethrin discriminating dosage and mortality scored 24h post-exposure: survival rates of wild-caught healthy males were 5/10 An. funestus, 1/9 An. rivulorum and 0/2 An. parensis; survival rates of laboratory-reared adult progeny from 19 An. funestus females averaged 14% (after 1h exposure to 1% permethrin 25:75cis:trans on papers in WHO test kits) and 27% (after 30 min in a bottle with 25 microg permethrin 40:60cis:trans). Anopheles funestus families showing >20% survival in these two resistance test procedures numbered 5/19 and 12/19, respectively. Progeny from 15 of the families were tested on 4% DDT impregnated papers and gave 100% mortality. Finding these proportions of pyrethroid-resistant An. funestus, associated with a malaria upsurge at Ndumu, has serious implications for malaria vector control operations in southern Africa.
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              Safety of pyrethroid-treated mosquito nets.

               N Nakashima,  A Aitio,  M Zaim (2000)
              The use of insecticide treated nets (ITNs) for personal protection against malaria vector Anopheles mosquitoes (Diptera: Culicidae) has become popular during the past decade. With the precautions outlined in this paper, field use of pyrethroids--at concentrations recommended for treatment of mosquito nets--poses little or no hazard to people treating the nets or to users of the treated nets. With frequent exposure to low concentrations of pyrethroids, the risk of toxicity of any kind is remote. Pyrethroids entering the systemic circulation are rapidly metabolized to much less toxic metabolites. Toxicologically, pyrethroids have a useful characteristic--the production of skin paraesthesia--which gives an early indication of exposure. This reversible symptom of exposure is due to transient stimulation of peripheral sensory nerves and not a toxic effect. In the retail market, for home use, the provision of proper packaging and labelling, with clear instructions on safe and effective use of the product are most important. Because many domestic users of pyrethroid 'home treatment kits' for ITNs may not be fully literate, it is essential that 'instructions for use' should be portrayed via pictograms with supporting text in appropriate local language(s).
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                Author and article information

                Journal
                Malar J
                Malaria Journal
                BioMed Central (London )
                1475-2875
                2006
                11 September 2006
                : 5
                : 77
                Affiliations
                [1 ]Organisation de Coordination pour la lutte contre les Endémies en Afrique Centrale (OCEAC), Yaoundé, Cameroun
                [2 ]Institut de Recherche pour le Développement (IRD), UR016, Montpellier, France
                [3 ]Université de Yaoundé I, Yaoundé, Cameroun
                [4 ]Institute of Medical Research and Studies of Medicinal Plants (IMPM), Ministry of Scientific Research and Innovation, Yaoundé, Cameroun
                Article
                1475-2875-5-77
                10.1186/1475-2875-5-77
                1584243
                16961938
                Copyright © 2006 Chouaibou et al; licensee BioMed Central Ltd.

                This is an Open Access article distributed under the terms of the Creative Commons Attribution License ( http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

                Categories
                Research

                Infectious disease & Microbiology

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