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      Reduced Efficacy of Insecticide-treated Nets and Indoor Residual Spraying for Malaria Control in Pyrethroid Resistance Area, Benin


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          These tools may no longer be effective for malaria control in parts of Benin.


          The pyrethroid knockdown resistance gene ( kdr) has become widespread in Anopheles gambiae in West Africa. A trial to test the continuing efficacy of insecticide-treated nets (ITN) and indoor residual spraying (IRS) was undertaken in experimental huts at 2 sites in Benin, 1 where kdr is present at high frequency (Ladji), the other where An. gambiae is susceptible (Malanville). Holes were made in the nets to mimic worn nets. At Malanville, 96% of susceptible An. gambiae were inhibited from blood-feeding, whereas at Ladji feeding was uninhibited by ITNs. The mortality rate of An. gambiae in ITN huts was 98% in Malanville but only 30% at Ladji. The efficacy of IRS was equally compromised. Mosquitoes at Ladji had higher oxidase and esterase activity than a laboratory-susceptible strain, but this fact did not seem to contribute to resistance. Pyrethroid resistance in An. gambiae appears to threaten the future of ITN and IRS in Benin.

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          Identification of a point mutation in the voltage-gated sodium channel gene of Kenyan Anopheles gambiae associated with resistance to DDT and pyrethroids.

          A field trial of permethrin-impregnated bednets and curtains was initiated in Western Kenya in 1990, and a strain of Anopheles gambiae showing reduced susceptibility to permethrin was colonized from this site in 1992. A leucine-phenylalanine substitution at position 1014 of the voltage-gated sodium channel is associated with resistance to permethrin and DDT in many insect species, including Anopheles gambiae from West Africa. We cloned and sequenced a partial sodium channel cDNA from the Kenyan permethrin-resistant strain and we identified an alternative substitution (leucine to serine) at the same position, which is linked to the inheritance of permethrin resistance in the F(2) progeny of genetic crosses between susceptible and resistant individuals. The diagnostic polymerase chain reaction (PCR) developed by Martinez-Torres et al. [(1998) Insect Mol Biol 7: 179-184] to detect kdr alleles in field populations of An. gambiae will not detect the Kenyan allele and hence reliance on this assay may lead to an underestimate of the prevalence of pyrethroid resistance in this species. We adapted the diagnostic PCR to detect the leucine-serine mutation and with this diagnostic we were able to demonstrate that this kdr allele was present in individuals collected from the Kenyan trial site in 1986, prior to the introduction of pyrethroid-impregnated bednets. The An. gambiae sodium channel was physically mapped to chromosome 2L, division 20C. This position corresponds to the location of a major quantitative trait locus determining resistance to permethrin in the Kenyan strain of An. gambiae.
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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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              Historical review of malarial control in southern African with emphasis on the use of indoor residual house-spraying.

              Indoor residual house-spraying (IRS) mainly with dichlorodiphenyltrichloroethane (DDT) was the principal method by which malaria was eradicated or greatly reduced in many countries in the world between the 1940s and 1960s. In sub-Saharan Africa early malarial eradication pilot projects also showed that malaria is highly responsive to vector control by IRS but transmission could not be interrupted in the endemic tropical and lowland areas. As a result IRS was not taken to scale in most endemic areas of the continent with the exception of southern Africa and some island countries such as Reunion, Mayotte, Zanzibar, Cape Verde and Sao Tome. In southern Africa large-scale malarial control operations based on IRS with DDT and benzene hexachloride (BHC) were initiated in a number of countries to varying degrees. The objective of this review was to investigate the malarial situation before and after the introduction of indoor residual insecticide spraying in South Africa, Swaziland, Botswana, Namibia, Zimbabwe and Mozambique using historical malarial data and related information collected from National Malaria Control Programmes, national archives and libraries, as well as academic institutions in the respective countries. Immediately after the inception of IRS with insecticides, dramatic reductions in malaria and its vectors were recorded. Countries that developed National Malaria Control Programmes during this phase and had built up human and organizational resources made significant advances towards malarial control. Malaria was reduced from hyper- to meso-endemicity and from meso- to hypo-endemicity and in certain instances to complete eradication. Data are presented on the effectiveness of IRS as a malarial control tool in six southern African countries. Recent trends in and challenges to malarial control in the region are also discussed.

                Author and article information

                Emerg Infect Dis
                Emerging Infectious Diseases
                Centers for Disease Control and Prevention
                February 2007
                : 13
                : 2
                : 199-206
                [* ]London School of Hygiene and Tropical Medicine, Cotonou, Benin, West Africa
                []Institut de Recherche pour le Developpement, Montpellier, France
                []University of Abomey-Calavi, Cotonou, Benin
                [§ ]Centre de Recherche Entomologique, Cotonou, Benin
                []London School of Hygiene and Tropical Medicine, London, United Kingdom
                Author notes
                Address for correspondence: Mark Rowland, London School of Hygiene and Tropical Medicine, Keppel St, London WC1E 7HT, United Kingdom; email: mark.rowland@ 123456lshtm.ac.uk


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