[When Urban and Agricultural Activities Favor the Proliferation of Mosquito Nuisance and Vectors of Pathogens to Humans]. / Quand les activités urbaines et agricoles favorisent la pullulation des moustiques nuisants et vecteurs d'agents pathogènes pour l'Homme.

Big cities have thrived on all continents, so have domestic and industrial wastes not to mention the often irrational use of agricultural inputs (fertilizers and pesticides) detrimental to plants and animals. One hundred and eighty million tons of fertilizers and 2.4 million tons of pesticides are spread every year worldwide. Such pollutions, whether urban or rural, have a significant impact on the biology of mosquitoes. Today some urban spaces have properly become a land of plenty for mosquitoes. The combined use of fertilizer and pesticides in the country, quite paradoxically also favor their proliferation. Ironically the very reasons that account for the multitudes of mosquitoes are the exact reasons responsible for the depletion of biodiversity.

Les grandes villes se sont multipliées sur tous les continents en générant des pollutions domestiques et industrielles et dans les campagnes, l'utilisation souvent irraisonnée des intrants agricoles (engrais et pesticides) déciment les plantes et les animaux. Cent quatre-vingt millions de tonnes d'engrais et 2,4 millions de tonnes de pesticides sont déversées chaque année dans le monde. Ces pollutions, qu'elles soient urbaines ou rurales, ont un impact considérable sur la biologie des moustiques. Aujourd'hui, certains espaces urbains sont devenus de véritables nids à moustiques et, dans les campagnes, l'usage combiné des engrais et des pesticides favorise, paradoxalement, leur prolifération. L'ironie de cette histoire du monde moderne est que les différents facteurs qui favorisent la pullulation des moustiques sont ceux là mêmes qui déciment une bonne partie de la biodiversité.

Risk factors of exposure to Aedes albopictus bites in mainland France using an immunological biomarker.

In recent decades, the invasive Aedes albopictus vector has spread across Europe and is responsible for numerous outbreaks of autochthonous arboviral disease. The aim of this study was to identify epidemiological and sociological risk factors related to individual levels of exposure to Aedes albopictus bites. A multidisciplinary survey was conducted with volunteer blood donors living in areas either colonised or not by Aedes albopictus in mainland France. Individual levels of exposure were evaluated by measuring the IgG level specific to Aedes albopictus saliva. The most striking risk factors concerned the localisation and characteristics of the dwelling. Individuals living in areas colonised prior to 2009 or recently colonised (between 2010 and 2012) had higher anti-salivary gland extract IgG levels compared with those who were living in areas not yet colonised by Ae. albopictus. The type of dwelling did not seem to impact the level of exposure to Aedes bites. People living in apartments had a higher anti-salivary gland extract IgG level than those living in individual houses but the difference was not statistically significant. Interestingly, the presence of air conditioning or window nets was associated with a noticeable reduction in bite intensity.

[Sugar-Supplemented Substrates Contaminated by Agricultural Inputs Favor the Proliferation of Aedes albopictus (Diptera: Culicidae)]. / Les substrats sucrés contaminés par les intrants agricoles favorisent la prolifération du moustique Aedes albopictus (Diptera : Culicidae).

Originating from South-East Asian tropical forests, Aedes albopictus (Skuse) (Diptera: Culicidae) is a mosquito of great genetic plasticity, which allowed it to adapt to a large variety of environments including the urban ones. Yet many a city nowadays comprises peri urban zones close to land devoted to farming. The sites where Ae. albopictus live and reproduce, located on the fringe of nectarfilled blossoming crops (orchards, colza and sunflower fields) are often polluted by chemical inputs such as fertilizers and pesticides. In this laboratory study we have assessed the impact of sugar-supplemented solutions contaminated by NPK fertilizer and/or by diflubenzuron or pyriproxyfen insecticides on adults of Ae. albopictus mosquitoes. The results have shown that the females feeding on a sweetened solution containing NPK fertilizer laid a significantly greater number of eggs. However when the sweetened solutions contained diflubenzuron or pyriproxyfen insecticides, the actions of both combinations resulted in a noticeable reduction of the numbers of eggs laid and the ones hatching. Finally when the sweetened solutions had been jointly polluted by the fertilizer and either one of the larvicides, the sterilizing effect of the insecticides was completely obliterated regarding both the numbers of eggs laid and the ones hatching. We are led to conclude that the somewhat inconsistent use of pesticides and fertilizers in agriculture bring about new ecological systems favorable to the proliferation of mosquitoes.

Originaire des forêts tropicales d'Asie du Sud-Est, Aedes albopictus (Skuse) (Diptera : Culicidae) est un moustique doté d'une grande plasticité génétique qui lui a permis de s'adapter à des environnements très divers, dont les milieux urbains. Or nombreuses sont les villes aujourd'hui où les quartiers périurbains s'installent à proximité de territoires ruraux à vocation agricole. Lorsque les lieux de vie et de reproduction d'Ae. albopictus se situent en lisière de cultures dont les fleurs sont riches en nectar (à l'exemple des arbres fruitiers, du colza et du tournesol), les adultes mâles et femelles se nourrissent d'exsudats sucrés souvent souillés par des intrants chimiques (engrais et pesticides). Nous avons mesuré dans cette étude de laboratoire, les impacts que généraient des solutions sucrées contaminées par un engrais NPK et/ou par les insecticides diflubenzuron et pyriproxyfen. Les résultats révèlent que lorsque les adultes d'Ae. albopictus absorbent une solution sucrée contenant un engrais NPK, les nombres d'oeufs pondus par les femelles s'en trouvent significativement augmentés. Quand une solution sucrée est au contraire contaminée par les insecticides diflubenzuron ou pyriproxyfen, l'action des deux composés provoque une nette diminution des nombres d'oeufs pondus mais aussi de ceux qui éclosent. Enfin, lorsque les réserves en sucre sont polluées à la fois par l'engrais et l'un ou l'autre des deux larvicides, l'effet stérilisant induit par les deux insecticides disparaît totalement, tant au niveau des nombres d'oeufs pondus que des nombres d'oeufs qui éclosent. Ces observations nous enseignent que l'utilisation pas toujours raisonnée des pesticides et des engrais en agriculture crée des situations écologiques nouvelles, favorables à la pullulation des moustiques.

The combination of NPK fertilizer and deltamethrin insecticide favors the proliferation of pyrethroid-resistant Anopheles gambiae (Diptera: Culicidae).

In this laboratory study, we investigated how the biological cycle of Anopheles gambiae s.s. (VKPR strain) would be like when grew in an environment containing more or less plant matter (2.5 or 5 g/l) and fertilizer (8-12-8 or 17-23-17 mg/l). Half of the environments studied were not exposed to insecticide (control) whereas the other half was submitted to deltamethrin treatment at the concentration of 0.015 mg/l. The bioassays showed that 2.5 g/l of plant matter in water are not sufficient to feed the hundred larvae, each breeding site contains. Treating these breeding sites with deltamethrin reversed the situation as it decreased the competition for food resources and allowed the surviving larvae to share the small amount of food enabling them to pursue their development until adults. If the introduction of NPK in untreated sites has not improved the nutritive qualities of the water, in the treated sites it multiplied the number of emerging adults by 2.5. In the waters containing 5 g/l of plant matter, the larvae did not undergo feeding competition and the impact of insecticide followed of a more traditional selection scheme that expressed itself by a lower number of emerging adults. In these environments treated or nontreated where plant matter is abundant, adding NPK brings food supplement to the larvae therefore increases the survival rate of An. gambiae. To conclude, whether in habitats with little or much plant matter, NPK presence in water results in larger adults with generally, more soluble proteins.

[Influence of plant matter and NPK fertilizer on the biology of Aedes aegypti (Diptera: Culicidae)]. / Influence des matières végétales et des engrais NPK sur la biologie de Aedes aegypti (Diptera : Culicidae).

In this laboratory study, we investigated the attractiveness of gravid Aedes aegypti females for aquatic habitats containing either NPK fertilizer or plant matter or a mixture of both. The development of larvae, adult emergence and weight of emerged adults were measured for each group and physicochemical analysis of water was made for the dosage of minerals and organic compounds. After 23 days experiment, NPK solution remained as attractive as the mixture of NPK + plant matter but did not ensure suitable development of larvae. The plant matter infusion showed less attractiveness for laying eggs than NPK but provided larvae with sufficient organic nutrients for their development. The combination of both NPK + plant matter provided the greatest attractiveness for gravid females and sufficient organic substance allowing mosquito larvae to grow. Physicochemical analyses of water containing NPK showed minerals only (NH4(+)+NO3(-)+P+K) whereas plant matter showed high content of carbon and nitrogen. The NPK + plant matter mixture contained both organic and minerals elements that favoured the proliferation of bacteria and then the development of mosquito larvae. These findings could lead to the development of new traps that could attract females mosquitoes and killed hatched larvae if mix with appropriate larvicides.

[Aedes aegypti oviposition in response to NPK fertilizers]. / Influence des engrais de type NPK sur l'oviposition d'Aedes aegypti.

Fertilizers are mineral associations intended to bring to the plants nutritive complements necessary to their growth. Modern fertilizers (NPK) combine the three basic elements which are the nitrogen (N), phosphorus (P) and potassium (K). In this study, we investigated in tunnel apparatus the influence of aqueous solutions containing low, moderate and high concentrations of NPK on the oviposition of Aedes aegypti. The results showed that the solutions containing moderate concentrations (NK = 17-33 mg/l and P = 23-47 mg/l) attracted significantly more gravid females than distilled water (P < 0.001). Conversely, the solutions containing either low or high concentrations of NPK (NK = 8 mg/l and P = 12 mg/l; NK = 50 mg/I and P = 70 mg/l) did not induce significant attraction (P > 0.05). These findings suggest that NPK fertilizers may influence the egg-laying behaviour of Ae. aegypti in field situations.

Lethal and behavioural effects of three synthetic repellents (DEET, IR3535 and KBR 3023) on Aedes aegypti mosquitoes in laboratory assays.

The knock-down, mortality and 'irritancy' effects of three synthetic repellents (DEET, IR3535 and KBR 3023) on Aedes aegypti (L) (Diptera: Culicidae) were evaluated in the laboratory in the absence of animal bait. Filter paper tests were carried out to assess the knock-down effect (KDt(50) and KDt(95)) and mortality (LC(50) and LC(95)) induced by each repellent. 'Irritancy' tests were carried out to compare the flight response (time to first take-off, or FT) to increasing concentrations of repellents (2-7%) and at five distances from the treated surface (0-40 mm). DEET had an insecticidal effect (KDt(50) = 9.7 min at 7%; CL(50)= 1165 mg/m(2)), whereas IR3535 and KBR 3023 did not. Relative to an untreated control, IR3535 was an irritant (relative irritancy or RI > 1) at doses of 5% and 7% (RI = 17.7 and 9.9, respectively), whereas DEET was an irritant at lower concentrations (RI = 12.3 at 2% DEET). KBR 3023 was the weakest irritant over the same range of concentrations (RI(max) = 3.6 at 6%). DEET was more of an irritant (RI(20) = 9.4) than IR3535 (RI(20) = 2.9) over a range of distances (0-20 mm), and KBR 3023 was not an irritant unless mosquitoes made contact with the treated surface. All three repellents had a significant effect on mosquitoes, but DEET exhibited a more complex mode of action than the others due to its insecticidal properties. The repellents do not behave as a single class of compounds with a common mode of action, but most probably affect different physiological systems in insects. The physiological and molecular mechanisms of repellents, especially DEET, should be investigated to ensure a better use of these molecules for skin applications and/or for treating materials against mosquitoes.

[Effect of pyrethoid and organophosphate bi-treated nets on the behaviour of Kdr+ Anopheles gambiae mosquitoes]. / Comportement d'Anopheles gambiae Kdr+ face a des moustiquaires bi-imprégnées d'insecticides pyréthri- noïde et organophosphoré.

The efficacy of mosquito nets treated with bifenthrin (pyrethroid) and chlorpyrifos-methyl (organophosphate), alone and in mixture, was evaluated against the pyrethroid-resistant strain (VKPR) of Anopheles gambiae using the tunnel test technique. Results first showed that bifenthrin and chlorpyrifos-methyl kept a good excito-repellency effect against resistant mosquitoes despite the low doses applied on nets. With the mixture, irritancy significantly increased, which consequently limited the duration of exposure of females to impregnated surface. This phenomenon most probably explained the absence of synergism between bifenthrin and chlorpyrifos-methyl concerning the insecticidal activity of this mixture. This study demonstrated the importance of the mosquito behaviour in the global effectiveness of insecticide treated materials and in the development of alternative strategies for vector control. Concerning the prospective insecticide mixtures, not only will we have to consider the insecticidal properties of each insecticide selected, but also their excito-repellency which affects the time of duration of mosquitoes on the treated surface.

[Resistance of Culex quinquefasciatus (Say, 1823) to deltamethrin and the use of impregnated mosquito nets in an urban area of Bouaké, Côte d'Ivoire]. / Résistance de Culex quinquefasciatus (Say, 1823) à la deltaméthrine et l'utilisation de la moustiquaire imprégnée en milieu urbain de Bouaké, Côte d'Ivoire.

The efficacy of pieces of nets treated with two formulations of deltamethrin (concentrated suspension at 1% and tablets at 25%) against two strains of Culex quinquefasciatus ("S-Lab", sensitive strain and "Bouaké", local strain) has been evaluated at the laboratory by experimental tunnels tests. The observed mortality of the sensitive strain has been 2-3 time more important than that of local strain. This observation seems to be linked to Culex quinquefasciatus of Bouaké pyrethrinoids-resistance. The excito-repellency effect of the two formulations has been lower for the local strain than for the sensitive strain. It has also been more important with the tablet at 25% than with concentrated suspension at 1%. The blood feeding rate has been more important for the local strain than for sensitive strain and it has been lower with the tablet at 25% than with the concentrated suspension at 1%.

Synergism between permethrin and propoxur against Culex quinquefasciatus mosquito larvae.

To see if synergism occurs between carbamate and pyrethroid insecticides, we tested permethrin and propoxur as representatives of these two classes of compounds used for mosquito control. Larvicidal activity of both insecticides was assessed separately and together on a susceptible strain of the mosquito Culex quinquefasciatus (Diptera: Culicidae) by two methods. When mixed at a constant ratio (permethrin : propoxur 1 : 60 based on LC50) and tested at serial concentrations to plot dose/mortality regression, significant synergy occurred between them (co-toxicity coefficient = 2.2), not just an additive effect. For example, when the mixture gave 50% mortality, the same concentrations of permethrin and propoxur alone would have given merely 2 x 1% mortality. When a sublethal dose (LC0) of permethrin or propoxur was added to the other (range LC10-LC95), synergism occurred up to the LC80 level. Synergistic effects were attributed to the complementary modes of action by these two insecticide classes acting on different components of nerve impulse transmission. Apart from raising new possibilities for Culex control, it seems appropriate to consider using such mixtures or combinations for insecticide-treated mosquito nets in situations with insecticide-resistant Anopheles malaria vectors.

Resistance to carbosulfan in Anopheles gambiae from Ivory Coast, based on reduced sensitivity of acetylcholinesterase.

Resistance to carbosulfan, a carbamate insecticide, was detected in field populations of the malaria vector mosquito Anopheles gambiae Giles (Diptera: Culicidae) from two ecologically contrasted localities near Bouaké, Ivory Coast: rural M'bé with predominantly M form of An. gambiae susceptible to pyrethroids; suburban Yaokoffikro with predominantly S form of An. gambiae highly resistant to pyrethroids (96% kdr). The discriminating concentration of 0.4% carbosulfan (i.e. double the LC100) was determined from bioassays with the susceptible An. gambiae Kisumu strain. Following exposure to the diagnostic dosage (0.4% carbosulfan for 1 h), mortality rates of female An. gambiae adults (reared from larvae collected from ricefields) were 62% and 29% of those from M'bé and Yaokoffikro, respectively, 24 h post-exposure. Exposure for 3 min to netting impregnated with the operational dosage of carbosulfan 200 mg/m2 gave mortality rates of 88% of those from M'bé and only 12.2% for Yaokoffikro. In each case the control untreated mortality rate was insignificant. Biochemical assays to detect possible resistance mechanism(s) revealed the presence of insensitive AChE in populations of An. gambiae at both localities, more prevalent in the S form at Yaokoffikro than in M form at M'bé, as expected from bioassays results. Our study demonstrates the need to monitor carbamate resistance among populations of the An. gambiae complex in Africa, to determine its spread and anticipate vector control failure if these insecticides are employed.

Efficacy of mosquito nets treated with a pyrethroid-organophosphorous mixture against Kdr- and Kdr+ malaria vectors (Anopheles gambiae).

In order to prevent the resistance of Anopheles gambiae s.l. to pyrethroids from spreading too quickly and to lengthen the effectiveness of insecticide impregnated mosquito nets, it has recently been suggested to use mixtures of insecticides that have different modes of action. This study presents the results obtained with tulle mosquito nets treated with bifenthrin (a pyrethroid) and chlorpyrifos-methyl (an organophosphorous) both separately and in mixture on two strains of An. gambiae, one sensitive to all insecticides, and the other resistant to pyrethroids. The values of KDt50 and KDt95 and the mortality induced with the mixture of bifenthrin (25 mg/m2) and chlorpyrifos-methyl (4.5 mg/m2) show a significant synergistic effect on the strain of An. gambiae susceptible to insecticides. However, the tested combination does not induce any synergistic effect on the VKPR strain selected with permethrin, but only enhances the effectiveness of the two insecticides taken separately.

Efficacy of mosquito nets treated with insecticide mixtures or mosaics against insecticide resistant Anopheles gambiae and Culex quinquefasciatus (Diptera: Culicidae) in Côte d'Ivoire.

Only pyrethroid insecticides have so far been recommended for the treatment of mosquito nets for malaria control. Increasing resistance of malaria vectors to pyrethroids threatens to reduce the potency of this important method of vector control. Among the strategies proposed for resistance management is to use a pyrethroid and a non-pyrethroid insecticide in combination on the same mosquito net, either separately or as a mixture. Mixtures are particularly promising if there is potentiation between the two insecticides as this would make it possible to lower the dosage of each, as has been demonstrated under laboratory conditions for a mixture of bifenthrin (pyrethroid) and carbosulfan (carbamate). The effect of these types of treatment were compared in experimental huts on wild populations of Anopheles gambiae Giles and the nuisance mosquito Culex quinquefasciatus Say, both of which are multi-resistant. Four treatments were evaluated in experimental huts over six months: the recommended dosage of 50 mg m(-2) bifenthrin, 300 mg m(-2) carbosulfan, a mosaic of 300 mg m(-2) carbosulfan on the ceiling and 50 mg m(-2) bifenthrin on the sides, and a mixture of 6.25 mg m(-2) carbosulfan and 25 mg m(-2) bifenthrin. The mixture and mosaic treatments did not differ significantly in effectiveness from carbosulfan and bifenthrin alone against anophelines in terms of deterrency, induced exophily, blood feeding inhibition and overall mortality, but were more effective than in earlier tests with deltamethrin. These results are considered encouraging, as the combination of different classes of insecticides might be a potential tool for resistance management. The mixture might have an advantage in terms of lower cost and toxicity.

Insecticide mixtures for mosquito net impregnation against malaria vectors.

Insecticides belonging to the pyrethroid family are the only compounds currently available for the treatment of mosquito nets. Unfortunately, some malaria vector species have developed resistance to pyrethroids and the lack of alternative chemical categories is a great concern. One strategy for resistance management would be to treat mosquito nets with a mixture associating two insecticides having different modes of action. This study presents the results obtained with insecticide mixtures containing several proportions of bifenthrin (a pyrethroid insecticide) and carbosulfan (a carbamate insecticide). The mixtures were sprayed on mosquito net samples and their efficacy were tested against a susceptible strain of Anopheles gambiae, the major malaria vector in Africa. A significant synergism was observed with a mixture containing 25 mg/m2 of bifenthrin (half the recommended dosage for treated nets) and 6.25 mg/m2 of carbosulfan (about 2% of the recommended dosage). The observed mortality was significantly more than expected in the absence of any interaction (80% vs 41%) and the knock-down effect was maintained, providing an effective barrier against susceptible mosquitoes.

[An experimental tool essential for the evaluation of insecticides: the testing huts]. / Un outil expérimental indispensable à l'évaluation des insecticides: les cases-pièges.

The following study analyses the potentialities of the experimental huts built in M'be Valley (Côte d'Ivoire) where the evaluations of the insecticide products have been carried out for many years in line with the WHOPES protocol on the methodology of stage 2 assays. Starting a testing station first requires a good knowledge of the sensitivity of Anopheles gambiae to the main insecticide families. Then thanks to the experimental huts the efficacy of the various means of treatment can be compared with the one in untreated huts; this study focuses on house spraying using 100 mg a.i./m2 and bednets impregnated with lambda-cyhalothrin at a dose of 15 mg a.i./m2. The fipronil used in house spraying doesn't show any repellent effect, however it does have an irritating effect that increases the natural exophily of An. gambiae females entering the testing huts. The blood-feeding rate recorded in the treated huts was reduced to 24% and to 38% mortality rate consisting mainly of a 24 hours delayed mortality. The bednets treated with lambda-cyhalothrin have greatly reduced the contact between man and vector since the entry rate of An. gambiae females was cut down by 68% compared to the control. The exophily of this anopheles was twofold greater with the impregnated bednets and the blood-feeding rate reduced to 47%. Finally the global mortality rate, two thirds of immediate mortality, one third of delayed mortality, reached 35%. The experimental huts in the M'be Valley therefore provide essential information regarding the selection of the most efficacious insecticides against An. gambiae. This experimental method must be extended to other sites in order to finalize ever more selective and appropriate means of control against nuisance and disease-vector mosquitoes.

Combined pyrethroid and carbamate 'two-in-one' treated mosquito nets: field efficacy against pyrethroid-resistant Anopheles gambiae and Culex quinquefasciatus.

A new approach is proposed in the treatment of mosquito nets, using a 'two-in-one' combination of pyrethroid and non-pyrethroid insecticides applied to different parts of bednets. The objectives are mainly to overcome certain limitations of pyrethroid-impregnated bednets currently recommended for malaria control purposes. Apart from developing alternatives to pyrethroid dependency, we sought to counteract pyrethroid irritant effects on mosquitoes (excito-repellency) and resistance to pyrethroids. The idea takes advantage of the presumed host-seeking behaviour of mosquitoes confronted by a net draped over a bed, whereby the mosquito may explore the net from the top downwards. Thus, nets could be more effective if treated on the upper part with residual non-irritant insecticide (carbamate or organophosphate) and with a pyrethroid on the lower part. Sequential exposure to different insecticides with distinct modes of action is equivalent to the use of a mixture as a potential method of managing insecticide resistance. We also intended to improve the control of nuisance mosquitoes, especially Culex quinquefasciatus Say (Diptera: Culicidae) that often survive pyrethroids, in order to encourage public compliance with use of insecticide-treated nets (ITNs). Polyester bednets were pretreated with residual pyrethroid (bifenthrin 50 mg/m2 or deltamethrin 25 mg/m2) on the lower half and with carbamate (carbosulfan 300 mg/m2) on the upper half to minimize contact with net users. Unreplicated examples of these 'two-in-one' treated nets were field-tested against wild mosquitoes, in comparison with an untreated net and bednets treated with each insecticide alone, including PermaNet wash-resistant formulation of deltamethrin 50 mg/m2. Overnight tests involved volunteers sleeping under the experimental bednets in verandah-trap huts at Yaokofikro, near Bouaké in C te d'Ivoire, where the main malaria vector Anopheles gambiae Giles, as well as Culex quinquefasciatus Say, are highly resistant to pyrethroids. Efficacy of these ITNs was assessed in the huts by four entomological criteria: deterrency and induced exophily (effects on hut entry and exit), blood-feeding and mortality rates (immediate and delayed). Overall, the best impact was achieved by the bednet treated with carbosulfan alone, followed by 'two-in-one' treatments with carbosulfan plus pyrethroid. Blood-feeding rates were 13% An. gambiae and 17% Cx. quinquefasciatus in huts with untreated nets, but only 3% with carbosulfan ITNs, 7-11% with combined ITN treatment, 6-8% An. gambiae and 12-14% Cx. quinquefasciatus with pyrethroid alone. Mosquitoes that entered the huts were killed sooner by nets with combined treatment than by pyrethroid alone. Mortality-rates in response to ITNs with carbosulfan (alone or combined with pyrethroid) were significantly greater for Cx. quinquefasciatus, but not for An. gambiae, compared to ITNs with only pyrethroid. About 20% of sleepers reported potential side-effects (headache and/or sneezing) from use of ITN treated with carbosulfan alone. Further development of this new 'two-in-one' ITN concept requires a range of investigations (choice of effective products, cost-benefit analysis, safety, etc.) leading to factory production of wash-resistant insecticidal nets treated with complementary insecticides.

Olyset Net efficacy against pyrethroid-resistant Anopheles gambiae and Culex quinquefasciatus after 3 years' field use in C te d'Ivoire.

Pyrethroid-impregnated bednets are advocated for personal protection against malaria vectors. To avoid the need for periodic re-treatment, it would be advantageous to have nets that retain insecticidal efficacy for years and withstand repeated washing. Such a type of commercially produced bednet with permethrin 2% incorporated in polyethylene fibres (trademark Olyset Net supplied by Sumika Life-Tech Co., Osaka, Japan) was evaluated against mosquitoes in veranda-trap huts at Yaokoffikro, near Bouaké, C te d'Ivoire, by standard WHOPES phase II procedures. Four Olyset Nets were compared with a standard untreated polyester net as control. They comprised three examples previously used in a village for over 3 years (one washed, one dirty, one very dirty) and a previously unused Olyset Net, newly unwrapped, from the same original batch. Bioassays with 3 min exposure of susceptible Anopheles gambiae Giles (Diptera: Culicidae) gave >99% mortality of female mosquitoes tested on the 'new' Olyset Net. The used Olyset Nets gave mortality rates averaging 83% for the washed net, 85% for the dirty net and 55% for the very dirty net (within 24-h following 3 min exposure). Thus, Olyset Nets were found to remain remarkably effective against susceptible An. gambiae for at least 3 years under field conditions. Wild pyrethroid-resistant populations of Culex quinquefasciatus Say and An. gambiae (savanna cytotype with 96% kdr) were assessed during June-August 1999 for their responses to sleepers protected by nets in the experimental huts. With regard to hut entry by foraging female mosquitoes, Olyset Nets showed some deterrency against An. gambiae (44% reduction by the new net, approximately 20% by the dirty nets, none by the washed net), but not against Cx. quinquefasciatus. Among mosquitoes entering the hut with untreated control net, 30-34% tried to leave (exophily) but were caught in the verandah trap. The permethrin repellency of Olyset Nets increased exophily by 19% for An. gambiae and 14% for Cx. quinquefasciatus. Blood-feeding rates were 16% An. gambiae and 35% Cx. quinquefasciatus in the hut with sleeper under the untreated net (showing considerable prevention of biting), 22-26% of both species in huts with washed or dirty used Olyset Nets (not significantly different from control), while the biting success rate of Cx. quinquefasciatus (but not kdr An. gambiae) was more than halved by the 'new' Olyset Net. Mortality rates of pyrethroid-resistant An. gambiae and Cx. quinquefasciatus from the huts were, respectively, 3% and 8% with the untreated polyester net, 27.5% and 17% with the 'new' Olyset, 15% and 17.5% with the washed Olyset, 16-25% and 17-20% with dirty old Olyset Nets. Kill differences between nets are significantly different for both An. gambiae and Cx. quinquefasciatus. Unfortunately the washed used Olyset Net showed least activity against resistant mosquitoes, despite its greatest activity against susceptible An. gambiae. In each case there was evidence that a high proportion of mosquitoes failed to feed through the net (many of them dying from starvation when they could not leave the closed hut), with indications that dirty Olyset nets enhanced this protective value.

[Impact of pyrethrin resistance on the efficacity of impregnated mosquito nets in the prevention of malaria: results of tests in experimental cases with deltamethrin SC]. / Impact de la résistance aux pyréthrinoïdes sur l'efficacité des moustiquaires imprégnées dans la prévention du paludisme: résultats des essais en cases expérimentales avec la deltaméthrine SC.

The effects of impregnated bednets treated with deltamethrin at a dosage of 25 mg a.i./m2 were evaluated at two testing stations in Côte d'Ivoire. The first one was located in Yaokoffikro, where Anopheles gambiae s.s. are resistant to pyrethroids (including deltamethrin), and the second in M'be, close to a large rice-growing area where An. gambiae s.s. are susceptible pyrethroids. In both situations, treating bednets with deltamethrin was very effective in limiting contact between man and vector. 72% fewer female An. gambiae entered the huts in the susceptible area, whereas a decrease of 43% was recorded in the resistant area, indicating that deltamethrin still has a certain repellent effect on resistant populations of An. gambiae s.s. Overall mortality induced by bednets treated with insecticide was significantly higher in the resistant area (56.4% as versus 44.3%). An explanation for this apparent paradox is that the mosquitoes being less repelled by the insecticide remain on the treated material for longer periods of time and most of them eventually die. The results of this study indicate that bednets treated with deltamethrin are an effective prophylactic measure even in areas where An. gambiae s.s. are resistant to pyrethroids and should still be considered as a practical means of personal protection against malaria even in pyrethroid resistance areas with high frequency of kdr resistance genes.

Modifications of pyrethroid effects associated with kdr mutation in Anopheles gambiae.

Effects of knockdown resistance (kdr) were investigated in three pyrethroid-resistant (RR) strains of the Afrotropical mosquito Anopheles gambiae Giles (Diptera: Culicidae): Kou from Burkina Faso, Tola and Yao from Côte d'Ivoire; compared with a standard susceptible (SS) strain from Kisumu, Kenya. The kdr factor was incompletely recessive, conferring 43-fold resistance ratio at LD50 level and 29-fold at LD95 level, as determined by topical application tests with Kou strain. When adult mosquitoes were exposed to 0.25% permethrin-impregnated papers, the 50% and 95% knockdown times (KdT) were 23 and 42 min for SS females, compared with 40 and 62 min for RS (F1 Kou x Kisumu) females. On 1% permethrin the KdT50 and KdT95 were 11 and 21 min for SS compared with 18 and 33 min for RS females. Following 1 h exposure to permethrin (0.25% or 1%), no significant knockdown of Kou RR females occurred within 24 h. Permethrin irritancy to An. gambiae was assessed by comparing 'time to first take-off' (TO) for females. The standard TO50 and TO95 values for Kisumu SS on untreated paper were 58 and 1044 s, respectively, vs. 3.7 and 16.5 s on 1% permethrin. For Kou RR females the comparable values were 27.3 s for TO50 and 294 s for TO95, with intermediate RS values of 10.1 s for TO50 and 71.9 s for TO95. Thus, TO values for RS were 2.7-4.4 times more than for SS, and those for RR were 7-18 times longer than for SS. Experiments with pyrethroid-impregnated nets were designed to induce hungry female mosquitoes to pass through holes cut in the netting. Laboratory 'tunnel tests' used a bait guinea-pig to attract mosquitoes through circular holes (5 x 1 cm) in a net screen. With untreated netting, 75-83% of laboratory-reared females passed through the holes overnight, 63-69% blood-fed successfully and 9-17% died, with no significant differences between SS and RR genotypes. When the netting was treated with permethrin 250mg ai/m2 the proportions that passed through the holes overnight were only 10% of SS vs. 40-46% of RR (Tola & Kou); mortality rates were 100% of SS compared with 59-82% of RR; bloodmeals were obtained by 9% of Kou RR and 17% of Tola RR, but none of the Kisumu SS females. When the net was treated with deltamethrin 25 mg ai/m2 the proportions of An. gambiae that went through the holes and blood-fed successfully were 3.9% of Kisumu SS and 3.5% of Yaokoffikro field population (94% R). Mortality rates were 97% of Kisumu SS vs. 47% of Yaokoffikro R. Evidently this deltamethrin treatment was sufficient to kill nearly all SS and half of the Yaokoffikro R An. gambiae population despite its high kdr frequency. Experimental huts at Yaokoffikro were used for overnight evaluation of bednets against An. gambiae females. The huts were sealed to prevent egress of mosquitoes released at 20.00 hours and collected at 05.00 hours. Each net was perforated with 225 square holes (2 x 2 cm). A man slept under the net as bait. With untreated nets, only 4-6% of mosquitoes died overnight and bloodmeals were taken by 17% of SS vs. 29% of Yaokoffikro R (P<0.05). Nets treated with permethrin 500 mg/m2 caused mortality rates of 95% Kisumu SS and 45% Yao R (P<0.001) and blood-feeding rates were reduced to 1.3% of SS vs. 8.1% of Yao R (P<0.05). Nets treated with deltamethrin 25 mg/m2 caused mortality rates of 91% Kisumu SS and 54% Yao R (P<0.001) and reduced blood-feeding rates to zero for SS vs. 2.5% for Yao R (P>0.05). (ABSTRACT TRUNCATED)