Biomedical subjects

Vincent Corbel

Publications and source records attributed to Vincent Corbel.

11 recordsLinked to original sources

PubMed2006

Synergism between insecticides permethrin and propoxur occurs through activation of presynaptic muscarinic negative feedback of acetylcholine release in the insect central nervous system.

Although synergism between pesticides has been widely documented, the physiological mechanisms by which an insecticide synergizes another remains unclear. Toxicological and electrophysiological studies were carried out on two susceptible pest species (the mosquito Culex quinquefasciatus and the cockroach Periplaneta americana) to understand better the physiological process involved in pyrethroid and carbamate interactions. Larval bioassays were conducted with the susceptible reference strain SLAB of C. quinquefasciatus to assess the implication of multi-function oxidases and non-specific esterases in insecticide detoxification and synergism. Results showed that the general theory of synergism (competition between pesticides for a common detoxification enzyme) was unlikely to occur in the SLAB strain since the level of synergy recorded between permethrin and propoxur was unchanged in the presence of piperonyl butoxide and tribufos, two inhibitors of oxidases and esterases, respectively (synergism ratios were similar with and without synergists). We also showed that addition of a sub-lethal concentration of nicotine significantly increased the toxicity of permethrin and propoxur at the lower range of the dose-mortality regression lines, suggesting the manifestation of important physiological disruptions at synaptic level. The effects of both permethrin and propoxur were studied on the cercal-afferent giant-interneuron synapses in the terminal abdominal ganglion of the cockroach P. americana using the single-fibre oil-gap method. We demonstrated that permethrin and propoxur increased drastically the ACh concentration within the synaptic cleft, which thereby stimulated a negative feedback of ACh release. Atropine, a muscarinic receptor antagonist, reversed the effect of permethrin and propoxur mixtures. This demonstrates the implication of the presynaptic muscarinic receptors in the negative feedback regulation process and in synergism. Based on these findings, we propose a cascade of molecular events explaining the occurrence of synergistic effects between pyrethroid and carbamate on many susceptible insects including C. quinquefasciatus, a mosquito of medical importance.

Acetylcholine↗

PubMed2006

Laboratory evaluation of pyriproxyfen and spinosad, alone and in combination, against Aedes aegypti larvae.

In this study, the efficacy of pyriproxyfen and spinosad, alone and in combination, was evaluated against the dengue vector Aedes aegypti (L.). Larval bioassays were carried out on susceptible mosquito larvae to determine the concentration-mortality responses of mosquitoes exposed to each insecticide alone and in mixture. Synergism between pyriproxyfen and spinosad was determined by the calculation of a combination index (CI) by using the isobologram method. For pyriproxyfen, LC50 and LC95 were 1.1 x 10(-4) (1.0 x 10(-4)-1.1 x 10(-4)) and 3.2 x 10(-4) (2.9 x 10(-4)-3.6 x 10(-4)) mg/liter, respectively. Pyriproxyfen acted at very low concentrations by inhibiting the adult emergence of Ae. aegypti (97% inhibition rates at 3.3 x 10(-4) mg/liter). Spinosad activity was -500 times lower than that of pyriproxyfen against the Bora strain, with LC50 and LC95 values estimated at 0.055 (0.047-0.064) and 0.20 (0.15-0.27) mg/liter, respectively. A binary mixture of pyriproxyfen and spinosad was realized at the ratio 1:500 by considering the values of the LC50 obtained for each product. The LC50 and LC95 of the mixture were 0.019 (0.016 - 0.022) and 0.050 (0.040 - 0.065) mg/liter, respectively. The mixture combined both the larvicidal activity of spinosad and the juvenoid action of pyriproxyfen. From the LC70 to LC99 a significant synergism effect was observed between the two insecticides (CI ranged from 0.74 to 0.31). This strong synergism observed at high concentrations allows a reduction by five and nine-fold of pyriproxyfen and spinosad amounts to kill almost 100% mosquitoes. Combination of pyriproxyfen and spinosad may then represent a promising strategy to improve mosquito control in situations with insecticide-resistant Aedes dengue vectors.

Aedes↗

PubMed2005

Experimental hut evaluation of bednets treated with an organophosphate (chlorpyrifos-methyl) or a pyrethroid (lambdacyhalothrin) alone and in combination against insecticide-resistant Anopheles gambiae and Culex quinquefasciatus mosquitoes.

BACKGROUND: Pyrethroid resistant mosquitoes are becoming increasingly common in parts of Africa. It is important to identify alternative insecticides which, if necessary, could be used to replace or supplement the pyrethroids for use on treated nets. Certain compounds of an earlier generation of insecticides, the organophosphates may have potential as net treatments. METHODS: Comparative studies of chlorpyrifos-methyl (CM), an organophosphate with low mammalian toxicity, and lambdacyhalothrin (L), a pyrethroid, were conducted in experimental huts in Côte d'Ivoire, West Africa. Anopheles gambiae and Culex quinquefasciatus mosquitoes from the area are resistant to pyrethroids and organophosphates (kdr and insensitive acetylcholinesterase Ace.1R). Several treatments and application rates on intact or holed nets were evaluated, including single treatments, mixtures, and differential wall/ceiling treatments. RESULTS AND CONCLUSION: All of the treatments were effective in reducing blood feeding from sleepers under the nets and in killing both species of mosquito, despite the presence of the kdr and Ace.1R genes at high frequency. In most cases, the effects of the various treatments did not differ significantly. Five washes of the nets in soap solution did not reduce the impact of the insecticides on A. gambiae mortality, but did lead to an increase in blood feeding. The three combinations performed no differently from the single insecticide treatments, but the low dose mixture performed encouragingly well indicating that such combinations might be used for controlling insecticide resistant mosquitoes. Mortality of mosquitoes that carried both Ace.1R and Ace.1S genes did not differ significantly from mosquitoes that carried only Ace.1S genes on any of the treated nets, indicating that the Ace.1R allele does not confer effective resistance to chlorpyrifos-methyl under the realistic conditions of an experimental hut.

Acetylcholinesterase↗

PubMed2005

[Malaria: what about exposure prophylaxis?].

Malaria would not exist without mosquito bites. The growing resistance to antimalarial drugs has led to an increasing role of personal protection measures. However, insect repellents are not evaluated as medicinal products, and many products available on the market contain active substances that have been poorly evaluated or present in insufficient doses. The efficacy of the most active insect repellents, DEET and icaridin/KBR 3023, like that of medicinal products, is limited by the subject's compliance, bearing in mind that the Anopheles bite is painless and that humans are primarily bothered by the nuisance factor of the mosquito.

Animals↗

PubMed2005

Assessment of laboratory and field assays of sunlight-induced killing of mosquito larvae by photosensitizers.

The effectiveness of light-induced killing of mosquito larvae in the presence of photosensitizers was studied with larvae of Aedes aegypti (L.), Anopheles stephensi (Liston), and Culex quinquefasciatus Say grown in the laboratory and of Cx. quinquefasciatus grown under field conditions. Tested photosensitizers included xanthene, chlorin, and porphyrin derivatives. All the larvae were treated at the fourth instar. Preliminary laboratory experiments showed a light-induced lethal effect of Rose Bengal (RB) on three species of mosquito larvae. Compared with other photosensitizers, RB seemed to be more efficient at even lower concentration than chlorin (e6) and chlorophyllin on Ae. aegypti larvae. Among the four porphyrin derivatives, i.e., chloroquinoline tetraphenyl propioamidoporphine, tetraphenyl porphine tetrasulfonate, hematoporphyrin (HP), and tetraphenylporphinepropionic acid porphine, HP was the only effective photosensitizer on Ae. aegypti larvae. The best conditions for field tests using RB were conducted on Cx. quinquefasciatus in Bobo-Dioulasso, Burkina Faso. The mortality induced by RB varied from 80 to 96% obtained with unfiltered cesspit water to 0.4 to 6.7% in cesspits with a heavy load of organic materials, thus providing the basis for further developments of this technique under field conditions.

Aedes↗

PubMed2005

Combination of a non-pyrethroid insecticide and a repellent: a new approach for controlling knockdown-resistant mosquitoes.

Although pyrethroid-treated materials are a promising tool for the prevention and the control of dengue in the tropics, the development of pyrethroid resistance in the main mosquito vector (Aedes aegypti) may negate their use for personal and/or community protection. In that context, the efficacy of a mixture of a repellent (N,N-diethyl toluamide [DEET]) and a non-pyrethroid insecticide (propoxur) was investigated under laboratory conditions against both pyrethroid-susceptible and pyrethroid-resistant mosquitoes with the knockdown resistance (kdr) mutation. The results showed that a combination of propoxur and DEET induced a knockdown effect and mortality as high as deltamethrin (a standard pyrethroid) against the susceptible strain, and significantly higher efficacy against the pyrethroid-resistant strain. This could be explained mainly by the existence of a strong synergistic interaction between DEET and propoxur in mosquitoes. This study constitutes a first step towards an alternative strategy for improving mosquito control in areas with pyrethroid resistance.

Animals↗

PubMed2004

Dosage-dependent effects of permethrin-treated nets on the behaviour of Anopheles gambiae and the selection of pyrethroid resistance.

BACKGROUND: The evolution and spread of pyrethroid resistance in Anopheles gambiae s.s, the major malaria vector in sub-Saharan Africa, is of great concern owing to the importance of pyrethroid-treated nets in the WHO global strategy for malaria control. The impact of kdr (the main pyrethroid-resistance mechanism) on the behaviour of An. gambiae is not well understood. The objective of this study was to determine whether high or low doses of permethrin differ in their resistance-selection effects. METHODS: The effect of permethrin treatment was assessed under laboratory conditions using the tunnel test technique against susceptible, heterozygous and homozygous genotypes. Experimental huts trials were then carried out in Benin to assess the level of personal protection conferred by nets treated with a variety of permethrin concentrations and their impact on the selection for kdr allele. RESULTS: Tunnel tests showed that nets treated with permethrin at 250 and 500 mg/m2 induced higher mortality and blood feeding reduction among susceptible and heterozygous (RS) females as compared to the lower concentration (100 mg/m2). The experimental hut trials showed that the best personal protection was achieved with the highest permethrin concentration (1,000 mg/m2). Mosquito genotyping revealed a non-linear relationship in the survival of kdr susceptible and resistant genotypes with permethrin dosage. Higher dosages (> or =250 mg/m2) killed more efficiently the RS genotypes than did lower dosages (50 and 100 mg/m2). CONCLUSION: This study showed that nets treated with high permethrin concentrations provided better blood feeding prevention against pyrethroid-resistant An. gambiae than did lower concentrations. Permethrin-treated nets seem unlikely to select for pyrethroid resistance in areas where the kdr mutation is rare and present mainly in heterozygous form.

Animals↗

PubMed2004

Efficacy of insecticide mixtures against larvae of Culex quinquefasciatus (Say) (Diptera: Culicidae) resistant to pyrethroids and carbamates.

The efficacy of insecticide mixtures of permethrin (pyrethroid) and propoxur (carbamate) was tested by larval bioassays on two strains of Culex quinquefasciatus (Say), one resistant to pyrethroids and the other resistant to carbamates. The method consisted in combining one insecticide at the highest concentration causing no mortality (LC0) with increasing concentrations of the second one. The concentration-mortality regression lines were determined for permethrin and propoxur alone and in combination, and synergism ratios (SR) were calculated in order to determine the magnitude of an increase or decrease in efficacy with use of the mixtures. With the pyrethroid-resistant strain (BK-PER), the results showed that propoxur at LC0 significantly enhanced the insecticidal activity of permethrin (SR50 = 1.54), especially on the upper range of the concentration-mortality regression. Conversely, when permethrin at LC0 was tested with propoxur against the carbamate resistant strain (R-LAB), an antagonistic effect was observed (SR50 = 0.67). With the BK-PER strain, an increased oxidative detoxification (MFO) appeared to be the main mechanism responsible for the synergistic interaction. Nevertheless, antagonism in the R-LAB strain is probably due to a physiological perturbation implying different target sites for pyrethroid (ie sodium channel) and carbamate insecticides [ie acetylcholinesterase (EC 3.3.3.7) and choline acetyltransferase (EC 2.3.1.6)].

Animals↗

PubMed2004

Dinotefuran: a potential neonicotinoid insecticide against resistant mosquitoes.

Because pyrethroid, organophosphate, and carbamate resistance is more and more developed in mosquitoes of medical importance, there is an urgent need for alternative insecticides for vector control. Dinotefuran, a new neonicotinoid insecticide commercialized by Mitsui Chemicals (Tokyo, Japan), could be a useful candidate in public health because it shows low mammalian toxicity and great insecticidal activity against a broad range of pests. In this study, the intrinsic toxicity of dinotefuran was evaluated by larval bioassay and topical application against different mosquito strains of Anopheles gambiae Giles, Culex quinquefasciatus Say, and Aedes aegypti L. having none, one, or several resistance mechanisms, respectively, to insecticides. The results showed that dinotefuran was less toxic than most of the commonly used insecticides (e.g., deltamethrin, carbosulfan, and temephos) against the susceptible mosquitoes tested (between 6- and 100-fold at the LD50 level). However, the toxicity of dinotefuran was not strongly affected by the presence of common resistance mechanism, i.e., kdr mutation and insensitive acetylcholinesterase (resistance ratio [RR] from 1.3 to 2.3). More interestingly, the carbamate-resistant strain of Cx. quinquefasciatus was significantly more affected by dinotefuran than the susceptible strain (RR = 0.70), probably because the insensitive acetylcholinesterase is less efficient to degrade nicotinic substrates than normal acetylcholinesterase. Despite the relatively low toxicity of dinotefuran against susceptible mosquitoes, the absence of cross-resistance with common insecticides (pyrethroids, carbamates, and organophosphates) makes neonicotinoids potential candidates for disease vector control, especially in area where mosquitoes are resistant to insecticides.

Aedes↗

PubMed2004

Topical applications of pyrethroid and organophosphate mixtures revealed positive interactions against pyrethroid-resistant Anopheles gambiae.

The efficacy of a binary mixture of bifenthrin (pyrethroid) and chlorpyrifos-methyl (organophosphate) has been tested against susceptible and pyrethroid-resistant strains of Anopheles gambiae Giles, the major malaria vector in sub-Saharan Africa. Dose-mortality regression lines were determined for each insecticide alone and in mixtures by topical applications on adult female mosquitoes. A combination index (CI) was used to quantify the interactions occurring between the pyrethroid and organophosphate insecticides. The results revealed synergism at high doses against both susceptible (0.7 > CI > 0.3) and pyrethroid-resistant (0.9 > CI > 0.7) An. gambiae. These results suggest that insecticide mixtures may be an alternative strategy for vector control, especially in areas where mosquitoes are resistant to pyrethroids.

Animals↗

PubMed2003

Evidence for selection of insecticide resistance due to insensitive acetylcholinesterase by carbamate-treated nets in Anopheles gambiae s.s. (Diptera: Culicidae) from Côte d'Ivoire.

Pyrethroid-treated nets are an efficient tool for reducing malaria transmission and morbidity. The recent evolution of pyrethroid resistance in several Anopheles species represents a major threat for the future success of roll back malaria in Africa. The possible use of nonpyrethroid insecticides, such as carbamates, on nets is a promising alternative solution because these insecticides are effective against susceptible and pyrethroid-resistant populations of Anopheles and Culex mosquitoes. Unfortunately, carbamate resistance as a result of insensitive acetylcholinesterase has recently been detected in Anopheles gambiae s.s. populations from Côte d'Ivoire. Using biochemical assays on surviving Anopheles mosquitoes from an experimental hut trial, we showed evidence for selection for an insensitive acetylcholinesterase mechanism by carbamate impregnated bednets. However, no such selection has been found with nets treated with pyrethroid alone or pyrethroid/carbamate "two-in-one" -treated nets. Because pyrethroid-impregnated nets were suspected to select for the Kdr mutation in An. gambiae, we propose that use of two-in-one nets could be a promising alternative strategy for the management of insecticide resistance in malaria vectors.

Acetylcholinesterase↗