Insecticide resistance in the West African malaria vector Anopheles gambiae and investigation of alternative tools for its delay

There is a current policy to eliminate malaria in the African continent. Pyrethroid-incorporated Long Lasting Insecticidal Nets (LLINs) and/or Indoor Residual Spraying (IRS) are the chemical weapons being deployed to achieve that goal. Rather worryingly, resistance to pyrethroids is well documented in the major vectors of malaria in Africa, and could decimate the contribution that vector control can make to any successful elimination agenda over the next decade. DDT (Dichlorodiphenyltrichloroethane) for IRS is cost effective but undesirable because of its environmental impact. There is a need to identify pyrethroid resistance mechanisms in the areas being scaled up, evaluate their direct impact on the efficacy of these tools and identify novel tools that might have potential as alternatives to pyrethroids and DDT for net or indoor residual treatments. This thesis reports that pyrethroid, organophosphate (Ops) & carbamate resistance is present in the Mopti (M) and Savanah (S) molecular forms of the major vector of malaria, Anopheles gambiae, in two West African countries, Benin and Ivory Coast where LLINs are currently being deployed,. Mechanisms for pyrethroids include elevated oxidase activities and the knock down resistance (kdr) gene at high frequency (>80%) whereas an insensitive acetylcholinesterase conferred Ops and carbamate resistance. Experimental hut tests in Southern Benin showed that the efficacy of Insecticide Treated Net (ITN) and IRS with the pyrethroid lambdacyalothrin was seriously compromised by pyrethroid resistance in the M form of An. gambiae, as opposed to the North where there is no record of pyrethroid resistance. This type of pyrethroid resistance, now spreading through West African populations of the M molecular form of An. gambiae, appears to have major operational significance in other areas such as Bioko, Niger and Burkina Faso. The alternative candidate insecticides, indoxacarb (an oxadiazine), chlorfenapy (a pyrrole) and chlorpyrifos methyl (an organophosphate) were evaluated, in the laboratory and/or in the field. In the laboratory, indoxacarb and chlorfenapyr on netting were more toxic than permethrin over the same dosage range (100-500mg/m2). Toxic activity was rather slow and bloodfeeding of mosquitoes was uninhibited in the presence of either insecticide. The experimental hut tests conducted in Southern Benin indicated that chlorfenapyr has high potential for IRS, killing 82.9% of pyrethroid resistant An. gambiae and 69% of Culex quinquefasciatus. Likewise, IRS with chlorpyrifos methyl CS (Capsule Suspension) was very efficacious at the same site in Benin. It killed 95.5% of pyrethroid resistant An. gambiae that entered a hut and showed activity on walls that lasted for more than 9 months without significant decay. If applied at high coverage, chlorpyrifos methyl CS should show higher, more-sustained levels of malaria transmission control than that achievable with DDT or pyrethroids. The feasibility of applying synthetic insect repellents on bednets (RTNs) to control insecticide resistant mosquitoes was explored in the second country, Ivory Coast. The results of tests done in experimental huts showed that formulations of volatile DEET (N,N-diethyl-3-methylbenzamide) and ethyl butylacetylaminopropionate (IR3535) on nets reduced bloodfeeding and the entry rate of mosquitoes into huts. An unexpected result was the 69-76% mortality of An. gambiae and 51-61% mortality of Cx. quinquefasciatus in huts containing RTNs. The DEET-based product provided better efficacy but was short-lived. Further treatment of netting with a formulation of DEET in which the repellent is gradually released from a capsule that binds the repellent strongly, showed that the formulation repels, inhibits blood-feeding and kills mosquitoes for a period of at least 6 months under laboratory conditions. Application of repellents to nets warrants further investigation as well as their development as alternatives to pyrethroids. Because it will not be possible to go for malaria elimination with the current tools, based on pyrethroids only, the results obtained with chlorfenapyr, indoxacarb and chlorpyrifos methyl should encourage further studies aiming at supplementing pyrethroids for vector control in areas where malaria mosquitoes are resistant to pyrethroids. <br/

Experimental hut comparisons of nets treated with carbamate or pyrethroid insecticides, washed or unwashed, against pyrethroid-resistant mosquitoes.

The efficacy against mosquitoes (Diptera: Culicidae) of a bednet treated with carbamate insecticide [carbosulfan capsule suspension (CS) 200 mg/m(2)] was compared with four types of pyrethroid-treated nets in veranda-trap huts at Yaokoffikro near Bouaké, Côte d'Ivoire, where the malaria vector Anopheles gambiae Giles carries the kdr gene (conferring pyrethroid resistance) at high frequency and Culex quinquefasciatus Say is also pyrethroid resistant. Pyrethroids compared were lambdacyhalothrin CS 18 mg/m(2), alphacypermethrin water dispersible granules (WG) 20 mg/m(2), deltamethrin 50 mg/m(2) (Permanet) and permethrin emulsifiable concentrate (EC) 500 mg/m(2). Insecticidal power and personal protection from mosquito bites were assessed before and after the nets were used for 8 months and hand washed five times in cold soapy water. Before washing, all treatments except permethrin significantly reduced blood-feeding and all had significant insecticidal activity against An. gambiae. The carbosulfan net gave significantly higher killing of An. gambiae than all pyrethroid treatments except the Permanet. Against Culex spp., carbosulfan was more insecticidal and gave a significantly better protective effect than any of the pyrethroid treatments. After washing, treated nets retained various degrees of efficacy against both mosquito genera - but least for the carbosulfan net. Washed nets with three types of pyrethroid treatment (alphacypermethrin, lambdacyhalothrin, permethrin) gave significantly higher mortality rates of Culex than in huts with the same pyrethroid-treated nets before washing. After five washes, the Permanet, which is sold as a long-lasting insecticidal product, performed no better than the other nets in our experimental conditions

Combining indoor residual spraying with chlorfenapyr and long-lasting insecticidal bed nets for improved control of pyrethroid-resistant Anopheles gambiae: an experimental hut trial in Benin.

BACKGROUND: Neither indoor residual spraying (IRS) nor long-lasting insecticidal nets (LLINs) are able to fully interrupt transmission in holoendemic Africa as single interventions. The combining of IRS and LLINs presents an opportunity for improved control and management of pyrethroid resistance through the simultaneous presentation of unrelated insecticides. METHOD: Chlorfenapyr IRS and a pyrethroid-impregnated polyester LLIN (WHO approved) were tested separately and together in experimental huts in southern Benin against pyrethroid resistant Anopheles gambiae and Culex quinquefasciatus. The bed nets were deliberately holed with either six or 80 holes to examine the effect of increasing wear and tear on protectiveness. Anopheles gambiae were genotyped for the kdr gene to assess the combination's potential to prevent the selection of pyrethroid resistance. RESULTS: The frequency of kdr was 84%. The overall mortality rates of An. gambiae were 37% and 49% with the six-hole and 80-hole LLINs, respectively, and reached 57% with chlorfenapyr IRS. Overall mortality rates were significantly higher with the combination treatments (82-83%) than with the LLIN or IRS individual treatments. Blood feeding (mosquito biting) rates were lowest with the 6-hole LLIN (12%), intermediate with the 80-hole LLIN (32%) and highest with untreated nets (56% with the 6-hole and 54% with the 80-hole nets). Blood feeding (biting) rates and repellency of mosquitoes with the combination of LLIN and chlorfenapyr IRS showed significant improvement compared to the IRS treatment but did not differ from the LLIN treatments indicating that the LLINs were the primary agents of personal protection. The combination killed significantly higher proportions of Cx. quinquefasciatus (51%, 41%) than the LLIN (15%, 13%) or IRS (32%) treatments. CONCLUSION: The chlorfenapyr IRS component was largely responsible for controlling pyrethroid-resistant mosquitoes and the LLIN component was largely responsible for blood feeding inhibition and personal protection. Together, the combination shows potential to provide additional levels of transmission control and personal protection against pyrethroid-resistant mosquitoes, thereby justifying the additional resources required. Chlorfenapyr has potential to manage pyrethroid resistance in the context of an expanding LLIN/IRS strategy

Measurement of overall insecticidal effects in experimental hut trials

BACKGROUND: The 'overall insecticidal effect' is a key measure used to evaluate public health pesticides for indoor use in experimental hut trials. It depends on the proportion of mosquitoes that are killed out of those that enter the treated hut, intrinsic mortality in the control hut, and the ratio of mosquitoes entering the treatment hut to those entering the control hut. This paper critically examines the way the effect is defined, and discusses how it can be used to infer effectiveness of intervention programmes. FINDINGS: The overall insecticidal effect, as defined by the World Health Organization in 2006, can be negative when deterrence from entering the treated hut is high, even if all mosquitoes that enter are killed, wrongly suggesting that the insecticide enhances mosquito survival. Also in the absence of deterrence, even if the insecticide kills all mosquitoes in the treatment hut, the insecticidal effect is less than 100%, unless intrinsic mortality is nil. A proposed alternative definition for the measurement of the overall insecticidal effect has the desirable range of 0 to 1 (100%), provided mortality among non-repelled mosquitoes in the treated hut is less than the corresponding mortality in the control hut. This definition can be built upon to formulate the coverage-dependent insecticidal effectiveness of an intervention programme. Coverage-dependent population protection against feeding can be formulated similarly. CONCLUSIONS: This paper shows that the 2006 recommended quantity for measuring the overall insecticidal effect is problematic, and proposes an alternative quantity with more desirable propertie

Antioxidant and anti-hyperglycaemic activity of Euphorbia hirta L. on Wistar rats

The objective of this study was to investigate the antioxidant capacity and anti-hyperglycemic activity of aqueous macerate extracts of Euphorbia hirta L., in the treatment of hypertension associated with diabetes. The research focused on the aqueous maceration of the aerial part or totum. The search and evaluation of the antioxidant activity was done according to the method of Blois based on the reduction of 2,2-diphenyl- - picrylhydrazyl (DPPH). The search for antihyperglycaemic activity was performed on rats having received a glucose overdose. This activity was compared to those of glyburide and metformin. The totum showed a percentage inhibition of 63.66 ± 0.54% for 1 mg/mL of extract and an IC50 of 0.031 ± 0.001 μg/mL .A 200 mg/kg extract has shown a very good antihyperglycaemic activity; while on normal blood sugar, the activity was not important compared with glyburide and metformin. These results show the presence of antioxidant and normoglycaemic substances in aqueous macerated of Euphorbia hirta L.Keywords: Anti-hyperglycaemic, antioxidant, Euphorbia hirta, glyburide, metformi

Loss of protection with insecticide-treated nets against pyrethroid-resistant Culex quinquefasciatus mosquitoes once nets become holed: an experimental hut study

ABSTRACT: BACKGROUND: An important advantage of pyrethroid-treated nets over untreated nets is that once nets become worn or holed a pyrethroid treatment will normally restore protection. The capacity of pyrethroids to kill or irritate any mosquito that comes into contact with the net and prevent penetration of holes or feeding through the sides are the main reasons why treated nets continue to provide protection despite their condition deteriorating over time. Pyrethroid resistance is a growing problem among Anopheline and Culicine mosquitoes in many parts of Africa. When mosquitoes become resistant the capacity of treated nets to provide protection might be diminished, particularly when holed. An experimental hut trial against pyrethroid-resistant Culex quinquefasciatus was therefore undertaken in southern Benin using a series of intact and holed nets, both untreated and treated, to assess any loss of protection as nets deteriorate with use and time. RESULTS: There was loss of protection when untreated nets became holed; the proportion of mosquitoes blood feeding increased from 36.2% when nets were intact to between 59.7% and 68.5% when nets were holed to differing extents. The proportion of mosquitoes blood feeding when treated nets were intact was 29.4% which increased to 43.6-57.4% when nets were holed. The greater the number of holes the greater the loss of protection regardless of whether nets were untreated or treated. Mosquito mortality in huts with untreated nets was 12.9-13.6%; treatment induced mortality was less than 12%. The exiting rate of mosquitoes into the verandas was higher in huts with intact nets. CONCLUSION: As nets deteriorate with use and become increasingly holed the capacity of pyrethroid treatments to restore protection is greatly diminished against resistant Culex quinquefasciatus mosquitoes

La haie vive, une technique durable et peu onéreuse pour protéger les cultures et terroirs ruraux

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