The online version of this article (doi:10.1186/1475-2875-11-24) contains supplementary material, which is available to authorized users.
This work was partially funded by BayerCropScience and they provided the bendiocarb for the IRS. BayerCropScience played no role in the experimental design or interpretation of the results.
HR and SN conceived the study. All authors participated in the study design. CJ and AS oversaw field collections, bioassays and conducted laboratory work. CJ wrote a draft of the manuscript. CJ and PCDJ performed the statistical analysis. All authors read and approved the final manuscript.
The operational impact of insecticide resistance on the effectiveness of long-lasting insecticide nets (LLINs) and indoor residual spraying (IRS) is poorly understood. One factor which may prolong the effectiveness of these tools in the field is the increase in insecticide susceptibility with mosquito age. In this study, LLINs and IRS were tested against young (three to five days) and old (17-19 days) pyrethroid resistant Anopheles gambiae s.l. from Burkina Faso.
Blood-fed adult Anopheles gambiae s.l. were collected from south-west Burkina Faso and identified to species/form level. Cohorts of the F1 progeny of An. gambiae s.s. S-forms were exposed to deltamethrin (0.05%) at three to five or 17-19 days post-emergence and tested for the frequency of the resistance allele 1014F. Isofemale lines of the M, S- form of An. gambiae s.s. and Anopheles arabiensis were exposed in WHO cone tests to either a) LLINs deployed in households for two years or (b) bendiocarb sprayed walls.
Mortality rates in response to deltamethrin (0.05%) increased from levels indicative of strong resistance in three to five day old F1 mosquitoes, to near full susceptibility in the 17-19 day old cohort. On exposure to LLINs sampled from the field, the mortality rate in isofemale lines was higher in older mosquitoes than young (OR = 5.28, CI 95% = 2.81-9.92), although the mortality estimates were affected by the LLIN tested. In general, the LLINs sampled from the field performed poorly in WHO cone bioassays using either laboratory susceptible or field caught mosquito populations. Finally, there was a clear relationship between mortality and age on exposure to bendiocarb-sprayed walls, with older mosquitoes again proving more susceptible (OR = 3.39, CI 95% = 2.35-4.90).
Age is a key factor determining the susceptibility of mosquitoes to insecticides, not only in laboratory studies, but in response to field-based vector control interventions. This has important implications for understanding the epidemiological impact of resistance. If mosquitoes old enough to transmit malaria are still being suppressed with available insecticides, is resistance potentially having less of an impact than often assumed? However, the poor performance of LLINs used in this study in Burkina Faso, is a cause for concern and requires urgent investigation.
Additional file 1: Details the rationale and model parameters for the LLIN and IRS experiments and includes the following tables and figures. Table A1 The Akaike's Information Criteria (AIC) for model selection for the An. gambiae s.s. isofemale LLIN and IRS experiments. Table A2 The Akaike's Information Criteria (AIC) for model selection for the An. gambiae s.s. isofemale LLIN experiment excluding data for An. arabiensis. Table A3 Estimates of parameters, standard errors, 95% confidence intervals, z and p values for mortality in response to LLINs. Table A4 Estimates of parameters, standard errors, 95% confidence intervals, z and p values for mortality in response to bendiocarb-sprayed walls. Table A5 Individual estimates of parameters from re-fitted model with Age:Treatment interaction term for each treatment reference category. Figure A1 Mortality of isofemale lines in response to bendiocarb treated walls pooled by age. (DOC 119 KB)12936_2011_2029_MOESM1_ESM.DOC
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- Aging partially restores the efficacy of malaria vector control in insecticide-resistant populations of Anopheles gambiae s.l. from Burkina Faso
Christopher M Jones
Wamdaogo M Guelbeogo
Paul CD Johnson
- BioMed Central
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