Pyrethroid Resistance Reduces the Efficacy of Space Sprays for Dengue Control on the Island of Martinique (Caribbean)

The mosquito Aedes aegypti is the major vector of the Dengue virus in human populations and is responsible of serious outbreaks worldwide. In most countries, vector control is implemented by the use of insecticides to reduce mosquito populations. During epidemics, insecticides of the pyrethroid family (blocking the voltage gated sodium channel protein in the nerve sheath) are used by space spraying with vehicle mounted thermal foggers to kill adult mosquitoes. Unfortunately some populations of Ae. aegypti have become resistant to these insecticides, leading to operational challenges for public health services. In Martinique (French West Indies), resistance to pyrethroids was detected in the 1990s. The present study assessed the impact of this resistance on the efficacy of vector control operations in 9 localities of Martinique. Here we showed that the resistance strongly reduces the efficacy of pyrethroid-based treatments, thus emphasizing the urgent need for alternative insecticides or tools to reduce dengue transmission

Evolution of resistance under insecticide selection pressure in Culex pipiens quinquefasciatus (Diptera, Culicidae) from Martinique

International audiencePopulation surveys of Culex pipiens quinquefasciatus carried out in 1991 and 1999 were compared with data collected in 1990 before the beginning of the control program against this mosquito. Larval samples collected in 1999 displayed resistance to the four tested insecticides: permethrin, propoxur, temephos, and chlorpyrifos. Temepbos resistance ratio at LC50 (RR50) ranged between 8.1- and 42-fold compared with 2.9- and 4.6-fold in 1990, and chlorpyrifos RR50 ranged between 8.6- and 123-fold compared with 6.4- and 19-fold in 1990. This increased resistance to organophosphorus insecticides was associated with a sharp decrease of susceptible genotypes at two loci (Ester and ace-1), as well as to an allele replacement at the Ester locus

Route of space spraying and individual houses where traps and cages were placed.

<p>The efficacy of synergized pyrethrins and deltamethrin against <i>Aedes aegypti</i> was evaluated in field conditions by conducting space spray applications using 4x4 vehicles mounted with a thermal fogger in 9 localities of Martinique. In each site, 3 rounds of treatment were made at 2 day intervals. In each locality, 5 sentinel cages containing female mosquitoes were placed inside and outside the houses (white X). Electric mosquito traps were also placed on the inside and outside of the same houses. The locality shown is “Place d'Armes”, which was treated with BanoleW (Control).</p

Mean number of females per trap placed outside houses before and after treatments.

<p>This figure shows estimated densities of <i>Ae. aegypti</i> females in the 9 treated sites based on trap data (see <a href="http://www.plosntds.org/article/info:doi/10.1371/journal.pntd.0001202#pntd-0001202-t002" target="_blank">Table 2</a> for data analysis). Four electric traps equipped with mosquito attractant were placed outside houses on terraces where they were protected from sun and rain. Traps were deployed at 11:00 AM and collected 24 hours later. Abundance (± SE) was estimated weekly starting 2 weeks before and ending 3 weeks after treatments (T).</p

Mean number of females per trap placed inside houses before and after treatments.

<p>This figure shows the estimated densities of <i>Ae. aegypti</i> female in the 9 treated sites based on trap data (see <a href="http://www.plosntds.org/article/info:doi/10.1371/journal.pntd.0001202#pntd-0001202-t002" target="_blank">Table 2</a> for data analysis). Four electric traps equipped with mosquito attractant were placed inside houses and distributed in rooms occupied by the inhabitants. Traps were deployed at 11:00 AM and collected 24 hours later. Abundance (± SE) was estimated weekly starting 2 weeks before and ending 3 weeks after treatments (T).</p

Mortality rates in WHO tube tests.

<p>This figure shows the mortality rates (± SE) of <i>Aedes aegypti</i> collected in the 9 localities selected for the space treatments and that of the laboratory susceptible strain (Bora) when exposed to pyrethrum (1%) or deltamethrin (0.05 %) in WHO tube tests. Data were analyzed using the AOD package of R.</p

Repeated measures split-plot analysis of variance for the productivity of larval sites^*.

<p>*N  =  number, DFnum  =  numerator degrees of freedom, DFden  =  denominator degrees of freedom, C.V.  =  coefficient of variance</p

Productivity of <i>Aedes aegypti</i> in the treated areas.

<p>This figure shows the productivity (±SE) (Weighted Breteau Index) of <i>Aedes aegypti</i> of the 9 localities before and after treatments (T). The density of <i>Ae. aegypti</i> larvae in each locality was estimated from 7 entomological surveys (see data analysis, <a href="http://www.plosntds.org/article/info:doi/10.1371/journal.pntd.0001202#pntd-0001202-t003" target="_blank">Table 3</a>).</p

Repeated measures split-plot analysis of variance for the number of female mosquitoes caught in traps^*.

<p>*N  =  number, DFnum  =  numerator degrees of freedom, DFden  =  denominator degrees of freedom, C.V.  =  coefficient of variance. The data analyzed were the log(mean number of females +1) caught per location.</p