Abstract
BackgroundOrganophosphates and pyrethroids are used widely in Brazil to control Aedes aegypti, the main vector of dengue viruses, under the auspices of the National Programme for Dengue Control. Resistance to these insecticides is widespread throughout Brazil. In Ceará the vector is present in 98% of districts and resistance to temephos has been reported previously. Here we measure resistance to temephos and the pyrethroid cypermethrin in three populations from Ceará and use biochemical and molecular assays to characterise resistance mechanisms.ResultsResistance to temephos varied widely across the three studied populations, with resistance ratios (RR95) of 7.2, 30 and 192.7 in Juazeiro do Norte, Barbalha and Crato respectively. The high levels of resistance detected in Barbalha and Crato (RR95 ≥ 30) imply a reduction of temephos efficacy, and indeed in simulated field tests reduced effectiveness was observed for the Barbalha population. Two populations (Crato and Barbalha) were also resistant to cypermethrin, whilst Juazeiro do Norte showed only an altered susceptibility. The Ile1011Met kdr mutation was detected in all three populations and Val1016Ile in Crato and Juazeiro do Norte. 1011Met was significantly associated with resistance to cypermethrin in the Crato population. Biochemical tests showed that only the activity of esterases and GSTs, among the tested detoxification enzymes, was altered in these populations when compared with the Rockefeller strain.ConclusionsOur results demonstrate that two A. aegypti populations from Ceará are under strong selection pressure by temephos, compromising the field effectiveness of this organophosphate. Our results also provide evidence that the process of reducing resistance to this larvicide in the field is difficult and slow and may require more than seven years for reversal. In addition, we show resistance to cypermethrin in two of the three populations studied, and for the first time the presence of the allele 1016Ile in mosquito populations from northeastern Brazil. A significant association between 1011Met and resistance was observed in one of the populations. Target-site mechanisms seem not to be implicated in temephos resistance, reinforcing the idea that for the studied populations, detoxification enzymes most likely play a major role in the resistance to this insecticide.
Highlights
Organophosphates and pyrethroids are used widely in Brazil to control Aedes aegypti, the main vector of dengue viruses, under the auspices of the National Programme for Dengue Control
Temephos has been used extensively over the period 2002-2009 in both Crato and Barbalha and in these municipalities resistance ratios have increased over this period with, for example, an increase in RR to temephos from 9.0 to 192.7 in Crato (21 fold increase) and from 7.5 to 30 in Barbalha (4 fold increase)
In Juazeiro do Norte where temephos was substituted by Bacillus thuringiensis israelensis (Bti) for larval control the RR to temephos decreased by 30% over this period (10.4 to 7.2) (Table 2)
Summary
Organophosphates and pyrethroids are used widely in Brazil to control Aedes aegypti, the main vector of dengue viruses, under the auspices of the National Programme for Dengue Control Resistance to these insecticides is widespread throughout Brazil. In Brazil, the wide distribution of Aedes aegypti - vector of the three circulating dengue virus serotypes (DENV-1, -2 and -3) - and the lack of a specific treatment or vaccine for dengue, are considered serious public health issues. This situation is exacerbated by the recent. In 2009, case numbers were lower than in the previous year, the mortality rate for dengue hemorrhagic fever was 34.6% and among severe dengue cases was 45% [2], significantly higher than the average mortality elsewhere in Brazil [2]
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