Here we showed that Ae. aegypti populations from Roraima State, Brazil collected in 2016 and 2018, were resistant to the pyrethroid deltamethrin and is the process of becoming resistant to the organophosphate malathion.
The larvicide organophosphate temephos was distributed from the MoH to Roraima State between 2004 and 2013 [7] and the IR monitoring network detected resistance to temephos in Ae. aegypti from Boa Vista in 2002, 2003, 2004 and 2007 [5, 18], but not in 2010 [4]. Mosquitoes from Pacaraima were resistant to temephos in 2011 [19]. The disseminated resistance to tempephos in the whole country made the MoH substitute temephos for the biolarvicide Bacillus thuringiensis var. israelensis (Bti) and later for IGR insecticides, such as the benzophenyl urea inhibitors of chitin synthesis diflubenzuron and novaluron, which started being applied in Roraima in 2010 [7].
Resistance to pyrethroid in Ae. aegypti from Roraima State was already high in previous evaluations. The first registers were in 2007 and 2010, when the capital Boa Vista was detected as resistant to pyrethroid [4, 18]. In 2011 Pacaraima presented the second most high resistance ration (RR95 = 60.3) detected that year in the whole country [7]. Here we showed that even two years after pyrethroid governmental application was substituted by malathion, resistance to pyrethroid is maintained in the four localities evaluated.
The Brazilian MoH started replacing pyrethroids for the organophosphate malathion in ultra-low volume-based applications against Ae. aegypti in 2010 [7], although pyrethroids are still used in campaigns against other vectors, such as anophelines, phlebotomines and tritomines. In Roraima State the first stock of malathion was received in December 2015. Here we showed that all the four populations collected in 2016, although not 100% killed to malathion 0.7%, had mortality above 90%, then not classified as resistant. The rate of mortality decreased by under 90% in Bonfim and Pacaraima two years later and were therefore classified as resistant to malathion. Differently for what occurs with pyrethroids, which are the active ingredient of household sprays and other governmental campaigns, the probable only source of organophosphate pressure was the governmental campaigns with malathion at that moment. The larvicide temephos is in theory not applied in Roraima since 2013, when was definitely substituted by IGRs [7]. The most recent data about temephos resistance in Roraima indicated resistance ratios (RR50) of 2.0 in Boa Vista (2010) and 4.3 in Pacaraima (2011), which were not considered high levels of resistance [4, 18]. Anyway, we cannot reject the possibility that mechanisms prior selected by temephos and pyrethroids are conferring cross resistance to malathion, as reported in some classical studies. For instance, a laboratory strain of Culex quinquefasciatus selected for temephos resistance in larvae developed cross resistance to several organophosphate adulticides, including malathion [20]. On the other hand, in Guadeloupe and Saint Martin Caribbean islands, Ae. aegypti populations developed high levels of resistance to temephos (8.9 to 33.1-fold) but low levels to malathion (1.7 to 4.4-fold) [21]
In regard to the possible mechanisms selected for insecticide resistance, alteration in the activity of GST and Esterase enzymes were detected in Boa Vista (2007) and Pacaraima (2011). Reduced activity of acetylcholinesterase enzyme was also observed in Pacaraima (2011) [7]. Lines of Ae. aegypti Brazilian populations that acquired resistance to malathion though selection pressure in the laboratory exhibited increased activity of GST, MFO P450 and esteresas, as determined by bichoemical analyses [22]. Over expression of genes related to metabolic resistance were detected in Ae. aegypti populations from Caribe, such as French Guyana and French West Indies islands [21, 23]. These metabolic related mechanisms however are difficult to associate with a specific class of insecticide. On the other hand, those high levels of resistance to pyrethroids in Roraima could be partially justified by the absence of the wild-type NaVS haplotype, already noted 2010 and 2011 in Boa Vista and Pacaraima [8]. The wild-type NaVS haplotype is still missing, with predominance of the double kdr NaVR2 (1016Ile + 1534Cys), except in Rorainópolis, where the NaVR1 (1016Val + 1534Cys) predominates. We corroborated that NaVR2 confreres higher levels of resistance to pyrethroid [15], once homozygotes R2R2 individuals were exclusively among survivals in bioassays with Boa Vista and Bonfim. The remaining high levels of resistance to pyrethroid even after the change for malathion in Roraima can be associated with the high prevalence of domestic use of insecticides, all composed of pyrethroids and easily acquired in local markets, as reported in other Brazilian states [17, 24]. In addition, we cannot neglect the migration of Ae. aegypti resistant populations from the neighbor countries and measures adopted against other vector borne diseases. In the border between Pacaraima (Brazil) and Santa Helena (Venezuela) there is an intense control of malaria, where pyrethroids are employed against Anopheles even in the urban area, thus submitting also Ae. aegypti to that selection pressure. In Bonfim it is also usual to find pyrethroid sprays acquired in Lethen City, in the Guyanese side of the border.
We speculate that the kdr NaVR2 haplotype in Ae. aegypti populations in that region should have migrated mostly from Venezuela, once it has been registered in Roraima State at least since 2010, when was absent or under low frequencies in the neighbors Amazonas and Pará States [8]. Interestingly, Rorainópolis is the only municipality in Roraima with occurrence of Ae. albopictus, however registered in areas in the border with Amazonas State, where that species has been registered since 2015 (CGVS-SESAU-RR). Therefore, it seems that the ecological transition zone between the biomes Amazon Forest to the south and the savannah-like Lavrado to the north has been limiting the dispersion of Ae. aegypti towards Amazonas State, whilst Ae. albopictus is avoided to invade the capital Boa Vista.
Besides new promising tools of Ae. aegypti control, such as Wolbachia, RIDL and pyriproxyfen autodissemination stations [25] currently in advance tested in Brazil [26, 27, 28], insecticides should play an important role in a long term in order to rapid decrease the densities of a target population and consequently mitigate the cycle of the arboviruses. Therefore it is of prime importance that a constant monitoring of insecticide resistance be inherent of the chemical control strategy in order to guide authorities about which products should have the best efficacy.