Phenotypic and genotypic resistance to commonly used insecticides in Aedes aegypti among four cities in southern Ecuador
Insecticide resistance (IR) can undermine efforts to control vector species of public health importance. Aedes aegypti is the main vector of resurging diseases in the Americas such as yellow fever and dengue, as well as recently emerging chikungunya and Zika viruses, which have caused unprecedented epidemics in the region. Vector control remains the primary public health intervention to prevent outbreaks of Aedes transmitted diseases. In many high-risk regions, like southern Ecuador, we have limited information on IR in Ae. aegypti. In this study, IR status in Ae. aegypti was measured across four cities in El Oro Province in Ecuador using phenotypic assays and genetic screening for alleles associated with resistance to pyrethroid insecticides. Bottle bioassays showed significant inter-seasonal variation in resistance to deltamethrin, a pyrethroid insecticide commonly used by the Ministry of Health, and alpha cypermethrin, as well as differences in deltamethrin resistance between cities. There was also a significant difference in phenotypic response to the organophosphate, Malathion, between two of the cities during the second sampling season. Genotyping showed moderate to high frequencies of the V1016I and F1534C resistance alleles in all four cites. Frequency of resistance genotypes varied significantly between cities in the first sampling season, and not in the later seasons, suggesting a possible selective response to vector control activity. Overall, resistance levels were highest in Machala, a city where dengue is hyperendemic and where insecticide use has historically been more intense than in the other cities included in the study. Despite statistically significant evidence that resistance alleles conferred phenotypic resistance, there was not precise correspondence between these indicators. We found that 17.6% of F1534C and 45.6% of V1016I mutant mosquitoes were susceptible in the bottle bioassays. This study shows there is spatiotemporal variability in IR in southern Ecuador, and serves as an initial examination of the genotypic and phenotypic indicators of IR in this region, providing important information to guide the vector control interventions by the public health sector.