Insecticide resistance in Aedes aegypti from Tapachula, Mexico: Spatial variation and response to historical insecticide use

Abstract

Background Insecticide use continues as the main strategy to control Aedes aegypti, the vector of dengue, Zika, chikungunya, and yellow fever. In the city of Tapachula, Mexico, mosquito control programs switched from pyrethroids to organophosphates for outdoor spatial spraying in 2013. Additionally, the spraying scheme switched from total coverage to focused control, prioritizing areas with higher entomological-virological risk. Five years after this strategy had been implemented, we evaluated the status and variability of insecticide resistance among Ae. aegypti collected at 26 sites in Tapachula. Methodology/Principal findings We determined the lethal concentrations at 50% of the tested populations (LC50) using a bottle bioassay, and then, we calculated the resistance ratio (RR) relative to the susceptible New Orleans strain. Permethrin and deltamethrin (pyrethroids), chlorpyrifos and malathion (organophosphates), and bendiocarb (carbamate) were tested. The frequencies of the substitutions V1016I and F1534C, which are in the voltage-gated sodium channel and confer knockdown-resistance (kdr) to pyrethroid insecticides, were calculated. Despite 5 years having passed since the removal of pyrethroids from the control programs, Ae. aegypti remained highly resistant to permethrin and deltamethrin (RR > 10-fold). In addition, following 5 years of chlorpyrifos use, mosquitoes at 15 of 26 sites showed moderate resistance to chlorpyrifos (5- to 10-fold), and the mosquitoes from one site were highly resistant. All sites had low resistance to malathion (< 5-fold). Resistance to bendiocarb was low at 19 sites, moderate at five, and high at two. Frequencies of the V1016I ranged from 0.16–0.71, while C1534 approached fixation at 23 sites (0.8–1). Resistance profiles and kdr allele frequencies varied across Tapachula. The variability was not associated with a spatial pattern at the scale of the sampling. Conclusion/Significance Mosquito populations respond to selection pressure at a focal scale in the field. Spatial variation across sites highlights the importance of testing multiple sites within geographical regions.

Current enzyme-mediated insecticide resistance status of Aedes aegypti populations from a dengue-endemic city in southern Mexico

“…levels. 17 The correlation between previously published Malathion RRs and AChEi (r=0.459) was statistically significant (p=0.03), but not for other enzymes. The correlations between the RRs of Clorpirifos, Bendiocarb, Deltamethrin and Permethrin and the levels of the six enzymes were not statistically significant (p>0.05).…”

“…Tapachula, Chiapas is located in southern Mexico on the border with Guatemala; it is a cosmopolitan city because of its high immigration activity, which has likely made it endemic to dengue fever with outbreaks occurring throughout the year. Aedes aegypti were collected from 22 sites in Tapachula 17 between January to April of 2018 and 2020 using 12 ovitraps per site (figure 1). 18 The ovitrap consisted of a one-liter plastic container coated with a 15 cm-wide strip of #615 filter paper that was replaced weekly for four continuous months in order to collect sufficient eggs.…”

“…aegypti populations made in 2018 in Tapachula, where low to moderate resistance to OP and CARB was reported, while they were still highly resistant to PYR. 17 This work is part of a research project which aim was to identify the resistance and mechanisms in Ae. aegypti, and how they responded to the historical use of insecticides in Tapachula from 2018 to 2021, where mosquito were collected twice a year in 26 sites.…”

“…aegypti, and how they responded to the historical use of insecticides in Tapachula from 2018 to 2021, where mosquito were collected twice a year in 26 sites. Preliminary results of the insecticide resistance diagnosis for the first year collected is already published, 17 while the insecticide resistance behavior thorough the years is being prepared for publication. Here was investigated whether Ae.…”

“…For this objective, biochemical assays were undertaken in two mosquito samples of the same collection sites, one at the beginning (2018) and another at the half of the study (2020). Additionally for the year 2018, whether the resistance to insecticides already reported 17 correlates with the enzyme results was investigated in the same mosquito populations.…”