Small-scale field assessment against the dengue vector Aedes aegypti using the auto-dissemination approach in an urban area of Vientiane, Lao PDR.

BACKGROUND: In Lao PDR, dengue fever is the most important vector borne disease and vector control remains the principal method to fight against Aedes aegypti the primary transmitter mosquito species. Vector control management programs need new strategies in addition to conventional larviciding and adulticiding interventions in the country. In this study, we examined the In2Care® Mosquito Trap's efficacy using insecticide auto-dissemination strategy. The insecticide pyriproxyfen, present in powder form inside the trap station, contaminates the body of gravid female mosquitoes visiting the traps and is later on disseminated via the mosquitoes in breeding sites surrounding the traps. We tested the attractiveness of the Traps, their efficacy to reduce the larval and adult abundance, and the impact on emergence rates. Specifically, we tested if the servicing interval of the In2Care® Mosquito Trap could be extended to 12 weeks. METHODS: Two black plastic ovitrap buckets and two BG® sentinel traps were placed in the premises of the Science campus of Vientiane Capital located in an urban area to measure weekly the larval and adult relative abundance of Aedes mosquitoes from 2017 to 2019. Twenty-five In2Care® Mosquito Traps were evenly distributed in this area and two studies of 12 weeks were implemented during January and April 2018 and, July to October 2018 (dry and rainy season, respectively). Every 2 weeks, water samples from 5 In2Care® Traps were randomly selected and tested at the laboratory with Ae. aegypti larvae to measure the larval and pupal mortality. The relative abundance of Aedes mosquitoes in the BG traps® with the presence of In2Care® Traps in 2018, was compared with the surveillance results obtained in 2017 and 2019 without In2Care® Traps. Every week, water samples from the ovitrap buckets were tested for Emergence Inhibition (EI). RESULTS: The In2Care® Traps were very attractive to gravid Ae. aegypti mosquitoes specifically during the rainy seasons with 96% of the traps colonized with larvae/pupae within four weeks. The bioassays showed 100% mortality in the water samples from the traps during the twelve weeks studies showing the good efficacy over time of the pyriproxyfen without additional servicing in the 12 week period. In addition, the larvicide was successfully disseminated into the ovitrap buckets placed in the treated area where 100% of EI during all weeks of intervention was measured. There was no significant effect of the treatment on adult abundance reduction in the treated area, probably due to recolonization of adult mosquitoes surrounding the field experiment. CONCLUSIONS: The observed potential of the In2Care® Mosquito Trap using the auto-dissemination strategy could lead to the use of this new tool in combination with conventional control methods against Dengue vectors in urban tropical areas. Large scale field trials should be implemented in Lao PDR to prove its efficacy for Public Health programs.

Detection of pyrethroid resistance mutations and intron variants in the voltage-gated sodium channel of Aedes (Stegomyia) aegypti and Aedes (Stegomyia) albopictus mosquitoes from Lao People's Democratic Republic.

In Lao People's Democratic Republic, Aedes aegypti (Linnaeus 1762) and Aedes albopictus (Skuse 1894) mosquitoes (Diptera: Culicidae) are vectors of arboviral diseases such as dengue. As the treatment for these diseases is limited, control of the vectors with the use of pyrethroid insecticides is still essential. However, mutations in the voltage-gated sodium channel (vgsc) gene giving rise to pyrethroid resistance are threatening vector control programs. Here, we analysed both Ae. aegypti and Ae. albopictus mosquitoes, which were collected in different districts of Laos (Kaysone Phomvihane, Vangvieng, Saysettha and Xaythany), for vgsc mutations commonly found throughout Asia (S989P, V1016G and F1534C). Sequences of the vgsc gene showed that the F1534C mutation was prevalent in both Aedes species. S989P and V1016G mutations were detected in Ae. aegypti from each site and were always found together. In addition, the mutation T1520I was seen in Ae. albopictus mosquitoes from Saysettha district as well as in all Ae. aegypti samples. Thus, mutations in the vgsc gene of Ae. aegypti are prevalent in the four districts studied indicating growing insecticide resistance throughout Laos. Constant monitoring programmes and alternative strategies for controlling Aedes should be utilized in order to prolong the effectiveness of pyrethroids thereby maximizing vector control.

Malaria and Dengue Mosquito Vectors from Lao PDR Show a Lack of the rdl Mutant Allele Responsible for Cyclodiene Insecticide Resistance.

The gamma-aminobutyric acid (GABA) receptor, RDL, plays important roles in neuronal signaling and is the target of highly effective insecticides. A mutation in RDL, commonly A296S, underlies resistance to several insecticides such as cyclodienes. Even though the use of cyclodienes has been banned, the occurrence of mutations substituting A296 is notably high in mosquitoes from several countries. Here, we report a survey investigating the prevalence of the Rdl mutant allele in mosquitoes from Laos, a country where mosquito-borne diseases such as malaria and dengue fever are health concerns. Anopheles and Aedes mosquitoes were collected from 12 provinces in Laos. Adult bioassays on Aedes aegypti (Linnaeus) (Diptera: Culicidae) and Aedes albopictus (Skuse) (Diptera: Culicidae) showed that all the populations tested were susceptible to dieldrin (4%) following WHO protocols. Exon 7 from a total of 791 mosquitoes was sequenced to identify the amino acid encoded for at 296 of RDL. Only one of these mosquitoes, Anopheles maculatus rampae Harbach and Somboon (Diptera: Culicidae) from Attapeu, carried the mutant allele being heterozygous for A296S. We therefore found a general lack of the Rdl mutant allele indicating that mosquitoes from Laos are not exposed to insecticides that act on the GABA receptor compared to mosquitoes in several other countries. Identifying the prevalence of the Rdl mutation may help inform the potential use of alternative insecticides that act on the GABA receptor should there be a need to replace pyrethroids in order to prevent/manage resistance.