Susceptibility status of the wild-caught Phlebotomus argentipes (Diptera: Psychodidae: Phlebotominae), the sand fly vector of visceral leishmaniasis, to different insecticides in Nepal

Abstract

Background Visceral leishmaniasis (VL) is targeted for elimination as a public health problem in Nepal by 2023. For nearly three decades, the core vector control intervention in Nepal has been indoor residual spraying (IRS) with pyrethroids. Considering the long-term use of pyrethroids and the possible development of resistance of the vector Phlebotomus argentipes sand flies, we monitored the susceptibility status of their field populations to the insecticides of different classes, in villages with and without IRS activities in recent years. Methodology/Principal findings Sand flies were collected from villages with and without IRS in five VL endemic districts from August 2019 to November 2020. The WHO susceptibility test procedure was adopted using filter papers impregnated at the discriminating concentrations of insecticides of the following classes: pyrethroids (alpha-cypermethrin 0.05%, deltamethrin 0.05%, and lambda-cyhalothrin 0.05%), carbamates (bendiocarb 0.1%) and organophosphates (malathion 5%). Pyrethroid resistance intensity bioassays with papers impregnated with 5× of the discriminating concentrations, piperonyl butoxide (PBO) synergist-pyrethroid bioassays, and DDT cross-resistance bioassays were also performed. In the IRS villages, the vector sand flies were resistant (mortality rate <90%) to alpha-cypermethrin and possibly resistant (mortality rate 90–97%) to deltamethrin and lambda-cyhalothrin, while susceptibility to these insecticides was variable in the non-IRS villages. The vector was fully susceptible to bendiocarb and malathion in all villages. A delayed knockdown time (KDT50) with pyrethroids was observed in all villages. The pyrethroid resistance intensity was low, and the susceptibility improved at 5× of the discriminating concentrations. Enhanced pyrethroid susceptibility after pre-exposure to PBO and the DDT-pyrethroid cross-resistance were evident. Conclusions/Significance Our investigation showed that P. argentipes sand flies have emerged with pyrethroid resistance, suggesting the need to switch to alternative classes of insecticides such as organophosphates for IRS. We strongly recommend the regular and systematic monitoring of insecticide resistance in sand flies to optimize the efficiency of vector control interventions to sustain VL elimination efforts in Nepal.

Molecular surveillance of insecticide resistance in Phlebotomus argentipes targeted by indoor residual spraying for visceral leishmaniasis elimination in India

“…Resistance to alpha-cypermethrin, the pyrethroid used for IRS in India, has yet to be demonstrated and was not recorded in tests performed annually between 2016-2019 PLOS NEGLECTED TROPICAL DISEASES [13]. However, pyrethroid resistance measured using the same Anopheles bioassay thresholds, which for alpha-cypermethrin (though not deltamethrin) are lower than for P. argentipes, [19] has recently emerged in Nepal, especially to alpha-cypermethrin, and appears to be more common in villages receiving IRS [20]. Though this might be linked to an earlier switch to pyrethroids in Nepal [20] than India, and operational impacts remain unclear, these results clearly highlight the need for vigilance.…”

“…However, pyrethroid resistance measured using the same Anopheles bioassay thresholds, which for alpha-cypermethrin (though not deltamethrin) are lower than for P. argentipes, [19] has recently emerged in Nepal, especially to alpha-cypermethrin, and appears to be more common in villages receiving IRS [20]. Though this might be linked to an earlier switch to pyrethroids in Nepal [20] than India, and operational impacts remain unclear, these results clearly highlight the need for vigilance. By monitoring frequencies of the 1014 kdr mutations across the sentinel site system in north-eastern India across a five-year period we sought evidence for changes which could provide early indication of changing resistance profiles.…”