Resistance of Anopheles stephensi to selected insecticides used for indoor residual spray and long-lasting insecticidal impregnated nets in Ethiopia

Abstract Background: Malaria, transmitted by the bite of infective female Anopheles mosquitoes, remains a global public health problem. The presence of invasive Anopheles stephensi, capable of transmitting Plasmodium vivax and P. falciparum, was first reported in Ethiopia in 2016. The ecology of this mosquito species differs from that of An. arabiensis, the primary Ethiopian malaria vector [1]. This study aimed to evaluate the efficacy of selected insecticides, which are used in indoor residual spraying (IRS) and selected long-lasting insecticidal treated nets (LLINs) for malaria vector control, against adults An. stephensi. Methods: Anopheles stephensi were collected as larvae and pupae from Awash Subah Kilo Town and Haro Adi village, Ethiopia. Adult female An. stephensi, reared from larvae and pupae collected from the field, aged 3-5 days were exposed to impregnated papers of IRS insecticides (propoxur 0.1%, bendiocarb 0.1%, pirimiphos-methyl 0.25%), and insecticides used in LLINs (alphacyper-methrin 0.05%, deltamethrin 0.05% and permethrin 0.75%), using diagnostic doses and WHO test tubes in a bio-secure insectary at Aklilu Lemma Institute of Pathobiology, Addis Ababa University (AAU-ALIPB). For each test and control tube, batches of 25 female An. stephensi mosquitoes were used to test each insecticide used in IRS. Additionally, cone tests were conducted to expose An. stephensi to four brands of LLINs, MAGNet™ (alpha-cypermethrin), PermaNet® 2.0 (deltamethrin), DuraNet© (alpha-cypermethrin) and SafeNet® (alpha-cypermethrin). A batch of ten sugar fed female mosquitoes aged 2-5 days were exposed to samples taken from five positions/sides of a net. The data from all replicates were pooled and descriptive statistics were used to describe features of the data. Results: All An. stephensi mosquitoes collected from Awash Subah Kilo Town and Haro Adi village (around Metehara) were resistant to all tested insecticides used in both IRS and LLINs. Of the tested LLINs, only MAGNet™ (alpha-cypermethrin active ingredient) caused 100% knockdown and mortality to An. stephensi at 60 minutes and 24 hours post exposure, while all other net brands resulted below the WHO cut- off points. All these nets, except SafeNet®, were collected during LLIN distribution for community members through the National Malaria Program, in December 2020. Conclusions: Anopheles stephensi is resistant to all tested insecticides used in IRS and in the tested LLIN brands, except MAGNet, did not cause in mortality as expected. This suggests that control of this invasive vector using existing adult malaria vector control methods may not be appropriate and that alternative strategies may be necessary.