Field evaluation of SumiLarv™ 2MR larvicide for controlling the invasive malaria vector Anopheles stephensi in Ethiopia: a randomised controlled trial

Abstract

The establishment of the invasive malaria vector Anopheles stephensi in Ethiopia threatens existing malaria control efforts. Unlike native vectors, An. stephensi thrives in both urban and rural areas, lacks clear seasonal peaks of abundance, and often breeds in artificial water containers. SumiLarv™ 2MR containing the larvicide pyriproxyfen (PPF) offers a promising method for controlling the immature stages of An. stephensi in such habitats. This habitat-randomised controlled trial evaluated the efficacy of SumiLarv™ 2MR against An. stephensi in Danan town, Somali region, Ethiopia. Entomological surveillance identified 102 large artificial containers (“birkas”) which were randomly allocated in a 1:1 ratio to receive either SumiLarv™ 2MR (treatment arm) or no treatment (control arm). SumiLarv™ 2MR was introduced into the birkas at a dose of one disc per 200 L of water. Morphological identification was performed on adult mosquitoes emerging from control containers, while molecular analysis was conducted on both morphologically identified adult mosquitoes and dead pupae collected from the control and treatment arm. The residual efficacy of PPF content of the remaining discs was also analysed. Inhibition of adult mosquito emergence was assessed every 2 weeks for 40 weeks to determine the duration of efficacy. The number of SumiLarv™ 2MR remained constant in the treatment arm throughout the study period. No significant differences were observed in water volume, pH, or temperature between the two study arms. Introduction of SumiLarv™ 2MR resulted in complete (100%) inhibition of adult emergence throughout the 40 weeks’ study period (t = 6.26, p < 0.05). Morphological identification of a sub-samples of adult mosquitoes emerging from control containers at different time points confirmed the presence of An. stephensi (99.85%) and An. gambiae s.l. (0.15%). Molecular analysis of morphologically identified adults from control containers and dead pupae from treatment containers confirmed that 98.91% were An.stephensi. Post-trial analysis showed that approximately 82.20% of the original PPF had been released from SumiLarv™ 2MR during study period. The high and sustained efficacy of SumiLarv™ 2MR demonstrated in this study suggests that it is a suitable tool for controlling An. stephensi in large artificial water storage containers.