Five Years, Two Mass Triple-Drug Administrations, and Ongoing Transmission: Using Mosquito and Human Indicators to Evaluate the Impact of Lymphatic Filariasis Interventions in Samoa

BackgroundFor lymphatic filariasis (LF) elimination, the World Health Organization recommends multiple rounds of mass drug administration (MDA). While LF antigen (Ag) is routinely used to monitor progress, recent evidence suggests more time-sensitive indicators are needed during the immediate post-MDA period. In Samoa, triple-drug MDA was distributed in 2018 and 2023. This study aimed to evaluate the impact of Samoas second round of triple-drug MDA on human and mosquito-based indicators. MethodologySurveys were conducted in eight primary sampling units (PSUs) in 2019 (7-9 months after the 2018 MDA), 2023 (4.5 years after the 2018 MDA) and 2024 (10 months after the 2023 MDA). Participants aged [≥]5 years from randomly selected households were tested for Ag and microfilariae (Mf). For molecular xenomonitoring (MX), mosquitoes were caught using BG-Sentinel traps at households, sorted into pools by species, and tested for filarial DNA using quantitative polymerase chain reaction (qPCR). Results and key findingsIn 2024, Ag prevalence was 10.3% (95% CI:7.3-14.5) vs 9.9% (95% CI: 3.5-21.0) in 2023 and 9.8% (95% CI:5.6-15.5) in 2019. Mf prevalence was 3.1% (95% CI: 1.8-5.5) in 2024 vs 5.1% (95% CI:1.3-12.4) in 2023 and 3.1% (95% CI:1.3-5.9) in 2019. Odds ratios (OR) of a positive test in 2024 vs 2023 showed no decrease for Ag (OR 1.0; 95% CI:0.7-1.6), but a potential reduction in the proportion of Ag-positive participants who were Mf-positive (OR 0.6; 95% CI: 0.2-1.4) and a significant reduction in the prevalence of qPCR-positive mosquitoes (OR 0.4; 95%CrI: 0.2-0.7). From 2019-2024, there were reductions in prevalence of qPCR-positive Aedes spp. (OR 0.5; 95% CrI:0.3-1.0) and Aedes polynesiensis (OR 0.4; 95% CrI: 0.2-0.8). ConclusionsIn Samoa, LF transmission continues despite two rounds of triple-drug MDA five years apart. Mosquito indicators can provide a more sensitive measure of MDA impact in the immediate post-intervention period and complement human indicators for long-term surveillance. Author summaryLymphatic filariasis is a mosquito-borne disease that can cause severe long-term disability. To eliminate this neglected tropical disease as a public health problem, the World Health Organization (WHO) recommends multiple rounds of mass drug administration (MDA) 12 months apart. Despite antigen (Ag) being the standard indicator for evaluating MDA impact, more time sensitive indicators, such as measuring microfilariae (Mf) in the blood or testing mosquitoes for filarial DNA (molecular xenomonitoring) may provide a more reliable assessment in the months following the intervention. Samoa distributed two rounds of triple-drug MDA in 2018 and 2023. We evaluated the impact of the second round on human and mosquito-based infection indicators including Ag, Mf, and qPCR-positive mosquitoes. We surveyed eight villages in Samoa in 2019 (7-9 months after the 2018 MDA), 2023 (4.5 years after the 2018 MDA) and 2024 (10 months after the 2023 MDA). The two rounds of MDA were insufficient to break the LF transmission cycle. Results from 2024 showed no decrease in Ag prevalence from either 2019 or 2023, although there was potentially a decrease in Mf prevalence. Molecular xenomonitoring (MX) provided an earlier signal of MDA impact than Ag, with a decrease in the prevalence of qPCR-positive mosquitoes observed within 10 months of the 2023 MDA.