Evaluating the impact of triple-drug mass drug administration using human and mosquito-based indicators of lymphatic filariasis in Samoa, 2019 – 2024

BACKGROUND: For lymphatic filariasis (LF) elimination, the World Health Organization recommends multiple rounds of mass drug administration (MDA) to interrupt transmission. 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 Samoa's second round of triple-drug MDA on human and mosquito-based indicators of transmission. METHODOLOGY: Surveys were conducted in eight primary sampling units (PSUs) in 2019 (6-9 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 presence of filarial DNA using quantitative polymerase chain reaction (qPCR). RESULTS AND KEY FINDINGS: In 2024, Ag prevalence was 10.3% (95% CI: 7.3-14.5) versus 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). CONCLUSIONS: In 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.