Progression of lymphatic filariasis antigenaemia and microfilaraemia over 4.5 years in antigen-positive individuals, Samoa 2019-2023

Objectives: The first round of triple-drug mass drug administration (MDA) for lymphatic filariasis (LF) in Samoa was in 2018. This study aims to i) examine progression of LF antigen (Ag) and microfilaria (Mf) in Ag-positive individuals from 2019-2023; and ii) compare Ag/Mf prevalence in household members of Mf-positive vs Mf-negative participants. Methods: In 2023, we tested Ag-positive participants (indexes) from a 2019 survey in Samoa, and their household members. We tested for Ag (Alere/Abbott Filariasis Test Strip) and Mf. We examined changes in Ag/Mf status in index participants and compared Ag/Mf prevalence between household members of Mf-positive and Mf-negative indexes. Results: We recruited 91 indexes and 317 household members. In 2023, all 17 Mf-positive indexes remained Ag-positive and 11/15 with Mf results (73.3%) were Mf-positive. Of 74 Mf-negative indexes, 79.7% remained Ag-positive in 2023 and 31.1% became Mf-positive. Household members of Mf-positive indexes were more likely to be Ag-positive (odds ratios 3.3, 95% CI 1.0-10.3) compared to those of Mf-negative indexes. Conclusion: Our results raise concerns regarding long-term effectiveness of a single-dose of triple-drug MDA for sustained clearance of Mf in Samoa. Guidelines for follow-up and treatment of Ag/Mf-positive people and household members are urgently required. (c) 2025 The Authors. Published by Elsevier Ltd on behalf of International Society for Infectious Diseases. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/)