Ongoing transmission of lymphatic filariasis in Samoa 4.5 years after one round of triple-drug mass drug administration

Background: Lymphatic filariasis (LF) remains a significant global issue. To eliminate LF as a public health problem, the World Health Organization (WHO) recommends multiple rounds of mass drug administration (MDA). In certain scenarios, including when elimination targets have not been met with two-drug MDA, triple-drug MDA (using ivermectin, diethylcarbamazine and albendazole) is recommended. In this study, we report on antigen (Ag) and microfilaria (Mf) prevalence in eight primary sampling units (PSUs) in Samoa 4.5 years after one round of triple-drug MDA. Methodology: In 2023, community surveys were conducted in eight PSUs that had been surveyed previously in 2018 (between 1.5 and 3.5 months post triple-drug MDA) and 2019 (six to eight-months post triple-drug MDA). Fifteen houses were randomly selected in each PSU with household members aged ≥ 5 years invited to participate. Blood samples were tested for Ag and Mf. Principal findings: Ag-positive participants were observed in six of the eight PSUs, and Ag prevalence was significantly above the 1% threshold in four PSUs. The presence of Mf-positive participants in five PSUs confirms the presence of residual active infections. Conclusions/Significance: This study provides evidence of persistent LF transmission in Samoa 4.5 years after one round of triple-drug MDA, confirming that one round was insufficient for interruption of transmission in this setting. Our findings highlight the negative impact of delaying MDA rounds, for example, due to public health emergencies. Author summary: The World Health Organization (WHO) recommends triple-drug mass drug administration (MDA) for the elimination of lymphatic filariasis (LF) as a public health problem for countries where targets have not been achieved using two-drug MDA, and where onchocerciasis is non-endemic. In 2018, Samoa was the first country to implement a national triple-drug MDA. This study surveyed 623 randomly selected participants across eight primary sampling units (PSUs) in Samoa, 4.5 years after the first round of triple-drug MDA. These results support the current WHO recommendations that multiple rounds of triple-drug MDA are required to interrupt transmission. A gap of 4.5 years between rounds of triple-drug MDA can result in a resurgence, with persistence of LF transmission and burden levels comparable to those observed before the MDA.