Epidemiology of Lymphatic Filariasis Antigen and Microfilaria in Samoa, 2019: 7–9 Months Post Triple-Drug Mass Drug Administration

The elimination of lymphatic filariasis (LF) as a public health problem remains an ongoing challenge in the Pacific region. This study reports on antigen (Ag) and microfilaria (Mf) prevalence in Samoa in 2019, 7-9 months after the completion of the first round of triple-drug mass drug administration (MDA). It evaluates the effectiveness of the intervention for reducing Ag prevalence to below a 2% threshold, and how this differs between 5-9-year-olds and ≥10-year-olds. We surveyed 30 randomly selected and five purposefully selected primary sampling units (PSUs) in Samoa in 2018 (1-3 months post-triple-drug MDA) and, again, in 2019. In each PSU, we conducted a community survey of 15-20 households and a convenience survey of 5-9-year-old children. A finger-prick blood sample was collected from all participants to test for Ag and Mf. Demographic details were also collected. There was no significant change in adjusted Ag prevalence in the 30 randomly selected PSUs between 2018 (3.9% [95% CI: 2.7-5.6%]) and 2019 (4.1% [95% CI 2.7-5.9%]). Significantly higher Ag prevalence was observed in participants aged ≥10 years (4.6%, 95% CIs 3.0-6.7%) compared to 5-9-year-olds (1.1%, 95% CIs 0.5-2.2%), supporting existing evidence that post-MDA surveillance should not be based on Ag prevalence among 6-7-year-olds. A single round of triple-drug MDA was insufficient to break LF transmission in Samoa 7-9 months post-MDA.