Impact of Triple-drug Mass Drug Administration on Prevalence of Antigen and Antibodies to Lymphatic Filariasis in Samoa, 2018-2019

BackgroundIn 2018, Samoa was the first country to distribute nationwide triple-drug mass drug administration (MDA) for lymphatic filariasis (LF) elimination. Prevalence of filarial antigen (Ag) is the main programmatic indicator used to define elimination target thresholds; however, anti-filarial antibodies (Ab) may provide more sensitive measures of transmission compared Ag. We aimed to investigate the utility of Ag and Ab to measure the impact of one round of triple-drug MDA on LF transmission in Samoa after 7-9 months. MethodsTwo community-based cross-sectional serosurveys of [≥]5-year-olds were conducted in 2018 (1-3 months post-MDA) and 2019 (7-9 months post-MDA) in 35 primary sampling units. Ag was detected by Alere Filariasis Test Strips (FTS). Multiplex bead assays (MBA) were used to detect Ab-seropositivity (Bm14 Ab, Wb123 Ab) using Ab-specific mean fluorescence intensity minus background (MFI-bg). Thresholds of seropositivity were determined using finite mixture models (FMM) for log-transformed MFI-bg, defined as the mean of the presumed seronegative class plus three standard deviations. Seroprevalence was adjusted for study design, age, and gender. FindingsA total of 3795 participants (mean age: 20.7; 49% male) were surveyed in 2018 and 4052 (mean age: 20.4; 48% male) in 2019. Between surveys, adjusted Bm14 Ab prevalence decreased (31.2% [95% CI=27.0-35.8] vs 26.3% [95% CI=22.5-30.4]; p=0.02). In 2019 vs 2018, lower odds of Bm14 Ab-positivity (aOR=0.76 [0.64-0.90]), Wb123 Ab-positivity (aOR=0.70 [0.55-0.89]), and dual positivity (aOR=0.68 [0.54-0.87]) were seen. Ag-positive, microfilariae (Mf)-positive participants had significantly higher mean MFI-bg for Wb123 Ab and Bm14 Ab compared to Ag-positive, Mf-negative participants. ConclusionSignificant reductions in Bm14 Ab seroprevalence suggest reduced LF transmission following one round of triple-drug MDA. Thus Abs may provide a more sensitive indicator of change compared to Ag. High MFI values could help identify persistent transmission in the absence of Ag testing. Author summaryLymphatic filariasis is a mosquito-borne disease that can cause lifelong disability and stigma. The main global strategy to eliminate this disease is to treat entire at-risk communities with preventive medicines, aiming to stop transmission altogether. Progress is usually measured using rapid tests that detect parasite antigens in blood. However, as infection becomes rarer, these tests may miss remaining cases, raising concerns that ongoing transmission could go undetected. In this study, we examined whether measuring antibodies in blood could provide earlier or more sensitive signals of infection following treatment. We analysed blood samples collected from communities in Samoa at two time points after a nationwide treatment campaign using a three-drug combination. We found clear reductions in some antibodies over time, even though antigen levels remained largely unchanged. People with active infection had much higher antibody levels than those without, suggesting antibodies may help identify areas where transmission is still occurring. Our findings show that antibody testing can add valuable information to current monitoring approaches and may help programmes better judge whether treatment is working. This is especially important at a time when funding for disease-specific surveillance is limited, and integrated approaches are increasingly needed to protect elimination gains.