Utility of antifilarial antibodies for post-validation surveillance of lymphatic filariasis in Tonga.

Post-validation surveillance (PVS) is essential to detect resurgence of lymphatic filariasis (LF) following elimination as a public health problem. Testing for antifilarial antibodies (Ab) alongside antigen (Ag) may increase sensitivity for identifying persistent transmission. We evaluated the utility of Ab-based serosurveillance for PVS in Tonga, which eliminated LF in 2017. A multi-pronged, risk-stratified cross-sectional survey incorporating community, school, an outpatient clinic, and volunteer recruitment was conducted in May-July 2024. Blood samples were tested for Ag using Alere™ Filariasis Test Strips and Ab against Bm14 Ag and Wb123 Ag using multiplex bead assays (MBA). Demographic associations with Ab seropositivity were assessed and pairwise comparisons were used to estimate mean differences in Ab median fluorescence intensity (MFI-bg) between Ag-positives and Ag-negatives. Ag and Ab results were available for 1,727 of 1,787 participants (97%; mean age 29 years). Prevalence was 2% for Ag, 3% for Bm14 Ab, and 5% for Wb123 Ab. Ab positivity was associated with male sex, age ≥ 19 years, and residence in outer islands. Ag-positives had significantly higher mean MFI-bg values for Bm14 Ab (p < 0.001) and Wb123 Ab (p < 0.001) than Ag-negatives. Wb123 Ab demonstrated the highest concordance with Ag status: 71.8% of Ag-positive participants were also Wb123 Ab-positive, while 96.4% of Ag-negative participants were Wb123 Ab-negative. Abs show promise as a complementary PVS tool and, when integrated into MBA platforms, have the potential to offer a cost- and resource-efficient approach to strengthening LF surveillance.