Persistent lymphatic filariasis transmission seven years after validation of elimination as a public health problem: a cross-sectional study in Tonga

Background: The World Health Organization (WHO) has validated 21 countries as having eliminated lymphatic filariasis (LF) as a public health problem. Post-validation surveillance (PVS) is required in countries where LF has been eliminated. Tonga eliminated LF in 2017, but no PVS strategy has been established. We aimed to identify any persistent LF transmission in Tonga in 2024 and provide evidence to support a PVS strategy.

Methods: A four-pronged, targeted, cross-sectional study was conducted in the Tongan districts of Tongatapu, Ha’apai, and Ongo Niuas in May–July 2024 in communities, primary schools, high schools, and an outpatient clinic. Participants were tested for LF antigen (Ag) and microfilariae (Mf). The outcome measure for persistent LF transmission was Ag-positivity.

Findings: Between 9 May and 19 July 2024, 1787 participants were recruited from 12 communities, 11 primary schools, five high schools, and one outpatient clinic. Overall, 39 participants (2·2%) were Ag-positive and five (0·3%) were Mf-positive. The highest Ag prevalence was in communities (4·0%; 95% confidence interval [95%CI]: 2·9–5·6), where all Mf-positives (n = 5) were identified. Ag-positivity was associated with male sex (adjusted odds ratio [aOR]:4·86; 95%CI: 2·25–10·46), older age (>50 years vs 5–10 years [aOR:7·51; 95%CI: 2·13–26·47]), and residing in Ha’apai (aOR:15·08; 95%CI: 5·41–42·05) and Ongo Niuas (aOR:10·85; 95%CI: 3·91–30·08).

Interpretation: We found persistent LF transmission in Tonga seven years post-validation. Community surveillance yielded the highest Ag and Mf prevalence. Efficiency of PVS could be improved by integrating surveillance activities into the existing health system and conducting community-based surveys, particularly among older males and in high prevalence areas.