São Tomé and Príncipe on the verge of eliminating lymphatic filariasis as a public health problem: evidence from IDA impact assessment surveys
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Background
Accelerated efforts to eliminate lymphatic filariasis (LF) as a public health problem have been supported by the introduction of the triple-drug regimen of ivermectin, diethylcarbamazine and albendazole (IDA) in endemic settings. In São Tomé and Príncipe, nationwide mass drug administration (MDA) with diethylcarbamazine and albendazole was implemented in 2018, followed by IDA in 2019 and 2020. This study assesses progress towards elimination using post-MDA impact assessment surveys conducted after cessation of treatment.
Methods
Cross-sectional surveys were conducted among adults aged 20 years and older in 2022 and again between December 2024 and January 2025. Circulating filarial antigen (CFA) was detected using the filarial test strip (FTS). Individuals who tested positive were examined for microfilaremia using nocturnal calibrated thick blood smear microscopy. Additionally, programme data on MDA coverage and morbidity were obtained from national surveillance records.
Results
Three rounds of nationwide MDA achieved high epidemiological coverage (86.4% in 2018, 74.2% in 2019 and 80.0% in 2020). The impact assessment surveys conducted in 2022 evaluated 14 132 adults, with 21 individuals (0.15%) testing positive for CFA, while the follow-up survey conducted between December 2024 and January 2025 assessed 14 653 adults and detected seven positive cases (0.05%). No microfilariae were detected among the 28 antigen-positive individuals examined using nocturnal calibrated thick blood smears. National morbidity records documented 190 cases of lymphoedema and nine cases of hydrocoele.
Conclusions
Infection indicators remain well below WHO decision thresholds, suggesting that LF transmission is unlikely to be sustained. São Tomé and Príncipe appears to be close to eliminating LF as a public health problem. However, strengthening morbidity management services will be essential to support the preparation of the national elimination dossier.
Author summary
Lymphatic filariasis is a parasitic disease transmitted by mosquitoes that can cause chronic swelling of the limbs (lymphoedema) and scrotal swelling (hydrocoele). In 2000, WHO launched the Global Programme to Eliminate Lymphatic Filariasis (GPELF) to interrupt transmission through MDA and provide care for affected individuals. Between 2018 and 2020, São Tomé and Príncipe implemented nationwide treatment campaigns using WHO-recommended drug combinations.
To evaluate the reduction in transmission, surveys were conducted among adults in 2022 and again from 2024 to 2025. Very few people tested positive for infection: 21 out of 14 132 adults (0.15%) in 2022 and seven out of 14 653 (0.05%) in the 2024–2025 follow-up survey. No microfilariae were detected among antigen-positive individuals, and national records reported relatively low numbers of lymphoedema (190 cases) and hydrocoele (nine cases). Taken together, the findings indicate that ongoing transmission is very unlikely. They also suggest that parasite transmission is unlikely to persist and that São Tomé and Príncipe is close to eliminating lymphatic filariasis as a public health problem. However, it remains necessary to enhance services for the management of chronic disease manifestations.
