Mayfield, Helen J., Sartorius, Benn, McLure, Angus, Curtis, Stephanie J., Martin, Beatris Mario, Sheridan, Sarah, Thomsen, Robert, Tofaeono-Pifeleti, Rossana, Viali, Satupaitea, Graves, Patricia M., and Lau, Colleen L. (2025) Reactive case detection can improve the efficiency of lymphatic filariasis surveillance compared to random sampling, Samoa 2023. PLoS Neglected Tropical Diseases, 2025. e0012622.

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Abstract

Background In Samoa, lymphatic filariasis (LF) remains endemic despite persistent elimination efforts. Targeted sampling based on locations of known infections could be an efficient strategy for locating infected individuals and residual infections to support these efforts. This cross-sectional study assesses the efficiency of reactive case detection versus random sampling for identifying LF antigen (Ag)-and microfilaria (Mf)-positive individuals in Samoa in varying Ag prevalence scenarios. Methodology In 2023, six primary sampling units (PSUs) were surveyed using random and targeted sampling strategies. PSUs were selected based on Ag prevalence in 2019; two PSUs each with low (3–5%), medium (6–7%) and high (13–17%) Ag prevalence. The randomly selected group included residents aged ≥5 years in 15 houses per PSU. The targeted group included residents aged ≥5 years in up to eight households within 200 metres of a household where Ag-positive resident(s) were identified in 2019. Blood samples were tested for Ag and examined for Mf. Principal findings The targeted sampling strategy (n = 400 people) identified more positives (57 Ag-positive, 23 Mf-positive) than the random sampling strategy (n = 494, 39 Ag-positive, 16 Mf-positive), with an overall targeted:random sampled case ratio of 1.8 (95% CI 1.3-2.5) for Ag and 1.8 (95% CI 1.1-3.1) for Mf. Efficiency gains were greatest in medium prevalence PSUs for both Ag-positives (ratio = 2.4, 95% CI 1.3-5.2) and Mf-positives (ratio = 2.6, 95% CI 0.9-12.8). Conclusions In Samoa, a targeted sampling strategy using reactive case detection was more efficient for locating Ag-positive and Mf-positive individuals compared to random sampling, with the highest efficiency gain in medium Ag prevalence settings. Our study demonstrates the value of testing household members of near neighbours of Ag-positive and helps to inform LF surveillance strategies in Samoa and the Pacific region by promoting more efficient resource allocation.

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