Reactive case detection can improve the efficiency of lymphatic filariasis surveillance compared to random sampling, Samoa 2023

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.

Fig 1. Location of the six primary sampling units (PSUs) in Samoa that were surveyed for lymphatic filariasis in 2023 using both random and targeted sampling strategies.

Spatial data on country, island, region, and village boundaries in Samoa were obtained from the Pacific Data Hub (pacificdata.org accessed on 8 July 2020) and DIVA-GIS (diva-gis.org, accessed on 12 August 2019) under an open access licence available at https://pacific-data.sprep.org/resource/public-data-license-agreement-0.

Fig 2. a) Percentage of Ag-positive households and b) adjusted Ag prevalence and 95% confidence intervals in the randomly selected (blue) and targeted (orange) groups for each 2019 Ag prevalence category (low [3-5%], medium [6-7%] and high [13-17%]) in six primary sampling units (PSUs) surveyed in 2023 in Samoa.

Fig 3. a) Percentage of Mf-positive households and b) adjusted Mf prevalence and 95% confidence intervals in the randomly selected (blue) and targeted (orange) groups for each 2019 Ag prevalence category (low [3-5%], medium [6-7%] and high [13-17%]) in six primary sampling units (PSUs) surveyed in 2023 in Samoa.

Fig 4. Ratio of a) Ag-positive individuals and b) Mf-positive individuals in the targeted group compared to the randomly selected group, including 95% confidence intervals.

Values presented for each 2019 Ag prevalence category (low [3-5%], medium [6-7%] and high [13-17%]) - in six primary sampling units (PSUs) surveyed in 2023 in Samoa.