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

## TL;DR

A targeted sampling method found more lymphatic filariasis cases than random sampling in Samoa, especially in areas with medium infection rates.

## Contribution

Demonstrates that reactive case detection is more efficient than random sampling for LF surveillance in medium prevalence settings.

## Key findings

- Targeted sampling identified more Ag- and Mf-positive individuals than random sampling.
- Efficiency gains were highest in medium Ag prevalence villages.
- Reactive case detection improved resource allocation for LF surveillance in Samoa.

## Abstract

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.

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.

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).

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.

Effective surveillance activities are essential for achieving elimination targets for lymphatic filariasis (LF). Cost-effective surveillance strategies are needed to locate infections for targeted treatment of individuals or high-risk communities. The aim of this study was to investigate the efficiency of targeted sampling using reactive case detection compared to random sampling for locating antigen (Ag)-positive and microfilaria (Mf)-positive individuals in Samoa and explore how differences in efficiency gains depend on the Ag prevalence in the village. In 2023, six villages with varying Ag prevalence in 2019 were surveyed using both random and targeted sampling strategies. For the randomly selected group, we selected 15 houses in each village and invited all household residents aged five years and over to participate. The targeted group consisted of residents in these same villages who lived within 200 metres of an Ag-positive participant from the 2019 survey. Finger prick blood samples were collected and tested for Ag and Mf. Overall, significantly more cases were identified per person tested using reactive case detection. This efficiency gain was greatest in the medium prevalence villages. These results show that targeted surveys of households neighbouring the residence of an Ag-positive person may be more efficient than random sampling, particularly in medium Ag prevalence villages. The findings could help improve the design of future LF surveys in Samoa and other Pacific Island countries.