Evaluation of sampling strategies for assessing lymphatic filariasis endemic status of a non-MDA district in South India.

BACKGROUND

India is moving towards the Lymphatic Filariasis (LF) elimination goal in 2027. Documentation on LF transmission status in the non-endemic and unsurveyed areas is crucial for WHO to certify that LF has been eliminated as a public health problem in the country. Appropriate sampling strategy is necessary to determine LF transmission status in the areas not under mass drug administration (MDA). We evaluated four different sampling strategies to identify the best tool(s) and indicator(s) that could be used to assess transmission interruption in a non-MDA district.

METHODOLOGY

This study was conducted in Salem district in Tamil Nadu, India, during the period from June 2022 to June 2023. Four different sampling strategies, namely: (i) School based Mini-TAS (Mini-sTAS, n = 480), (ii) Community based Mini-TAS (Mini-cTAS, n = 480), (iii) Molecular xenomonitoring surveys (MX, n = 7500), and (iv) Purposive sampling of five high-risk sites (human, n = 1500 and vector surveys, n = 3750), were evaluated for their ability to assess LF transmission status in the area. These strategies were compared with a large-scale community survey (n = 10200) in 30 randomly selected sites (villages/wards) assessing human infection in the study area. While Filariasis Test strips (FTS) were used to assess circulating filarial antigen (CFA), night blood smears from CFA positives were collected to assess microfilaraemia (Mf). Mosquito samples collected from MX surveys were subjected to polymerase chain reaction (PCR) assays to assess the infection in vectors.

RESULTS

The results of the large-scale survey showed that the overall prevalence of CFA was 0.2% (95% CI: 0.1%-0.3%), below the critical threshold of 2%. Mini-sTAS and Mini-cTAS both showed that the CFA prevalence among children was below the elimination threshold of 2%. MX surveys showed the vector infection prevalence of 0.03% (95% CI: 0.01%-0.09%). These three strategies showed that the district is non-endemic and corroborate the results of a large-scale community survey. However, under the purposive sampling strategy, in two high-risk sites, either human or vector infection prevalence was above the respective elimination thresholds. Further, the administrative blocks in which these sites were situated shared borders with known LF endemic districts.

CONCLUSIONS

The sampling strategies that may be recommended for a non-MDA or unsurveyed district to assess LF transmission status would be to use (i) school- or community-based Mini-TAS or (ii) conduct MX surveys to classify them as endemic or non-endemic based on the pre-defined thresholds by WHO. For further confirmation, serosurveys among adults may be conducted in five purposively selected high-risk sites to identify pockets of LF transmission, if any.