Lymphatic filariasis antigen and microfilaria epidemiology in Samoa in 2019, six months post triple-drug mass drug administration

BackgroundElimination of lymphatic filariasis (LF) as a public health problem remains an ongoing challenge in the Pacific region. In 2018, Samoa was the first country to implement a national triple-drug mass drug administration (MDA) using ivermectin, diethylcarbamazine, and albendazole (IDA). This study reports on antigen (Ag) and microfilaria (Mf) prevalence in Samoa in 2019, 6-8 months post-MDA, and evaluates the effectiveness of the intervention in reducing Ag prevalence to below a 2% threshold. We also compared the change in Ag prevalence pre- and post-MDA between 5-9-year-olds and [≥]10-year-olds to assess the suitability of young children as the target group for transmission assessment surveys (TAS). MethodologyWe surveyed 30 randomly selected and 5 purposefully selected primary sampling units (PSUs) in Samoa in 2018 (1.5-3.5 months post triple-drug MDA) and 2019 (6-8 months post triple-drug MDA). In each PSU, we conducted a community survey of 15-20 randomly selected households and a convenience survey of 5-9-year-old children. Demographic details were collected using an electronic questionnaire. A finger prick blood sample was collected from all participants to test for Ag and Mf. Prevalence estimates were adjusted for age, sex, and survey design. Principal FindingsThere was no significant change in adjusted Ag prevalence in the 30 randomly selected PSUs between 2018 (3.9% [95% CI: 2.7-5.6%]) and 2019 (4.1% [95% CI 2.7-5.9%]). In these PSUs, significantly higher Ag prevalence was observed in participants aged [≥]10 years (4.6%, 95% CIs 3.0-6.7%) compared to 5-9-year-olds (1.1%, 95% CIs 0.5-2.2%). Conclusions/SignificanceA single round of triple-drug MDA was insufficient to break LF transmission in Samoa 6-8 months post-MDA. Significantly higher Ag prevalence in participants [≥]10 years old also supports existing evidence that basing elimination thresholds on Ag prevalence among 6-7-year-olds may not be the most suitable strategy for post-MDA surveillance. Author SummaryElimination of lymphatic filariasis (LF) as a public health problem remains an ongoing challenge in many countries in the Pacific. Mass drug administration (MDA) is used to treat at-risk populations at the regional or national level with the aim of breaking transmission between humans and mosquitoes. In 2018, Samoa was the first country to distribute a national triple-drug MDA, combining ivermectin, diethylcarbamazine, and albendazole. In this study we compared the prevalence and clustering of antigen (Ag) and microfilaria (Mf) pre- and post-MDA. We observed evidence of ongoing LF transmission in Samoa 6-8 months post triple-drug MDA, with an estimated antigen prevalence in 30 randomly selected primary sampling units of 4.1% (95% CI 2.7-5.9%), and Mf-positive participants identified in all four administrative regions. This supports current guidance from the World Health Organization that multiple intervention rounds are needed to break the transmission cycle between humans and mosquitoes. Higher Ag prevalence in those aged [≥]10 years, suggests that surveillance of adults rather than children should be used to inform programmatic decisions for disease elimination. Based on the significant Ag clustering at the household level, targeted surveillance and treatment of household members of Ag-positive people is recommended.