Potential strategies for strengthening surveillance of lymphatic filariasis in American Samoa after mass drug administration: targeting older age groups, hotspots, and household members of infected persons

Under the Global Programme to Eliminate Lymphatic Filariasis (LF), American Samoa conducted mass drug administration (MDA) from 2000-2006. Despite passing Transmission Assessment Surveys (TAS) in 2011/2012 and 2015, American Samoa failed TAS-3 in 2016, with antigen (Ag) prevalence of 0.7% (95%CI 0.3-1.8%) in 6-7 year-olds. A 2016 community survey (Ag prevalence 6.2% (95%CI 4.4-8.5%) in age ≥8 years) confirmed resurgence. Here, we explore the potential of targeted strategies to strengthen post-MDA surveillance.Using Ag data plus new antibody data (Wb123, Bm14, Bm33) from the 2016 surveys, we identified risk factors for seropositivity using multivariable logistic regression. We compared TAS with strategies that targeted high-risk subpopulations (older ages, householders of Ag-positive TAS children [index children]) and/or known hotspots, and used NNTestav (average number needed to test to identify one positive) to compare sampling efficiency.Antibody prevalence in TAS-3 (n=1143) were 1.6% for Bm14 (95%CI 0.9-2.9%), 7.9% for Wb123 (95%CI 6.4-9.6%), and 20.2% for Bm33 (95%CI 16.7-24.3%); and in the community survey (n=2507), 13.9% for Bm14 (95%CI 11.2-17.2%), 27.9% for Wb123 (95%CI 24.6-31.4%), and 47.3% for Bm33 (95%CI 42.1-52.6%). Ag prevalence was 20.7% (95%CI 9.7-53.5%) in households of index children. Higher Ag prevalence was found in males (adjusted odds ratio [aOR] 3.01), age ≥18 years (aOR 2.18), residents of Fagali’i (aOR 15.81), and outdoor workers (aOR 2.61). Using Ag, NNTestav ranged from 142.5 for TAS, to <5 for households of index children. NNTestav was lower in older ages, and highest for Ag, followed by Bm14, Wb123 and Bm33 antibodies.We propose a multi-stage surveillance strategy, starting with population-representative sampling (e.g. TAS), followed by targeted strategies in subgroups and locations with low NNTestav. This approach could potentially improve the efficiency of identifying remaining infected persons and residual hotspots. The utility of antibodies in surveillance should also be explored.Lymphatic filariasis (LF) is a parasitic infection transmitted by mosquito bites. Globally, tens of millions are infected, with many disfigured and disabled by severe damage to their lymphatic systems, such as severe swelling of the legs (elephantiasis) or scrotum (hydrocele). The Global Programme to Eliminate LF (GPELF) aims to interrupt disease transmission through mass drug administration (MDA), and to control illness and suffering in affected persons. The World Health Organization recommends conducting Transmission Assessment Surveys (TAS) in school children aged 6 to 7 years, to determine if infection rates have dropped to levels where disease transmission is no longer sustainable. From 2000-2006, American Samoa conducted MDA and made significant progress towards eliminating LF. However, despite passing TAS in 2011/2012 and 2015, surveys in 2016 showed evidence of resurgence. Our study explored alternative surveillance strategies and compared their efficiency with TAS. Based on our findings, we recommended that in addition to TAS, strategies that target high-risk populations and hotspots would strengthen surveillance and help countries achieve their goals of LF elimination.