Modelling lymphatic filariasis elimination in American Samoa: GEOFIL predicts need for new targets and six rounds of mass drug administration

Background: As part of the global effort to eliminate the debilitating mosquito-borne disease lymphatic filariasis (LF), seven rounds of two-drug (diethylcarbamazine and albendazole) mass drug administration (MDA) were conducted in American Samoa over 2000–2006. However subsequent surveys demonstrated ongoing transmission prompting further rounds of three-drug (diethylcarbamazine, albendazole, and ivermectin) MDA starting in 2018. Methods: We extend GEOFIL, a spatially-explicit agent-based model of LF transmission to predict the probability and timing of the local elimination or resurgence of LF for different MDA scenarios starting in 2018: two-drug vs. three-drug MDA, two to seven annual rounds, and population coverage rates of 55–75%. We developed an interactive visualisation comparing the effect of MDA strategies on different outcomes. Results: At least six annual rounds of three-drug MDA treating 75% of the population were required to achieve LF elimination in American Samoa by 2035 in > 50% of simulations. In scenarios where MDA did not achieve elimination, prevalence doubled approximately every three years, even if MDA reduced antigen prevalence to <1% (the target recommended by the World Health Organisation). Prevalence in six- and seven-year-old children was approximately one quarter of the prevalence in the general population. Conclusion: The three rounds of three-drug MDA conducted in 2018, 2019, and 2021 may have come close to WHO targets but are unlikely to interrupt LF transmission in American Samoa without further interventions. The recommended post-MDA surveillance strategy of testing primarily six and seven-year-old children will delay detection of resurgence compared to population representative surveys. The recommended elimination targets (reducing antigen prevalence below 0.5%, 1%, or 2%) may not be sufficient to interrupt transmission in countries with LF epidemiology like American Samoa. Alternative surveillance strategies and interventions designed to identify and eliminate spatially localized residual transmission may need to be considered. Interactive visualisations may assist decision-makers to choose locally appropriate strategies

Lymphatic Filariasis in 2016 in American Samoa: Identifying Clustering and Hotspots Using Non-Spatial and Three Spatial Analytical Methods

Background: American Samoa completed seven rounds of mass drug administration from 2000–2006 as part of the Global Programme to Eliminate Lymphatic Filariasis (LF). However, resurgence was confirmed in 2016 through WHO-recommended school-based transmission assessment survey and a community-based survey. This paper uses data from the 2016 community survey to compare different spatial and non-spatial methods to characterise clustering and hotspots of LF. Method: Non-spatial clustering of infection markers (antigen [Ag], microfilaraemia [Mf], and antibodies (Ab [Wb123, Bm14, Bm33]) was assessed using intra-cluster correlation coefficients (ICC) at household and village levels. Spatial dependence, clustering and hotspots were examined using semivariograms, Kulldorf’s scan statistic and Getis-Ord Gi* statistics based on locations of surveyed households. Results: The survey included 2671 persons (750 households, 730 unique locations in 30 villages). ICCs were higher at household (0.20–0.69) than village levels (0.10–0.30) for all infection markers. Semivariograms identified significant spatial dependency for all markers (range 207–562 metres). Using Kulldorff’s scan statistic, significant spatial clustering was observed in two previously known locations of ongoing transmission: for all markers in Fagali’i and all Abs in Vaitogi. Getis-Ord Gi* statistic identified hotspots of all markers in Fagali’i, Vaitogi, and Pago Pago-Anua areas. A hotspot of Ag and Wb123 Ab was identified around the villages of Nua-Seetaga-Asili. Bm14 and Bm33 Ab hotspots were seen in Maleimi and Vaitogi-Ili’ili-Tafuna. Conclusion: Our study demonstrated the utility of different non-spatial and spatial methods for investigating clustering and hotspots, the benefits of using multiple infection markers, and the value of triangulating results between method

Comparison of immunochromatographic test (ICT) and filariasis test strip (FTS) for detecting lymphatic filariasis antigen in American Samoa, 2016

Circulating filarial antigen (Ag) prevalence, measured using rapid point-of-care tests, is the standard indicator used for monitoring and surveillance in the Global Program to Eliminate Lymphatic Filariasis. In 2015, the immunochromatographic test (ICT) was replaced with the filariasis test strip (FTS), which has higher reported sensitivity. Despite differences in sensitivity, no changes in recommended surveillance targets were made when the FTS was introduced. In 2016, we conducted lymphatic filariasis surveys in American Samoa using FTS, which found higher Ag prevalence than previous surveys that used ICT. To determine whether the increase was real, we assessed the concordance between FTS and ICT results by paired testing of heparinised blood from 179 individuals (63% FTS-positive). ICT had 93.8% sensitivity and 100% specificity for identifying FTS-positive persons, and sensitivity was not associated with age, gender, or presence of microfilariae. Based on these findings, if ICT had been used in the 2016 surveys, the results and interpretation would have been similar to those reported using FTS. American Samoa would have failed Transmission Assessment Survey (TAS) of Grade 1 and 2 children with either test, and community prevalence would not have been significantly different (4.1%, 95% CI, 3.3–4.9% with FTS vs. predicted 3.8%, 95%, CI: 3.1–4.6% with ICT)

GEOFIL: a spatially-explicit agent-based modelling framework for predicting the long-term transmission dynamics of lymphatic filariasis in American Samoa

In this study, a spatially-explicit agent-based modelling framework GEOFIL was developed to predict lymphatic filariasis (LF) transmission dynamics in American Samoa. GEOFIL included individual-level information on age, gender, disease status, household location, household members, workplace/school location and colleagues/schoolmates at each time step during the simulation. In American Samoa, annual mass drug administration from 2000 to 2006 successfully reduced LF prevalence dramatically. However, GEOFIL predicted continual increase in microfilaraemia prevalence in the absence of further intervention. Evidence from seroprevalence and transmission assessment surveys conducted from 2010 to 2016 indicated a resurgence of LF in American Samoa, corroborating GEOFIL’s predictions. The microfilaraemia and antigenaemia prevalence in 6-7-yo children were much lower than in the overall population. Mosquito biting rates were found to be a critical determinant of infection risk. Transmission hotspots are likely to disappear with lower biting rates. GEOFIL highlights current knowledge gaps, such as data on mosquito abundance, biting rates and within-host parasite dynamics, which are important for improving the accuracy of model predictions

An MHC-I Cytoplasmic Domain/HIV-1 Nef Fusion Protein Binds Directly to the μ Subunit of the AP-1 Endosomal Coat Complex

The down-regulation of the major histocompatibility complex class I (MHC-I) from the surface of infected cells by the Nef proteins of primate immunodeficiency viruses likely contributes to pathogenesis by providing evasion of cell-mediated immunity. HIV-1 Nef-induced down-regulation involves endosomal trafficking and a cooperative interaction between the cytoplasmic domain (CD) of MHC-I, Nef, and the clathrin adaptor protein complex-1 (AP-1). The CD of MHC-I contains a key tyrosine within the sequence YSQA that is required for down-regulation by Nef, but this sequence does not conform to the canonical AP-binding tyrosine-based motif Yxxphi, which mediates binding to the medium (micro) subunits of AP complexes. We previously proposed that Nef allows the MHC-I CD to bind the mu subunit of AP-1 (micro1) as if it contained a Yxxphimotif.Here, we show that a direct interaction between the MHC-I CD/Nef and micro1 plays a primary role in the down-regulation of MHC-I: GST pulldown assays using recombinant proteins indicated that most of the MHC-I CD and Nef residues that are required for the down-regulation in human cells contribute to direct interactions with a truncated version of micro1. Specifically, the tyrosine residue of the YSQA sequence in the MHC-I CD as well as Nef residues E62-65 and P78 each contributed to the interaction between MHC-I CD/Nef and micro1 in vitro, whereas Nef M20 had little to no role. Conversely, residues F172/D174 and V392/L395 of the binding pocket on micro1 for Yxxphi motifs were required for a robust interaction.These data indicate that the MHC-I cytoplasmic domain, Nef, and the C-terminal two thirds of the mu subunit of AP-1 are sufficient to constitute a biologically relevant interaction. The data also reveal an unexpected role for a hydrophobic pocket in micro1 for interaction with MHC-I CD/Nef

Detecting and confirming residual hotspots of lymphatic filariasis transmission in American Samoa 8 years after stopping Mass Drug Administration

The Global Programme to Eliminate Lymphatic Filariasis (LF) aims to eliminate the disease as a public health problem by 2020 by conducting mass drug administrations (MDA) and controlling morbidity. Once elimination targets have been reached, surveillance is critical for ensuring that programmatic gains are sustained and challenges include timely identification of residual areas of transmission. WHO guidelines encourage cost-efficient surveillance, such as integration with other population-based surveys. In American Samoa, where LF is caused by Wuchereria bancrofti, and Aedes polynesiensis is the main vector, the LF elimination program has made significant progress. Seven rounds of MDA (albendazole and diethycarbamazine) were completed from 2000 to 2006, and Transmission Assessment Surveys were passed in 2010/2011 and 2015. However, a seroprevalence study using an adult serum bank collected in 2010 detected two potential residual foci of transmission, with Og4C3 antigen (Ag) prevalence of 30.8% and 15.6%. We conducted a follow up study in 2014 to verify if transmission was truly occurring by comparing seroprevalence between residents of suspected hotspots and residents of other villages. In adults from non-hotspot villages (N=602), seroprevalence of Ag (ICT or Og4C3), Bm14 antibody (Ab) and Wb 123 Ab were 1.2% (95% CI 0.6-2.6%), 9.6% (95% CI 7.5-12.3%) and 10.5% (95% CI 7.6-14.3%), respectively. Comparatively, adult residents of Fagali'i (N=38) had significantly higher seroprevalence of Ag (26.9%, 95% CI 17.3-39.4%), Bm14 Ab (43.4%, 95% CI 32.4-55.0%), and Wb123 Ab 55.2% (95% CI 39.6-69.8%). Adult residents of Ili'ili/Vaitogi/Futiga (N=113) also had higher prevalence of Ag and Ab, but differences were not statistically significant. The presence of transmission was demonstrated by 1.1% Ag prevalence (95% CI 0.2% to 3.1%) in 283 children aged 7-13 years who lived in one of the suspected hotspost, including a 9 year old child. Our results provide field evidence that integrating LF surveillance with other surveys is effective and feasible for identifying potential hostpots, and conducting surveillance at worksites provides an efficient method of sampling large populations of adults

Potential use of antibodies to provide an earlier indication of lymphatic filariasis resurgence in post–mass drug ad ministration surveillance in American Samoa

Background: Under the Global Programme to Eliminate Lymphatic Filariasis (LF), American Samoa conducted 7 rounds of mass drug administration (MDA) between 2000 and 2006. The territory passed transmission assessment surveys (TASs) in 2011 (TAS-1) and 2015 (TAS-2). In 2016, the territory failed TAS-3, indicating resurgence. This study aims to determine if antibodies (Abs) may have provided a timelier indication of LF resurgence in American Samoa. Methods: We examined school-level antigen (Ag) and Ab status (presence/absence of Ag- and Ab-positive children) and prevalence of single and combined Ab responses to Wb123, Bm14, and Bm33 Ags at each TAS. Pearson chi-square test and logistic regression were used to examine associations between school-level Ab prevalence in TAS-1 and TAS-2 and school-level Ag status in TAS-3. Results: Schools with higher prevalence of Wb123 Ab in TAS-2 had higher odds of being Ag-positive in TAS-3 (odds ratio [OR] 24.5, 95% confidence interval [CI] 1.2–512.7). Schools that were Ab-positive for WB123 plus Bm14, Bm33, or both Bm14 and Bm33 in TAS-2 had higher odds of being Ag-positive in TAS-3 (OR 16.0–24.5). Conclusion: Abs could provide earlier signals of resurgence and enable a timelier response. The promising role of Abs in surveillance after MDA and decision making should be further investigated in other settings

Potential strategies for strengthening surveillance of lymphatic filariasis in American Samoa after mass drug administration: reducing ‘number needed to test’ by 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. Using data from the 2016 survey, this study aims to i) investigate antibody prevalence in TAS-3 and the community survey, ii) identify risk factors associated with being seropositive for Ag and anti-filarial antibodies, and iii) compare the efficiency of different sampling strategies for identifying seropositive persons in the post-MDA setting. 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%). Multivariable logistic regression was used to identify risk factors for being seropositive for Ag and antibodies. 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). Ag prevalence was 20.7% (95%CI 9.7–53.5%) in households of Ag-positive children identified in TAS-3. We used NNTestav (average number needed to test to identify one positive) to compare the efficiency of the following strategies for identifying persons who were seropositive for Ag and each antibody: i) TAS of 6–7 year-old children, ii) population representative surveys of older age groups, and iii) targeted surveillance of subpopulations at higher risk of being seropositive (older ages, householders of Ag-positive TAS children, and known hotspots). For 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 or population representative survey of older ages), followed by strategies that target subpopulations and/or locations with low NNTestav. This approach could potentially improve the efficiency of identifying remaining infected persons and residual hotspots. Surveillance programs should also explore the utility of antibodies as indicators of transmission

Genetic epidemiology of lymphatic filariasis in American Samoa after mass drug administration

Over 892 million people in 48 countries are at risk of infection by nematodes that cause lymphatic filariasis. As part of the Global Programme to Eliminate Lymphatic Filariasis, mass drug administration is distributed to communities until surveillance indicates infection rates are below target prevalence thresholds. In some countries, including American Samoa, lymphatic filariasis transmission persists despite years of mass drug administration and/or has resurged after cessation. Nothing is known about the population genetics of Wuchereria bancrofti worms in Polynesia, or whether local transmission is persisting and/or increasing due to inadequate mass drug administration coverage, expansion from residual hotspots, reintroduction from elsewhere, or a combination. We extracted DNA from microfilariae on blood slides collected during prevalence surveys in 2014 and 2016, comprising 31 pools of five microfilariae from 22 persons living in eight villages. We sequenced 1104 bp across three mitochondrial markers (ND4, COI, CYTB). We quantified parasite genetic differentiation using variant calls and estimated haplotypes using principal components analysis, F-statistics, and haplotype networks. Of the variants called, all but eight were shared across the main island of Tutuila, and three of those were from a previously described hotspot village, Fagali’i. Genotypic data did not support population genetic structure among regions or villages in 2016, although differences were observed between worms collected in Fagali’i in 2014 and those from 2016. Because estimated haplotype frequency varied between villages, these statistics suggested genetic differentiation, but were not consistent among villages. Finally, haplotype networks demonstrated American Samoan sequence clusters were related to previously published sequences from Papua New Guinea. These are, to our knowledge, the first reports of W. bancrofti genetic variation in Polynesia. The resurgent parasites circulating on the main island of American Samoa represent a single population. This study is the first step towards investigating how parasite population structure might inform strategies to manage resurgence and elimination of lymphatic filariasis

Triple-drug treatment is effective for lymphatic filariasis microfilaria clearance in Samoa

Following the first triple-drug mass drug administration (MDA) for lymphatic filariasis in Samoa in 2018, unexpected persistence of microfilaria (Mf) positivity in 18 (15%) of 121 antigen-positive persons was observed in a nationwide household survey 1–2 months later. Of the 18 Mf positive persons, 14 reported taking the MDA, raising concerns about MDA efficacy. In 2019, 5–6 months after the 2018 survey, a monitored treatment study was done to evaluate directly observed weight-based treatment in these Mf positive individuals. Mf presence and density were assessed before and 7 days after treatment, using 1 mL membrane filtered venous blood, and 60 uL thick blood films on slides prepared from venous or fingerprick blood. All 14 participants were still Mf positive on filters from venous blood pre-treatment samples, but two were negative by slide made from the same samples. Mf were cleared completely by day 7 in 12 of 13 participants followed up, and by day 30 in the remaining participant. Filtered blood using EDTA samples (to reduce clumping of Mf) is preferred over slides alone for improving the likelihood of detecting Mf and estimating their density. The triple-drug MDA strategy was effective at clearing Mf when given and taken at the correct dose