Current WHO recommendations for lymphatic filariasis (LF) surveillance advise programs to implement activities to monitor for new foci of transmission after stopping mass drug administration (MDA). A current need in the global effort to eliminate LF is to standardize diagnostic tools and surveillance activities beyond the recommended transmission assessment survey (TAS). TAS was first conducted in American Samoa in 2011 (TAS 1) and a repeat TAS was carried out in 2015 (TAS 2). Circulating filarial antigen (CFA) and serologic results from both surveys were analyzed to determine whether interruption of LF transmission has been achieved in American Samoa. A total of 1,134 and 864 children (5–10 years old) were enrolled in TAS 1 and TAS 2, respectively. Two CFA-positive children were identified in TAS 1, and one CFA-positive child was identified in TAS 2. Results of both surveys were below the threshold for which MDA was warranted. Additionally, 1,112 and 836 dried blood spots from TAS 1 and TAS 2, respectively were tested for antibodies to Wb123, Bm14 and Bm33 by luciferase immunoprecipitation system (LIPS) assay and multiplex bead assay. In 2011, overall prevalence of responses to Wb123, Bm14, and Bm33 was 1. 0%, 6. 8% and 12. 0%, respectively. In 2015, overall prevalence of positive Bm14 and Bm33 responses declined significantly to 3. 0% (p<0. 001) and 7. 8% (p = 0. 013), respectively. Although passing TAS 1 and TAS 2 and an overall decline in the prevalence of antibodies to Bm14 and Bm33 between these surveys suggests decreased exposure and infection among young children, there were persistent responses in some schools. Clustering and persistence of positive antibody responses in schools may be an indication of ongoing transmission. There is a need to better understand the limitations of current antibody tests, but our results suggest that serologic tools can have a role in guiding programmatic decision making.
Lymphatic filariasis (LF), endemic in 72 countries, is a debilitating mosquito-transmitted parasitic disease caused by filarial worms (Wuchereria bancrofti and Brugia spp.) [1]. In 1997, at the 50th World Health Assembly (WHA), a resolution was passed to eliminate LF as a public health problem by 2020 [2]. Shortly thereafter, in 1999, the Pacific Program for the Elimination of Lymphatic Filariasis (PacELF) was established to eliminate the disease in the Pacific Region through a strategy of annual rounds of mass drug administration (MDA) [3]. The following year, the Global Program to Eliminate Lymphatic Filariasis (GPELF) was established to assist all LF-endemic countries in achieving this elimination goal through the same MDA strategy. At the start of GPELF it was estimated that approximately 1. 4 billion people were at risk for infection. By the end of 2016, MDA had been implemented in 66 of 72 LF-endemic countries, with a cumulative total of 6. 7 billion treatments delivered since the start of GPELF [4]. After multiple rounds of MDA, LF elimination programs must be able to determine when it is appropriate to stop treatment. The World Health Organization (WHO) -recommended transmission assessment survey (TAS) was designed as a decision-making tool to determine when transmission of LF is presumed to have reached a level low enough that it cannot be sustained even in the absence of MDA [5]. In areas where W. bancrofti is the principal LF pathogen, infection is assessed in the TAS by measuring circulating filarial antigen (CFA). Since its integration into national programs in 2011, TAS has successfully been implemented across LF endemic countries, and based on the results, MDA has been discontinued in multiple locations. The global number of people requiring MDA has been reduced from 1. 4 billion in 2000 to 856. 4 million in 2016 [4]. Effective monitoring and evaluation (M&E) is not only necessary during the MDA period but important throughout the lifespan of the LF program, including after MDA has stopped. Current WHO recommendations for post-MDA surveillance include periodic surveys: repeating TAS twice at 2- to 3-year intervals after stopping MDA. Beyond the TAS, post-MDA surveillance guidance has not been standardized. Current WHO recommendations for surveillance advise programs to implement activities to monitor for new foci of transmission through the assessment of microfilaremia, antigenemia, or antibodies [5]. After effective MDA, microfilaremia and antigenemia begin to decline in populations and become increasingly difficult to detect [6]. Detection of antifilarial antibodies appears to provide the earliest indicator of filarial exposure [7], and the absence of detectable antibody responses may provide evidence that transmission has been interrupted. Surveys conducted in 1999 indicated that 17% of residents in 18 villages in American Samoa were infected with W. bancrofti [3]. This established American Samoa as one of the areas with the highest filarial infection levels in the Pacific Region and the only U. S. territory endemic for LF. The American Samoa Department of Health (DOH) started MDA in 2000. Annual MDA coverage was low (<50%) prior to 2003. After reassessment and modification of the communication and distribution strategies, the program treated an estimated 70% and 65% of the population in 2003 and 2004, respectively [8]. Results from surveys in four sentinel sites showed an overall decline in CFA levels from 13% in 2003 to 0. 95% in 2006 [9]. An island-wide survey was conducted in 2007, and CFA prevalence was 2. 3%, with the majority of the antigenemia detected in adults [10]. Because LF was presumed to be at very low levels, minimal programmatic activities were conducted from 2008–2010. In accordance with WHO recommendations, TAS 1 was conducted in American Samoa in 2011 and was repeated in 2015 (TAS 2). The DOH opted to include antifilarial antibody testing in both surveys to complement antigen testing. In this paper we report CFA and serologic results from the two TAS that were conducted to determine whether or not interruption of LF transmission has been achieved in American Samoa.
The surveys were approved by the DOH Institutional Review Board (IRB) and the U. S. Centers for Disease Control and Prevention (CDC) as program evaluation, non-research. In preparation for the TAS, survey details were described in a written document distributed to school officials and parents or guardians of potential participants. In accordance with DOH and Department of Education policies, parents or guardians provided written permission for participation of children. Additionally, children ≥7 years of age were asked to provide oral assent for their participation on the day of the survey. All data were collected electronically, and identifiable information was kept confidential and maintained by using a secure database with access restricted to essential survey personnel. American Samoa, a U. S. territory, is located in the South Pacific comprising of seven small islands and atolls. More than 90% of the total population live on the main island of Tutuila with the remainder of the residents dispersed on the adjacent island of Aunu’u and the outer Manu’a islands of Ta’u, Ofu and Olosega. Tutuila and Aunu’u comprised the evaluation unit for TAS. TAS 1 was carried out in February 2011 and TAS 2 was conducted in April 2015. Surveys were implemented according to WHO guidelines for conducting TAS in areas where Aedes spp. are the main LF vectors [5]. Because of high school enrollment rates (>95%), school-based surveys were conducted at both time points, and grades 1 and 2 were used as a proxy for the recommended age (6–7 years). Systematic sampling was recommended for both surveys, but due to low rates of consent, all children with signed consent forms were enrolled. The target sample sizes in 2011 and 2015 were 1,042 and 1,014, respectively. The critical cutoff, the maximum number of observed positive results that is consistent with a threshold of < 1%, for both surveys was six antigen-positive children. For both surveys approximately 160 μL of blood was collected via a single finger stick into an EDTA-coated blood collection tube (Ram Scientific, Yonkers, NY). One hundred microliters of blood was used for the detection of CFA by immunochromatographic card test (ICT) (Alere; Scarborough, ME). The cards were read at 10 min and marked as either positive or negative according to the manufacturer’s instructions. The remaining 60 μL of blood (10 μL per extension x 6 extensions) was spotted onto filter paper (Cellabs, Sydney, Australia), dried and stored at -20°C until shipped to National Institutes of Health (NIH) for antifilarial antibody testing by luciferase immunoprecipitation system (LIPS) assay [11] or CDC for testing by multiplex bead assay (MBA) [12–14] (described below). In TAS 1, IgG responses to Wb123 were determined by previously described LIPS assay [11]. One modification was made to accommodate the use of dried blood spots (DBS) instead of serum. DBS were eluted in 200 μl of PBS, and 40 μl of the eluted material was used for the assay. Cutoff values were calculated from receiver operator characteristic (ROC) curves using sera from W. bancrofti-infected patients and presumed negative sera from North Americans with no history of foreign travel. Antifilarial antibody responses to Bm14 [15] and Bm33 [16] for samples collected during TAS 1 and responses to Wb123, Bm14 and Bm33 for samples collected during TAS 2 were determined by previously described MBA [7,12–14]. Briefly, DBS were eluted to yield a sample dilution of 1: 400 in PBS buffer (pH 7. 2) containing 0. 3% Tween-20,0. 02% sodium azide, 0. 5% casein, 0. 5% polyvinyl alcohol (PVA), 0. 8% polyvinylpyrrolidone (PVP), and 3 μg/ml Escherichia coli extract. E. coli extract was added to the buffer to absorb antibodies to any residual E. coli proteins that may not have been eliminated in the antigen purification process. Samples having a coefficient of variation of >15% between duplicate wells for ≥2 positive LF antibody responses were repeated. The average of the median fluorescent intensity (MFI) values from the duplicate wells minus the background (bg) fluorescence from the buffer-only blank was reported as MFI-bg. Cutoff values were calculated from ROC curves using sera from W. bancrofti-infected patients and presumed negative sera from US citizens with no history of foreign travel. Parents or guardians of individuals who were ICT positive were notified of test results, and the children were offered a standard single dose of diethylcarbamazine (DEC) (6 mg/kg) and albendazole (400 mg). Analyses were performed in R version 3. 3. 0 [17] with the survey package [18] using a 5% level of significance. Because a high percentage of the American Samoa population of 1st and 2nd graders participated, samples from TAS 1 and TAS 2 were treated as clustered samples with a finite population correction. Differences in frequencies were evaluated with a Rao-Scott Χ2 statistic [19]. Confidence intervals for proportions utilize the incomplete beta function [20]. Changes in MFI were evaluated with the complex sampling version of Mood’s test for differences in medians [21].
A total of 1,134 children from 25 of 26 public and private elementary schools were enrolled in TAS 1; 50. 6% were male, and the mean age was 6. 8 years (range 5–10 years). Because written informed parental consent was required for participation, systematic sampling of children could not be applied as intended. All children with signed consent forms from parents/guardians were enrolled in the survey. One small private school (St. Theresa) was not sampled because of school officials refusal to participate. Demographic information was not available for 57 students from Tafuna Elementary. For 197 (17. 4%) children enrolled, no blood sample was collected or the quantity of blood collected was insufficient for testing by ICT. Of the samples tested, 2/937 (0. 2%, 95% upper confidence limit (CL) 0. 8%) were antigen positive. Both positive children were from the same school, Lupelele Elementary. Demographic information and number of samples tested by ICT are summarized by school in Table 1. By 2015, two elementary schools that had existed in 2011 were closed and six new schools had opened. A total of 864 children from all 30 public and private elementary schools were enrolled in TAS 2; 48. 4% were male, and the mean age was 7. 0 years. As with TAS 1, written parental consent was required for participation and all children with signed consent forms were enrolled in the survey. For 96 (11. 1%) children enrolled, no blood sample was collected or the quantity of blood collected was insufficient for ICT testing. Of the samples tested, 1/768 (0. 1%, 95% CL 0. 3%) was positive. The antigen-positive child was from Lupelele Elementary, the same school where the two antigen-positive children were identified in TAS 1 four years earlier. Demographic information and number of samples tested by ICT for TAS 2 are given in Table 1, stratified by school. In 2011, a total of 1,112 DBS were prepared for antibody testing. Overall prevalence of responses to Wb123, Bm14, and Bm33 was 1. 0%, 6. 8% and 12. 0%, respectively. There was at least one Wb123 antibody-positive child in 6/25 (24. 0%) schools. Distribution of responses to Bm14 and Bm33 responses was more widespread than responses to Wb123 with at least one antibody-positive child identified in 88. 0% and 80. 0% of schools, respectively. In 2015, a total of 836 DBS were collected for antibody testing. Overall prevalence of Bm14 and Bm33 responses declined significantly to 3. 0% (p<0. 001) and 7. 8% (p = 0. 013), respectively. The prevalence of Wb123 responses was 3. 6% in TAS 2, but results were not directly compared to those from TAS 1 because of the different testing platform used. The distribution of Wb123 responses was more widespread in TAS 2 than TAS 1 with at least one antibody-positive child identified in 12/30 (40. 0%) schools. Distribution of Bm14 responses was more focal in 2015 than in 2011 with at least one antibody-positive child identified in 12/30 (40. 0%) schools; only one of these 12 schools did not have any Bm14-positive children in TAS 1. Distribution of positive Bm33 responses was the most widespread (56. 7% of schools) of the three markers assessed, but was still more focally distributed in 2015 compared to 2011. Antibody responses are summarized by school in Fig 1 and Table 2. Change in MFI-bg for Bm14 and Bm33 was compared for the 22 schools included in both surveys. There were significant declines in the median quantitative MBA responses for Bm14 and Bm33 in 21/22 (95. 5%) schools. Median MFI-bg values are summarized in Table 3, stratified by school. All three ICT-positive children identified in the surveys had positive antibody responses to Wb123, Bm14 and Bm33. However, concordance of individual antibody responses for antigen-negative children was relatively poor at both time points. Only 21/161 (13. 0%) antibody-positive children in TAS 1 and 20/73 (27. 4%) positive children in TAS 2 had positive responses to at least two markers. Similar discordance was also observed for samples tested where the antigen status was unknown. Test concordance for antigen negative children is summarized in Table 4. Test concordance for children whose antigen status was unknown is summarized in Table 5.
The findings and conclusions in this report are those of the authors and do not necessarily represent the official position of the Centers for Disease Control and Prevention.
Lymphatic filariasis (LF), endemic in 72 countries, is a debilitating mosquito-transmitted parasitic disease caused by filarial worms. The Global Program to Eliminate Lymphatic Filariasis (GPELF) aims to interrupt transmission through mass drug administration (MDA) and to reduce suffering caused by the disease. At the start of GPELF in 2000 it was estimated that approximately 1. 4 billion people were at risk for infection. By the end of 2016, primarily through successful MDA programs, the global number of people requiring interventions was reduced to 856. 4 million. Current recommendations by the World Health Organization for LF surveillance advise programs to implement activities to monitor for new foci of transmission after stopping MDA. A current need in the global effort to eliminate LF is to standardize diagnostic tools and surveillance activities beyond the recommended transmission assessment survey (TAS). Two TAS were conducted in American Samoa; first in 2011 (TAS 1) and repeated in 2015 (TAS 2). In our evaluation, circulating filarial antigen and serologic results from both surveys were analyzed to determine whether interruption of LF transmission has been achieved in American Samoa. Despite passing TAS 1 and TAS 2, clustering and persistence of positive antibody responses in schools may be an indication of ongoing transmission. Results from our evaluation suggest that serologic tools can have a role in guiding programmatic decision-making.
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Comparison of antigen and antibody responses in repeat lymphatic filariasis transmission assessment surveys in American Samoa