Lymphatic filariasis (LF) is a neglected mosquito-borne parasitic disease caused by three species of filarial parasites—Wuchereria bancrofti, Brugia malayi and Brugia timori [1]. In May 1997, the World Health Assembly passed a resolution calling for the global elimination of LF as a public health problem by the year 2020. The World Health Organisation (WHO) subsequently launched the Global Programme to Eliminate Lymphatic Filariasis (GPELF) in 2000 to facilitate this aim [2]. In 1999, the Pacific Programme for the Elimination of Lymphatic Filariasis (PacELF) was formed to coordinate regional efforts toward elimination in the 22 Pacific Island countries and territories (PICTs) by 2020, utilising a strategy of annual mass drug administration (MDA) of a single dose of diethylcarbamazine (DEC) plus albendazole for the entire at-risk population [3].
The WHO criteria for ceasing MDA after a minimum of five effective annual rounds (coverage exceeding 65% of the total population) in areas where Aedes mosquitos are the primary vector is <1% antigenemia in a transmission assessment survey (TAS) of 6- to 7-year-old children. Critical cut-off values are calculated based on sample sizes designed so that a TAS evaluation unit (EU) has at least a 75% chance of passing if antigenemia is 0.5% and no more than 5% chance of passing if antigenemia is ≥1% [4]. Prior to the development of the TAS, PacELF had established separate criteria for ceasing MDA if <1% filarial antigenemia (<2% upper 95% CI) across all age groups, based on the results of a population-based cluster survey (C-survey).
American Samoa is an unincorporated territory of the USA. The population of 54,454 (in 2015) inhabits the islands of Tutuila, Aunu’u, Ofu-Olosega and Ta’u. The capital, Pago Pago, is located on Tutuila, the largest island, where >95% of the population reside. The territory was partitioned from neighbouring Samoa in 1899: however, the two communities continue to share strong family, cultural, linguistic and economic bonds. LF in American Samoa is caused by the diurnally sub-periodic Wuchereria bancrofti, transmitted predominantly by the day-biting mosquito Aedes polynesiensis, with the night-biting Aedes samoanus as a secondary vector [5,6,7].
LF prevalence surveys have been conducted in American Samoa since 1923, with microfilaremia (Mf) prevalence in pre-PacELF surveys as high as 21% in 1962 (n = 1000). MDA with DEC (6 mg/kg monthly for a total of 72 mg/kg over a year) was undertaken in 1963 and 1965. A follow-up survey in 1968 found Mf prevalence had decreased to 0.3% (n = 1053 across 13 villages) [3]. No further MDA interventions were recorded until the commencement of PacELF and the national elimination programme in 1999, when a nationwide convenience survey of 18 villages established a baseline prevalence of 16.5% (n = 3018) filarial antigenemia by immunochromatographic card test (ICT) [3]. Seven rounds of annual MDA followed during 2000–2006, targeting the whole population except pregnant women, children less than 2 years old and the severely ill [8].
MDA coverage in American Samoa was initially poor, ranging from 19% in 2000 to 49% in 2002. From 2002 to 2005, the national programme was independently evaluated using a variety of formative research methods including focus groups with drug distributors (programme directors, nurses, health assistants and volunteers); a multi-stage household cluster survey of community knowledge, attitudes and practices; and key informant interviews with church leaders. The programme evaluation resulted in significant changes to community mobilisation, behaviour change communication and drug delivery strategies. Notably, the evaluation identified the involvement of churches as a key driver of improved programme coverage, with over half of the population receiving treatment in conjunction with church attendance [9]. Coverage increased to 71% in 2003 and was sustained at a relatively high level in 2004 (65%), 2005 (67%) and 2006 (70%) [9].
A significant decrease in LF antigen prevalence was observed in sentinel villages between 2001 and 2006, coinciding with the improvements in MDA participation and coverage [10]. Two spot check village surveys in 2006 (a total of four villages) found higher prevalence outside of sentinel villages [8, 10, 11].
This paper reports on the results of a 2007 population-based PacELF C-survey and compares the proportion of antigen-positive adults in 2007 with those found in a 2010 study for corresponding villages. We aimed to identify potential risk factors for infection in 2007 and examine small-scale changes in antigenemia over time by comparing village-level adult seroprevalence between the 2007 and 2010 surveys. Furthermore, we aimed to reflect on how these small-scale variations in disease transmission may affect post-MDA surveillance practices and strategies.