Lymphatic filariasis epidemiology in Samoa in 2018: Geographic clustering and higher antigen prevalence in older age groups

Abstract

Background Samoa conducted eight nationwide rounds of mass drug administration (MDA) for lymphatic filariasis (LF) between 1999 and 2011, and two targeted rounds in 2015 and 2017 in North West Upolu (NWU), one of three evaluation units (EUs). Transmission Assessment Surveys (TAS) were conducted in 2013 (failed in NWU) and 2017 (all three EUs failed). In 2018, Samoa was the first in the world to distribute nationwide triple-drug MDA using ivermectin, diethylcarbamazine, and albendazole. Surveillance and Monitoring to Eliminate LF and Scabies from Samoa (SaMELFS Samoa) is an operational research program designed to evaluate the effectiveness of triple-drug MDA on LF transmission and scabies prevalence in Samoa, and to compare the usefulness of different indicators of LF transmission. This paper reports results from the 2018 baseline survey and aims to i) investigate antigen (Ag) prevalence and spatial epidemiology, including geographic clustering; ii) compare Ag prevalence between two different age groups (5–9 years versus ≥10 years) as indicators of areas of ongoing transmission; and iii) assess the prevalence of limb lymphedema in those aged ≥15 years. Methods A community-based cluster survey was conducted in 30 randomly selected and five purposively selected clusters (primary sampling units, PSUs), each comprising one or two villages. Participants were recruited through household surveys (age ≥5 years) and convenience surveys (age 5–9 years). Alere Filariasis Test Strips (FTS) were used to detect Ag, and prevalence was adjusted for survey design and standardized for age and gender. Adjusted Ag prevalence was estimated for each age group (5–9, ≥10, and all ages ≥5 years) for random and purposive PSUs, and by region. Intraclass correlation (ICC) was used to quantify clustering at regions, PSUs, and households. Results A total of 3940 persons were included (1942 children aged 5–9 years, 1998 persons aged ≥10 years). Adjusted Ag prevalence in all ages ≥5 years in randomly and purposively selected PSUs were 4.0% (95% CI 2.8–5.6%) and 10.0% (95% CI 7.4–13.4%), respectively. In random PSUs, Ag prevalence was lower in those aged 5–9 years (1.3%, 95% CI 0.8–2.1%) than ≥10 years (4.7%, 95% CI 3.1–7.0%), and poorly correlated at the PSU level (R-square = 0.1459). Adjusted Ag prevalence in PSUs ranged from 0% to 10.3% (95% CI 5.9–17.6%) in randomly selected and 3.8% (95% CI 1.3–10.8%) to 20.0% (95% CI 15.3–25.8%) in purposively selected PSUs. ICC for Ag-positive individuals was higher at households (0.46) compared to PSUs (0.18) and regions (0.01). Conclusions Our study confirmed ongoing transmission of LF in Samoa, in accordance with the 2017 TAS results. Ag prevalence varied significantly between PSUs, and there was poor correlation between prevalence in 5–9 year-olds and older ages, who had threefold higher prevalence. Sampling older age groups would provide more accurate estimates of overall prevalence, and be more sensitive for identifying residual hotspots. Higher prevalence in purposively selected PSUs shows local knowledge can help identify at least some hotspots.

Triple-Drug Treatment Is Effective for Lymphatic Filariasis Microfilaria Clearance in Samoa

“…For all blood samples with a positive FTS result, thick blood slides were prepared according to WHO guidelines as previously described [ 9 ]. Three 20 μL lines of blood were placed onto a single slide using a micropipette, and up to three slides were prepared if sufficient blood was available.…”

“…However, some countries are experiencing challenges including stagnation of progress, delay in MDA initiation, persistence of transmission hotspots, failing Transmission Assessment Surveys (TAS) during post-MDA surveillance, and resurgence (increasing LF antigen prevalence) after MDA had stopped [ 6 , 7 , 8 , 9 ]. These countries include Samoa and American Samoa which have efficient day-biting Aedes polynesiensis and other vectors, including night biting species, with exposure 24 h a day and few options for vector control.…”

“…A large population representative LF community survey as part of the Surveillance and Monitoring for Elimination of LF and Scabies in Samoa (SaMELFS) project was conducted in Oct-Nov 2018, 8–11 weeks after the August 2018 MDA, and showed age- and gender-standardized antigen prevalence of 4.0% (95% CI 2.8–5.6%) in 3940 participants aged 5 years and over [ 9 ]. The MDA coverage was reported to be very good with 80.2% of the total population reporting taking MDA [ 17 ].…”

“…The MDA coverage was reported to be very good with 80.2% of the total population reporting taking MDA [ 17 ]. Amongst the 122 Ag-positive participants, slides were available for 121 (99.2%) [ 9 ], and 18 (14.9%) were Mf-positive. Surprisingly, 14 of the 18 Mf-positive persons reported taking the IDA pills in 2018, raising concerns about the effectiveness of IDA in Samoa.…”

“…Surprisingly, 14 of the 18 Mf-positive persons reported taking the IDA pills in 2018, raising concerns about the effectiveness of IDA in Samoa. The estimated post-MDA Mf prevalence of 0.6% (0.3–1.0%) nationwide and 1.7% (0.7–4.1%) in known hotspots [ 9 ] also caused concern that IDA might not be as effective as expected in Samoa.…”