A comprehensive assessment of lymphatic filariasis in Sri Lanka six years after cessation of mass drug administration

Abstract

Background: The Sri Lankan Anti-Filariasis Campaign conducted 5 rounds of mass drug administration (MDA) with diethycarbamazine plus albendazole between 2002 and 2006. We now report results of a comprehensive surveillance program that assessed the lymphatic filariasis (LF) situation in Sri Lanka 6 years after cessation of MDA.

Methodology and principal findings: Transmission assessment surveys (TAS) were performed per WHO guidelines in primary school children in 11 evaluation units (EUs) in all 8 formerly endemic districts. All EUs easily satisfied WHO criteria for stopping MDA. Comprehensive surveillance was performed in 19 Public Health Inspector (PHI) areas (subdistrict health administrative units). The surveillance package included cross-sectional community surveys for microfilaremia (Mf) and circulating filarial antigenemia (CFA), school surveys for CFA and anti-filarial antibodies, and collection of Culex mosquitoes with gravid traps for detection of filarial DNA (molecular xenomonitoring, MX). Provisional target rates for interruption of LF transmission were community CFA <2%, antibody in school children <2%, and filarial DNA in mosquitoes <0.25%. Community Mf and CFA prevalence rates ranged from 0-0.9% and 0-3.4%, respectively. Infection rates were significantly higher in males and lower in people who denied prior treatment. Antibody rates in school children exceeded 2% in 10 study sites; the area that had the highest community and school CFA rates also had the highest school antibody rate (6.9%). Filarial DNA rates in mosquitoes exceeded 0.25% in 10 PHI areas.

Conclusions: Comprehensive surveillance is feasible for some national filariasis elimination programs. Low-level persistence of LF was present in all study sites; several sites failed to meet provisional endpoint criteria for LF elimination, and follow-up testing will be needed in these areas. TAS was not sensitive for detecting low-level persistence of filariasis in Sri Lanka. We recommend use of antibody and MX testing as tools to complement TAS for post-MDA surveillance.

Conflict of interest statement

The filarial antigen test used in this study uses reagents licensed from Barnes-Jewish Hospital, an affiliation of GJW. All royalties from sales of these tests go to the Barnes Jewish Hospital Foundation, a not for profit charitable organization (http://www.barnesjewish.org/giving/about-us). This does not alter our adherence to all PLOS policies on sharing data and materials.

Figures

Figure 1. Graphic summary of comprehensive filariasis surveillance data for Public Health Inspector areas in Sri Lanka.

Data shown are rates (% with 95% confidence limits as vertical lines). The dotted line in the top panel and the lower dotted lines in the two lower panels show the old provisional targets for interruption of transmission. The upper dotted lines in the two lower panels are recommended revised targets for the upper confidence limits for antibody rates in first and second grade primary school children and for filarial DNA rates in mosquitoes, respectively.

Figure 2. Distribution of households and mosquito collection sites tested for filariasis in Unawatuna PHI area in Galle district.

Panel A. Blue waypoints indicate households (HH) where all tested residents had negative filarial antigen tests; waypoints in red (CFA positivity) or yellow (microfilaremia and CFA positivity) indicate houses with at least one infected subject. Panel B shows molecular xenomonitoring results. Trap sites with no mosquito pools positive for filarial DNA are shown in blue, and traps with one or more positive mosquito pools are shown in red. Filarial DNA was detected in mosquitoes collected in 60% of the traps in this PHI.