Date Published: January 20, 2018
Publisher: The American Society of Tropical Medicine and Hygiene
Author(s): Kimberly Y. Won, Sana Sambou, Amanda Barry, Keri Robinson, Momodou Jaye, Bakary Sanneh, Abdoulie Sanyang, Katherine Gass, Patrick J. Lammie, Maria Rebollo.
A current need in the global effort to eliminate lymphatic filariasis (LF) is the availability of reliable diagnostic tools that can be used to guide programmatic decisions, especially decisions made in the final stages of the program. This study conducted in The Gambia aimed to assess antifilarial antibody levels among populations living in historically highly LF-endemic areas and to evaluate the use of serologic tools to confirm the interruption of LF transmission. A total of 2,612 dried blood spots (DBSs) collected from individuals aged 1 year and above from 15 villages were tested for antibodies to Wb123 by enzyme-linked immunosorbent assay (ELISA). A subset of DBS (N = 599) was also tested for antibodies to Bm14 by ELISA. Overall, the prevalence of Wb123 was low (1.5%, 95% confidence interval [CI] 1.1–2.1%). In 7 of 15 villages (46.7%), there were no Wb123-positive individuals identified. Individuals with positive responses to Wb123 ranged in age from 3 to 100 years. Overall, Bm14 prevalence was also low (1.5%, 95% CI 0.7–2.8%). Bm14 positivity was significantly associated with older age (P < 0.001). The low levels of antibody responses to Wb123 observed in our study strongly suggest that sustainable LF transmission has likely ceased in The Gambia. In addition, our results support the conclusion that serologic tools can have a role in guiding programmatic decision making and supporting surveillance.
Lymphatic filariasis (LF) is a mosquito-transmitted parasitic disease caused by three main species of filarial worms (Wuchereria bancrofti, Brugia malayi, and B. timori).1 In 1997, at the 50th World Health Assembly (WHA), a resolution was passed to eliminate LF as a public health problem by 2020 (WHA resolution 50.29).2 Shortly thereafter, in 2000, the Global Program to Eliminate Lymphatic Filariasis (GPELF) was organized to assist countries in achieving this goal.3 At the onset of GPELF, it was estimated that 120 million individuals were infected and that approximately 1.3 billion people throughout the tropics and subtropics were at risk of filarial infection.3 To reach the established elimination targets, LF programs set out to treat individuals in endemic areas through annual community-wide mass drug administration (MDA) for at least 5 years. By the end of 2015, MDA had been implemented in 63 of 73 LF-endemic countries, with a cumulative total of 6.2 billion treatments delivered since the launch of GPELF.4
A total of 4,481 individuals (aged 1–100 years) from the 15 villages were enrolled in the study. Of those enrolled, a total of 2,612 (58.2%) DBS from all the 15 villages were tested for antibodies to Wb123. There was no difference in age or sex between individuals not included for serologic testing and individuals with antibody results. Demographic information was not available for 161 (6.2%) samples with antibody results. Antibody prevalence for individuals with missing demographic data was not different from prevalence for those with available demographic information. There were no individuals who were antigen positive by ICT. Overall, the prevalence of positive Wb123 responses was low (1.5%, 95% confidence interval [CI] 1.1–2.1%). In 7 of 15 villages (46.7%), there were no antibody-positive individuals identified. Of the eight villages with at least one person with a positive Wb123 result, six (75%) were located in the Western Division (Figure 1). Individuals with positive responses to Wb123 ranged in age from 3 to 100 years. Wb123 results by community are summarized in Table 1. There was no statistically significant difference in Wb123 prevalence among the study villages once adjusted for age, sex, and clustering by village.
The results of TAS conducted in 2013 in The Gambia indicated that there was no LF transmission among 6- and 7-year-old children, and in 2016, The Gambia was removed from the WHO’s official list of LF-endemic countries.4 Although the absence of antigenemia among children in The Gambia was likely an indicator of interrupted transmission, there was no information collected from older age groups. Our study aimed to assess antifilarial antibody levels among populations living in historically highly LF-endemic areas in The Gambia and to evaluate the use of serologic tools to confirm the absence of LF transmission.