Achievements and challenges of lymphatic filariasis elimination in Sierra Leone

Abstract

Background: Lymphatic filariasis (LF) is targeted for elimination in Sierra Leone. Epidemiological coverage of mass drug administration (MDA) with ivermectin and albendazole had been reported >65% in all 12 districts annually. Eight districts qualified to implement transmission assessment survey (TAS) in 2013 but were deferred until 2017 due to the Ebola outbreak (2014-2016). In 2017, four districts qualified for conducting a repeat pre-TAS after completing three more rounds of MDA and the final two districts were also eligible to implement a pre-TAS.

Methodology/principal findings: For TAS, eight districts were surveyed as four evaluation units (EU). A school-based survey was conducted in children aged 6-7 years from 30 clusters per EU. For pre-TAS, one sentinel and one spot check site per district (with 2 spot check sites in Bombali) were selected and 300-350 persons aged 5 years and above were selected. For both surveys, finger prick blood samples were tested using the Filariasis Test Strips (FTS). For TAS, 7,143 children aged 6-7 years were surveyed across four EUs, and positives were found in three EUs, all below the critical cut-off value for each EU. For the repeat pre-TAS/pre-TAS, 3,994 persons over five years of age were surveyed. The Western Area Urban had FTS prevalence of 0.7% in two sites and qualified for TAS, while other five districts had sites with antigenemia prevalence >2%: 9.1-25.9% in Bombali, 7.5-19.4% in Koinadugu, 6.1-2.9% in Kailahun, 1.3-2.3% in Kenema and 1.7% - 3.7% in Western Area Rural.

Conclusions/significance: Eight districts in Sierra Leone have successfully passed TAS1 and stopped MDA, with one more district qualified for conducting TAS1, a significant progress towards LF elimination. However, great challenges exist in eliminating LF from the whole country with repeated failure of pre-TAS in border districts. Effort needs to be intensified to achieve LF elimination.

Fig 1. District-level treatment coverage during 2013–2016 in Sierra Leone NTDP: epidemiological coverage of the national reported treatment among the district total population projected from the 2015 national census; Census: epidemiological coverage of the national reported treatment among the district total population from the CDD census; IM: program coverage from independent monitoring evaluation.

Left panel: four repeat pre-TAS districts Bombali (Bom), Koinadugu (Koin), Kailahun (Kail) and Kenema (Ken); Right panel: eight TAS districts Bo, Pujehun (Puj), Bonthe (Bon), Moyamba (Moy), Port Loko (PLK), Kambia (Kam), Kono (Kon) and Tonkoulili (Ton).

Fig 2. Distribution of survey clusters and location of FTS positive cases in 8 TAS districts in Sierra Leone in 2017.

Fig 3. Location of repeat pre-TAS and pre-TAS survey sites and point-prevalence of FTS positives in 6 districts in Sierra Leone in 2017.