Impact assessment of onchocerciasis through lymphatic filariasis transmission assessment surveys using Ov-16 rapid diagnostic tests in Sierra Leone

Abstract

Abstract Background Sierra Leone is endemic with onchocerciasis in 14 of its 16 districts with high prevalence (47–88.5%) by skin snips at baseline. After 11 rounds of mass treatment with ivermectin with good coverage, an impact assessment was conducted in 2017 to assess the progress towards onchocerciasis elimination in the country. Methods A cluster survey was conducted, either integrated with lymphatic filariasis (LF) transmission assessment survey (TAS) or standalone with the LF TAS sampling strategy, in 12 (now 14) endemic districts. Finger prick blood samples of randomly selected children in Grades 1–4 were tested in the field using the SD Bioline Onchocerciasis IgG4 rapid tests. Results In total, 17,402 children aged 4–19 years in 177 schools were tested and data from 17,364 children aged 4–14 years (14,230 children aged 5–9 years) were analyzed. There were 346 children confirmed positive for Ov-16 IgG4 antibodies, a prevalence of 2.0% (95% CI: 1.8–2.2%) in children aged 4–14 years with prevalence increasing with age. Prevalence in boys 2.4% (95% CI: 2.1–2.7%) was higher than in girls 1.6% (95% CI: 1.4–1.9%). There was a trend of continued reduction from baseline and 2010. Using data from children aged 5–9 years, overall prevalence was 1.7% (95% CI: 1.5–1.9%). The site prevalence ranged from 0–33.3% (median prevalence = 0.0%): <2% in 127 schools, 2% – <5% in 34 schools and ≥ 5% in 16 schools. There was a significant difference in average prevalence between districts. Using spatial analysis, the Ov-16 IgG4 antibody prevalence was predicted below 2% in coastal areas and in the large parts of Koinadugu, Bombali and Tonkolili districts while high prevalence (> 5%) was predicted in some focal areas, centered in Karene, Kailahun, and Moyamba/Tonkolili. Conclusions Low Ov-16 IgG4 antibody prevalence was shown in most areas across Sierra Leone. In particular, low seroprevalence in 5–9 years old suggests that the infection was reduced to a low level after 11 rounds of treatment intervention. Sierra Leone has made major progress towards elimination of onchocerciasis. However, attention must be paid to those high prevalence focal areas.