Re-assessment of schistosomiasis in nine endemic districts with cluster sampling in Sierra Leone

Abstract

Background: Baseline mapping showed that schistosomiasis was highly/moderately endemic in nine districts in Sierra Leone. Mass drug administration (MDA) with praziquantel started in 2009, and after multiple rounds of treatment, an impact assessment was conducted in 2016 followed by a second re-assessment in 2022 using cluster sampling to provide more granular data for refining chiefdom (sub-district) treatment strategies.

Methods: On average, 20 rural villages were systematically selected per district by probability proportional to population size across the nine districts. Surveys were conducted in schools, and 24 school children aged between 5 and 14 years were randomly selected, with an equal number of boys and girls. One stool sample and one urine sample were collected per child. Two Kato-Katz slides were examined per stool for Schistosoma mansoni infection. Hemastix strips were used as a proxy for S. haematobium infection with urine filtration used for egg counts on hematuria-positive samples.

Results: In total, 4,736 stool samples and 4,618 urine samples were examined across 200 schools in 125 chiefdoms. Overall, the prevalence of S. mansoni was 16.3% (95% CI: 15.3-17.4%), while the overall prevalence of S. haematobium was 2.0% (95% CI: 1.6-2.4%) by hematuria. The prevalence of heavy infections for S. mansoni and S. haematobium was 1.5% (95% CI: 1.1-1.9%) and 0.02% (95% CI: 0.0-0.14%), respectively. Among 125 chiefdoms surveyed, the overall schistosomiasis prevalence was <10% in 65 chiefdoms, 10-49.9% in 47 chiefdoms, and ≥ 50% in 13 chiefdoms. There was a mixed relationship between schistosomiasis in school children and WASH access in schools.

Conclusion: Sierra Leone has made significant progress in reducing schistosomiasis prevalence across the country after a decade of MDA intervention. However, high prevalence remains in some hotspot chiefdoms. The next steps are for the national program to investigate and address any potential issues such as low coverage or poor knowledge of schistosomiasis risk behaviors and, where appropriate, consider broadening to community-wide treatment in hotspot chiefdoms or communities.

Keywords: Schistosoma haematobium; Schistosoma mansoni; Sierra Leone; cluster sampling; impact assessment; schistosomiasis.

Figure 1

Treatment coverage of school-aged children from 2016 to 2022 in Sierra Leone by health district.

Figure 2

Distribution and point prevalence of any schistosomiasis in 2022.

Figure 3

Trend of prevalence of any schistosomiasis over the years in Sierra Leone. S. haematobium and S. mansoni were evaluated separately at baseline, so the prevalence of S. mansoni was used as a proxy for any species infection in the figure.

Figure 4

Spatially smoothed contour maps of predicted schistosomiasis prevalence in 2022 (A), 2016 (B), and of S. mansoni only at baseline in 2008 (C) in Sierra Leone. The prevalence of any species infection was not available in 2008 as the survey for S. haematobium was conducted separately, and the baseline S. mansoni prevalence was used as a proxy.