Schistosomiasis amongst adolescent boys in non-lakeshore southern Malawi: Investigating local risk-factors within a nested community-based cross-sectional survey

Abstract

Background: Schistosomiasis is endemic to Malawi, where preventive chemotherapy by mass drug administration (MDA) has been the foundational public health strategy for over a decade. Despite ongoing control, our understanding of the contemporary epidemiology of schistosomiasis in rural Malawi is limited to infrequent school-based surveys, typically lacking evidence from community-based surveys particularly within non-lakeshore upland communities who may be perceived to be at lower risk. Methods: Between July and August 2022, we conducted a cross-sectional parasitological survey amongst a community-representative sub-sample of boys aged 10-15 years who had been randomly selected and recruited to the DeWorm3 endline survey in Namwera, Mangochi District. A total of 306 participants from 38 communities were assessed for S. mansoni by duplicate Kato-Katz thick smears. Of these, 243 (79.4%) subsequently provided a urine sample to be assessed by filtration for S. haematobium and 238 (77.8%) responded to a risk-factor questionnaire. A parallel malacological survey of eight locally important water contact sites was conducted. Results: The overall prevalence of egg-patent schistosomiasis was 50.6% (95% CI 44.2-57.1). The prevalence of S. haematobium was 47.7% (95% CI 41.3-54.2), of which 37.9% (n=44) were heavy intensity infections whereas the prevalence of S. mansoni was 6.5% (95% CI 4.0-9.9), with one moderate intensity infection (0.3%). There was strong evidence of a positive association between detected S. haematobium infection and reporting "red urine" (p<0.001) and 'bilharzia' (p=0.005). Biomphalaria spp. were found at two sites while Bulinus spp. were found at five sites. Conclusion: Despite multiple years of MDA at reportedly high coverage, we observed a high egg-patent prevalence with high prevalence of heavy intensity infections amongst boys aged 10–15 years. This evidences engrained and ongoing transmission requiring additional efforts to gain and sustain effective control. Our findings highlight the importance of epidemiological monitoring alongside a schistosomiasis control programme, particularly in areas historically perceived to be at lower risk.