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

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.