Interventions and strategies for urogenital schistosomiasis elimination in Zanzibar

Urogenital schistosomiasis is caused by Schistosoma haematobium and can lead to severe morbidity if untreated. The human-snail-human life cycle of S. haematobium can be interrupted with treatment, snail control and behavior change interventions. The Zanzibar islands, United Republic of Tanzania, have achieved low overall prevalence nowadays; thus, their historical interventions can guide other sub-Saharan African countries towards the same goal. At the same time, questions arise about whether large-scale treatment is still justified in areas with a very low prevalence. Novel strategies are now needed to map environmental factors and human infection, identify clusters, and address spatial heterogeneity of infection focally.

The current thesis addresses these questions in several ways. A systematic review of 100 years of schistosomiasis and snail-related research on Zanzibar was performed to identify interventions and their impact on the S. haematobium prevalence. The impact of seven years of mass drug administration (MDA) and of a 16-month treatment gap on S. haematobium prevalence was determined. Finally, as part of the SchistoBreak study (2020-2024) being implemented to develop novel strategies for the elimination of schistosomiasis, two cross-sectional surveys in schools and households utilized micro-mapping to assess the impact of test-treat-track-test-treat (5T) interventions as an alternative to MDA in low-prevalence areas.

A hundred years of interventions resulted in a low overall prevalence of <5% in 2020. After a 16-month gap of MDA, spatial heterogeneity of S. haematobium infection on the islands was observed and became more pronounced when the prevalence rebounded primarily in hotspot areas. A novel strategy for infection mapping within the SchistoBreak study demonstrated the feasibility of finding pre-randomized households in remote settings. After one year of 5T interventions in low-prevalence areas, no significant prevalence increase was revealed.

A combination of MDA, including treating adults and preschool-aged children, snail control, and behavior change measures are crucial to reducing the S. haematobium prevalence in hotspot areas. Environmentally friendly snail control and new intervention grounds for behavior change measures need to be explored. To compare the schistosomiasis prevalence across countries and to create a global prevalence map, micro-mapping guidelines by the World Health Organization are required. In low-prevalence settings, targeted interventions present alternatives to MDA; however, future studies need to assess the optimal interventions required to maintain or further reduce the prevalence towards interruption of transmission.