Abstract
BACKGROUND: The Zanzibar islands, Tanzania, have eliminated schistosomiasis as a public health problem since 2017 and need to rethink their intervention strategies to ensure that the progress made is sustained and advanced. We evaluated the performance of a novel surveillance-response approach for interrupting Schistosoma transmission on Pemba Island from 2020-2024. METHODOLOGY: In low-prevalence implementation units (IUs), surveillance-response interventions were implemented, which consisted of active and reactive case finding, treatment of S. haematobium-positive individuals, and reactive snail control. The performance of the surveillance-response interventions was measured by sensitivity, timeliness and impact on prevalence. Annual cross-sectional surveys were conducted in schools and households to estimate the total number of individuals infected with S. haematobium in the area and the proportion identified by the surveillance-response approach. Urogenital schistosomiasis was diagnosed by egg microscopy. PRINCIPAL FINDINGS: Among the 20 IUs in the study area, 15, 16, and 17 were considered low-prevalence IUs in the intervention periods in 2021, 2022, and 2023, respectively. Across the intervention periods, 4.6% (707/15509) among the schoolchildren included in active surveillance were tested S. haematobium-positive and treated. During reactive surveillance, at water bodies 8.2% (10/122) and in households 9.9% (45/454) of individuals were found infected and treated. Moreover, 47 among the 262 waterbodies were treated with molluscicide. The overall sensitivity of the surveillance-response approach across 2 periods, where complete surveillance data were available, was 23.0%. The timeliness of reactive interventions was 2 weeks. In the low-prevalence IUs, the prevalence in schoolchildren changed from 0.5% (7/1552) in 2021 to 0.4% (6/1653) in 2022, from 0.6% (12/2123) in 2022 to 0.7% (15/2240) in 2023, and from 0.4% (8/2287) in 2023 to 1.0% (27/2755) in 2024 after surveillance-response implementation. The respective prevalence in community members was 0.5% (14/2969) in 2021 and 0.7% (19/2928) in 2022, 0.6% (18/3175) in 2022 and 0.3% (10/2979) in 2023, and 0.4% (12/3257) in 2023 and 0.7% (22/3106) in 2024. CONCLUSION: Surveillance-response interventions maintained the low S. haematobium prevalence, but interruption of transmission was not achieved. The overall sensitivity of the approach was low. Timeliness was very good but required strong communication and collaboration between the surveillance-response teams. To work on a larger scale, with good coverage and improved sensitivity, elimination programs will need a large number of well-trained staff and adequate tools for surveillance and response activities, data management, and communication. TRIAL REGISTRATION: ISRCTN, ISCRCTN91431493. Registered 11 February 2020, ISRCTN - ISRCTN91431493: Novel intervention strategies for schistosomiasis elimination in Zanzibar.