# Evaluation of surveillance-response interventions for Schistosoma haematobium elimination on Pemba Island, Tanzania: A 4-year intervention study with repeated cross-sectional surveys

## TL;DR

A new surveillance-response strategy was tested on Pemba Island to maintain low schistosomiasis levels, but it did not fully eliminate the disease within four years.

## Contribution

The study evaluates a novel surveillance-response approach for sustaining schistosomiasis elimination in low-prevalence areas.

## Key findings

- Surveillance-response interventions maintained low S. haematobium prevalence but did not interrupt transmission.
- The overall sensitivity of the approach was 23.0%, indicating limited case detection capability.
- Timeliness of reactive interventions was 2 weeks, but required strong communication and collaboration.

## Abstract

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.

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.

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.

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.

ISRCTN, ISCRCTN91431493. Registered 11 February 2020, ISRCTN - ISRCTN91431493: Novel intervention strategies for schistosomiasis elimination in Zanzibar.

Schistosomiasis is a neglected tropical disease that is caused by infection with Schistosoma parasites, which are transmitted in natural freshwater bodies. On the Zanzibar islands, Tanzania, intense interventions, including large-scale treatment campaigns, have successfully reduced the number of Schistosoma infections and associated morbidity over the past decades. To fully eliminate schistosomiasis from Zanzibar, new intervention strategies need to be explored. Here, we assessed the performance of a novel targeted intervention approach termed “surveillance-response” in low-endemic areas on Pemba Island. We screened children for schistosomiasis in schools, treated the positives and tracked them to their homes and to freshwater bodies that they used, where we also tested present individuals and treated the positives. At the water bodies, we conducted surveys to detect intermediate host snails and sprayed a molluscicide if snails were present. In annual surveys, we found that the surveillance-response interventions maintained the already low number of schistosomiasis cases, but complete elimination was not achieved within 4 years. The ability of the approach to detect all schistosomiasis cases was low, but the timeliness from start to end of the procedures was very good. To conduct surveillance-response at a larger scale with good population coverage and improved sensitivity for case detection, schistosomiasis elimination programs will need a large number of well-trained staff and adequate tools for surveillance and response activities, data management, and communication.