Rapid diagnostic assay distinguishing the snail vectors Bulinus globosus and Bulinus nasutus for urogenital schistosomiasis transmission risk mapping in East Africa

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Abstract As schistosomiasis nears elimination in certain endemic areas, targeted risk mapping is crucial for identifying focal transmission areas. Despite long-standing control efforts, including mass drug administration, focal snail control and behavioural interventions, urogenital schistosomiasis persists in the Zanzibar archipelago. Efficient resource allocation requires identifying areas where human–freshwater contact overlaps with suitable snail hosts. Of the 4 Bulinus species potentially acting as vectors (intermediate hosts) for Schistosoma haematobium in Zanzibar, Bulinus globosus is responsible for most of the transmission, whilst Bulinus nasutus , a morphologically cryptic species to B. globosus , plays a minor role in transmission. To facilitate future distribution mapping of these 2 snail vectors, this study developed a rapid and low-cost diagnostic molecular assay to differentiate these snail species. A duplex PCR assay was designed to target the 5.8S rDNA and internal transcribed spacer 2, amplifying 2 different-sized fragments for B. globosus and B. nasutus . Twenty-one primer combinations were tested to determine those with the most consistent profiles. The final assay iteration provided consistent species-specific differentiation of Zanzibar B. globosus and B. nasutus . Closely related species from mainland Tanzania, B. productus and a currently undescribed Bulinus sp., were also tested and produced similar banding patterns to B. nasutus and B. globosus , respectively, indicating that the assay is most suitable for local differentiation of Zanzibar populations where these taxa do not occur sympatrically. The assay allows cheap and rapid differentiation of these hosts, whose allopatric distribution on Zanzibar may dictate the degree of schistosomiasis transmission across these Indian Ocean islands.

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