A pilot investigation of bovine schistosomiasis on Unguja Island, Zanzibar raises a new concern for elimination of urogenital schistosomiasis

Abstract

Our pilot parasitological investigation of cattle, supplemented with molecular DNA characterisation of encountered schistosomes, sheds first light upon bovine schistosomiasis on Unguja Island, Zanzibar. During February 2024, a total of 99 cattle were examined. Of these, 47 were exported animals from the Tanzanian mainland, designated for slaughter at two governmental abattoirs (Kisakasaka & Muwanda), and 52 were free-grazing animals sampled from four grazing locations within the island’s North and Central regions. Upon visual inspection of 31 cattle carcasses at Kisakasaka for adult worms, prevalence of schistosomiasis was 51.6%, whereas upon faecal miracidia hatching test (MHT) was 80.6%. At Muwanda, only faecal MHT was used, finding a much lower prevalence of 12.5%. In free-grazing animals, prevalence of schistosomiasis by MHT was 0.0%. At Muwanda the animal quarantine paddock was in disrepair, inclusive of a large pond now acting as a watering point. Here, numerous Bulinus forskalii sp. were found. Whilst no snails were observed to shed schistosome cercariae, molecular xenomonitoring did detect a pre-patent infection prevalence of 10.8%, with Schistosoma bovis firmly incriminated. Molecular DNA characterisation of adult schistosomes (n = 19) by real-time PCR and high resolution melt profiling, alongside DNA sequencing, also identified S. bovis although two worms were putative S. bovis-S. mattheei hybrids. Atypical intrauterine eggs of S. bovis were noted upon microscopy of a worm pair. A broader screen of 92 miracidia confirmed S. bovis and 3 miracidia as S. bovis-S. mattheei hybrids. Contrasting with Pemba Island, Zanzibar, where autochthonous transmission of S. bovis can occur, bovine schistosomiasis on Unguja Island currently appears restricted to imported animals alone. However, the seminal detection of putative S. bovis-mattheei hybrids, alongside the current inadequate quarantine facilities at Muwanda, raises a new concern that such hybrid schistosomes may escape and enter the island’s hinterland. Should this happen, surveillance and control of urogenital schistosomiasis on Unguja would be compromised and further complicated. We therefore strongly recommend immediate repair and improved maintenance of governmental animal quarantine facilities. Future epidemiological surveys of imported cattle are now well-justified, not only to better understand the full repertoire of hybrid schistosomes present but also to develop appropriate mitigating interventions.