African schistosomes; inter species interactions and hybridisation


B Webster1
1 Natural History Museum, UK


Schistosomes are parasitic trematodes that cause Schistosomiasis, the most important water-borne Neglected Tropical Disease burdening and inflicting unnecessary suffering on many poor rural communities. 206.4 million people require treatment in 78 tropical and sub-tropical developing countries. Schistosomiasis is also a major disease of animals, affecting large numbers of domestic livestock worldwide. There are 25 described schistosome species. Their two-host lifecycle includes an asexual stage in certain species of fresh water snails, governing their geographical distribution, and a sexual stage living in the blood vessels of species specific/preferred mammalian hosts. Asian schistosomiasis is highly zoonotic complicating control, whereas in Africa species specific host preferences exist. However, hybridisation between schistosome species is now frequently being reported across Africa, with the hybridisation between human and animal schistosomes raising concerns about zoonotic African schistosomiasis and the presence/emergence of highly virulent forms. Uniquely among trematodes, schistosomes are dioecious. The sexually dimorphic adult worms form pairs providing opportunity for inter species mating. Crossing experiments have demonstrated species preferences, competition and non-reciprocal mating with hybridisation readily occurring between closely related species. Hybrid vigour was observed with expanded snail host ranges, increased fecundity/vectoral capacity and heightened animal heterosis and pathology. In nature pre- and post-zygotic reproductive isolating barriers, such as host specificity, anatomical site of infection, distribution, mating preferences, competition and incompatibility, are thought to prevent prolific inter-species admixture. However, reports of suspected natural hybridisation date back to the 1940’s, with possible implications for disease pathology, host switching/range, drug efficacy and transmission dynamics. Due to recent advances in our ability to capture, preserve and genetically analyse schistosome larvae (eggs, miracidia, cercariae), natural hybrids/introgressed forms of human and livestock schistosome species combinations have been identified by discordance between mitochondrial cox1 (mtDNA) and nuclear ITS (nDNA) DNA. These include the widespread S. haematobium-bovis (Sh:Sb) hybrid, the more focal West African S. haematobium-curassoni (Sh:Sc) hybrid and the recently identified East African S. haematobium-mattheei (Sh:Sm) hybrid suggesting that zoonotic schistosome transmission and hybridisation may be a real risk in Africa. However, the dynamics of this hybridisation remain poorly understood with strong biases observed . Here we will present further molecular analyses of S. haematobium, S. bovis and the S. haematobium-bovis hybrids, which dispute random mating and prolific admixing of these populations.

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