To live or to die: competitive exclusion of Schistosoma japonicum by Exorchis sp. in Oncomelania hupensis.

Schistosomiasis, caused by blood flukes of the genus Schistosoma, is a neglected tropical disease whose transmission depends on freshwater snails. Oncomelania hupensis is the obligate intermediate host for Schistosoma japonicum, disrupting this snail-schistosome relationship is therefore a crucial strategy for disease control and elimination. However, current snail-directed control methods are limited and can adversely affect biodiversity, ecological balance, and environmental health, underscoring the need for deeper mechanistic understanding of factors that naturally regulate schistosome transmission. Notably, prior infection with Exorchis sp. has been shown to completely block subsequent infection by S. japonicum in O. hupensis through within-host competitive exclusion. Therefore, elucidating the biological dynamics and molecular mechanisms underlying this interference competition may thus contribute to the development of novel anti-schistosome strategies. This review systematically synthesizes current knowledge on the life cycle of Exorchis sp., the mechanisms driving its competitive dominance over S. japonicum, field surveys of its natural distribution, and the underlying immunological mechanisms. By integrating these facets, we aim to advance fundamental understanding of within-host trematode competition and its implications for schistosomiasis transmission ecology. We further discuss how insights into the molecular and immunological mechanisms of this interaction may inform future targeted interventions. Finally, this work presents a unique opportunity to investigate gastropod immunity and host-parasite co-evolutionary dynamics, thereby broadening our knowledge of molluscan immune competence and its role in shaping disease transmission.