Not all 'hybrids' matter: towards a nuanced understanding of Schistosoma species hybridization.

Hybridization between Schistosoma species has gained increasing attention in recent years, with reports from across Africa raising concerns about impacts on transmission, zoonotic spillover and control. While such interpretations often present 'hybrids' as emerging threats, they frequently outpace the supporting evidence. In this paper, we critically examine the assumptions and limitations underlying current discourse on Schistosoma hybridization. Drawing on experimental, field and genomic studies, we distinguish between transient hybridization, dead-end interactions and introgression, each with variable biological and epidemiological relevance. Advanced genomic analyses suggest the apparent rise in hybrid reports likely reflects expanded molecular surveillance rather than biological emergence, as many forms may have existed historically but went undetected. The absence of baseline or longitudinal data impedes testing of temporal trends, justifying enhanced surveillance while also cautioning against overreaction. Policy and funding responses should therefore remain proportionate and evidence-based. We argue that many hybridization scenarios do not warrant reactive control measures, especially where existing interventions remain effective. Overstating their significance risks misdirecting the already limited resources towards interventions, such as livestock treatment, that lack feasibility or justification. We propose a context-sensitive framework to distinguish fundamental evolutionary questions from actionable public health priorities. This article is part of the Royal Society Science+ meeting issue 'Parasite evolution and impact in action: exploring the importance and control of hybrid schistosomes in Africa and beyond'.