Seasonal dynamics and species-specific immune responses to <i>Prohemistomum vivax</i> encysted metacercariae infections in polycultured Nile tilapia ( <i>Oreochromis niloticus</i> ) and African catfish ( <i>Clarias gariepinus</i> )

Polyculture farming of Nile tilapia and African catfish is widely practiced in Egypt, yet parasitic infections threaten production sustainability. This study compared Prohemistomum vivax encysted metacercariae (EMC) infections in both species using morphological, molecular, phylogenetic, physiological, and immunological analyses. A total of 500 fish (250 species) were collected seasonally from semi-intensive earthen ponds in Kafr El Sheikh Governorate. Clarias gariepinus showed higher overall prevalence (60.0% vs. 40.0%), peaking in summer (72.8% vs. 68.5%). However, Oreochromis niloticus exhibited significantly higher infection intensities (38.0 ± 16.2 vs. 22.2 ± 7.1 EMC/field in summer; p < .05). ITS sequencing confirmed P. vivax identity (99.71% similarity). EMC infections altered blood biochemistry, indicating liver dysfunction and elevated stress. O. niloticus demonstrated stronger immune responses, with TNF-α (10.0-fold muscle, 8.0-fold liver) and IL-1β (11.0-fold muscle, 9.0-fold liver) upregulation exceeding C. gariepinus (p < .05). These findings reveal species-specific susceptibility patterns, supporting seasonally adapted, host-specific parasite management strategies for sustainable aquaculture and zoonotic risk mitigation.