Abstract
Parasitic infections caused by Clinostomum complanatum and Euclinostomum heterostomum pose significant threats to aquaculture systems and public health. This study examined their molecular, immunological, and pathological impacts in Egyptian Nile tilapia (Oreochromis niloticus), with COXI gene sequencing confirming both species and revealing genetic similarities to regional isolates, suggesting broad geographical distribution. Euclinostomum heterostomum-infected fish exhibited significant upregulation of immune-related genes (IFN-γ: 1.96-fold; IL-10: 2.06-fold; IL-12: 1.91-fold; IL-1β: 4.07-fold; CYP-1α: 2.07-fold) and elevated oxidative stress markers (SOD: 2.96-fold; CAT: 3.52-fold; GSH: 3.07-fold; TAC: 2.58-fold) compared to uninfected controls (p < 0.0001 for all comparisons). Similarly, Clinostomum complanatum infections triggered upregulation of IFN-γ (2.17-fold), IL-10 (2.09-fold), IL-12 (2.25-fold), IL-1β (2.51-fold), and CYP-1α (2.58-fold), alongside increased SOD (2.63-fold), CAT (2.70-fold), GSH (2.99-fold), and TAC (2.65-fold) compared to uninfected controls (p < 0.0001 for all comparisons). Histopathology revealed necrosis, fibrosis, and inflammation in gill and kidney tissues, with immunohistochemical staining confirming localized inflammatory markers. These findings demonstrate systemic immune activation and physiological stress caused by clinostomid infections, providing critical insights for aquaculture disease management strategies.