Sublethal Cypermethrin Exposure Induces Systemic Toxicity in Nile Tilapia ( Oreochromis niloticus ): An Integrated Biomarker and Quantitative Histopathological Assessment

Cypermethrin is a synthetic pyrethroid insecticide widely used in agriculture and is frequently detected in aquatic environments through surface runoff. This study evaluated the acute and sublethal toxicity of cypermethrin in Nile tilapia ( Oreochromis niloticus ) using an integrated biomarker approach. The 96‐h LC 50 value was estimated to be 0.668 ppm, and fish were subsequently exposed for 7 days to three sublethal concentrations corresponding to 25%, 50%, and 75% of the LC 50 . Hematological, biochemical, antioxidant, neurotoxicity, and histopathological responses were assessed, and tissue alterations were further quantified using ImageJ‐based histomorphometric analysis. Cypermethrin exposure induced significant hematological alterations, including decreased red blood cell count and hemoglobin level, and increased white blood cell (WBC) count, blood glucose, and serum protein levels. Antioxidant enzyme activities and malondialdehyde levels increased significantly, whereas total antioxidant capacity decreased, indicating oxidative stress. Acetylcholinesterase activity was significantly inhibited, suggesting neurotoxicity, while increased aspartate aminotransferase, alanine aminotransferase, and alkaline phosphatase activities indicated hepatic impairment. Histopathological examination revealed concentration‐dependent lesions in the liver, gill, and intestine, including vacuolation, hemorrhage, necrosis, lamellar fusion, and epithelial disruption. Quantitative histological analysis confirmed a significant increase in altered tissue area in cypermethrin‐exposed fish. These findings demonstrate that sublethal cypermethrin exposure induces systemic physiological and tissue‐level alterations in O. niloticus and support the use of quantitative histopathology as a complementary tool in aquatic ecotoxicological assessment.