Polyethylene nanoplastics intensify toxicity of potassium clavulanate in African catfish (Clarias gariepinus).

Nanoplastics (NPs) and antibiotics frequently co-occur in aquatic environments, yet their combined effects on fish health remain poorly understood. This study examined the individual and joint toxicity of polyethylene nanoplastics (PE-NPs; 20 and 80 nm, 5 mg L⁻¹) and potassium clavulanate (CA; 100 ng L⁻¹) in African catfish (Clarias gariepinus) during a 15-day exposure followed by a 15-day recovery phase. Exposure to PE-NPs (20 nm) alone significantly increased erythrocyte apoptosis (2.8-fold vs. control, p < 0.001), while CA alone also caused significant DNA damage (1.9-fold vs. control, p < 0.001). PE-NPs (80 nm) alone induced no notable alterations. Co-exposure to CA and PE-NPs, particularly the 20 nm particles, produced the strongest erythrocyte apoptosis, DNA damage, and declines in Hb, Hct, RBCs, and WBCs. These hematological and genotoxic effects persisted after recovery, indicating incomplete or delayed reversal of toxicity. Spleen histopathology showed vascular congestion, hemorrhage, fibrosis, and lymphoid depletion, especially in co-exposed fish. The results demonstrate a statistically significant, size-dependent synergistic interaction, suggesting that smaller PE-NPs amplify CA-induced cytotoxicity and genotoxicity through a "Trojan horse" mechanism. The persistence of these effects underscores the ecological risk of NP-pharmaceutical mixtures and the need to include such interactions in aquatic risk assessments.