The Effect of Oxytetracycline on the Immune Response and Expression of Immune‐Related Genes in European Sea Bass ( <i>Dicentrarchus labrax</i> , L. 1758)

Antibiotics are extensively used in aquaculture to treat and prevent bacterial diseases, but their application can lead to negative side effects such as immunosuppression. Oxytetracycline (OTC) is a widely used antibiotic in aquaculture for treating bacterial diseases due to its broad‐spectrum activity and economic advantage. This study appraises the effects of OTC treatment on the immune response of European sea bass ( Dicentrarchus labrax , L. 1758) by assessing immune‐related gene expression, serological parameters, hematological parameters, and histological findings. Fish were divided into three groups: control (fed with nonsupplemented OTC), 75 mg OTC kg −1 day −1 feed, and 100 mg OTC kg −1 day −1 feed for 21 days. Blood and tissue samples were collected for analysis on the 4th, 7th, 10th, 14th, and 21st day of OTC feeding. The results showed a decrease in immune response parameters such as nitro blue tetrazolium (NBT) positive cell, total white blood cells (WBCs), serum lysozyme activity, serum Ig‐M levels, and serum complement activity for OTC‐fed fish, while immune‐related genes ( Lys-g , Ig-M , interleukin ( IL )‐ 1β , C3 , and major histocompatibility complex [ MHC ] –IIα ) were significantly downregulated in kidney and spleen tissues, especially with prolonged exposure. The study hypothesized that long‐term use of antibiotics in fish disrupts the functioning of the immune system. As a result of the study, the hypothesis was confirmed, and using OTC for longer than the normal treatment period or in high doses caused negative changes in the immune system parameters and made fish more susceptible to other pathogens.