In vivo modulation of immune response and antioxidant defense in Atlantic cod, Gadus morhua following oral administration of oxolinic acid and florfenicol

Oxolinic acid and florfenicol are the commonly used antibiotics for the treatment of bacterial diseases in Atlantic cod, Gadus morhua. The changes in selected innate humoral immune response of the fish, bacterial proliferation in serum and transcriptional activity of selected immune and antioxidant defense-related genes following oral administration of these antimicrobial compounds were evaluated. Juvenile cod (75-100 g) were fed commercial feed coated with either florfenicol (10 mg kg^{- 1} fish, active ingredient) or oxolinic acid (20 mg kg^{- 1} fish, active ingredient) at a ration of 0.5% body weight for 10 days. Whole blood and serum samples were collected on the 10th day of feeding the antibiotics and at 3, 5 and 10 days after their withdrawal. Serum protein was significantly higher in fish at the 10th day post-withdrawal of both antibiotics. Florfenicol-fed fish had lower myeloperoxidase activity at 3 days post-withdrawal, while there were differential effects on alkaline phosphatase activity. Vibrio anguillarum and Aeromonas salmonicida were significantly reduced in the sera of antibiotics-fed fish until the 5th day post-withdrawal. Florfenicol could inhibit V. anguillarum better than oxolinic acid, while A. salmonicida was more susceptible than V. anguillarum upon treatment with both antibiotics. Furthermore, transcriptional profiles of selected genes related to bacterial defense, inflammation and antioxidant defense were dependent on the type of antibiotics that was administered and the time of sampling. These results indicate that oral administration of antibiotics modulates the immune response and antioxidant defense in Atlantic cod and these may, in turn, affect their ability to resist bacterial pathogens.