Modulation of immune response in Atlantic cod, Gadus morhua following oral administration of antimicrobials

Florfenicol and oxolinic acid are the commonly used chemotherapeutants for the treatment of bacterial diseases in Atlantic cod, Gadus morhua. In the present study, we evaluated the changes in the innate immune response of the fish, bacterial proliferation in serum and transcriptional upregulation of some immune-related genes following oral administration of these antibiotics. Juvenile cod (av. weight g) were offered a commercial feed coated with antibiotics (10 mg kg-1 feed) at a ration of 1% 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. Florfenicol-fed fish had lower myeloperoxidase activity at 3 days post-withdrawal, while there were differential effects on alkaline phosphatase activity. Proliferation of the bacterial pathogens, Vibrio anguillarum and Aeromonas salmonicida was affected by the type of antimicrobial used, but a significant reduction in bacterial count was observed until the 5th day post-withdrawal. Florfenicol could inhibit V. anguillarum better than oxolinic acid. A. salmonicida was more susceptible than V. anguillarum upon treatment with both chemotherapeutants. Furthermore, transcriptional profiles of selected genes related to bacterial defense, inflammation and antioxidant defense depended on the antibiotic applied and the time of sampling. These results indicate that administration of chemotherapeutants modulates the immune response in Atlantic cod and this may, in turn, affect their resistance to against bacterial pathogens.