Modulation of the immune parameters and expression of genes of gilthead seabream (Sparus aurata L.) by dietary administration of oxytetracycline

The oxytetracycline (OTC) is an antibiotic widely used in the aquaculture, principally in the treatment of infections caused by the pathogenic Gram-negative bacteria. In the present study, the consequences of the oral administration of OTC on the main humoral (natural haemolytic complement activity, IgM levels and seric peroxidase level) and cellular (respiratory burst, intracellular peroxidase level and phagocytosis) immune parameters of gilthead seabream (Sparus aurora. L) were evaluated after 7, 14 and 21 days of treatment Furthermore, the expression levels of immune-associated genes (MHC-II alpha., C3, IL-1 beta, COX-2 and beta-defensin) were analysed by real-time polymerase chain reaction in head-kidney and gut. Three experimental groups were established: control group (fed with non-supplemented OTC food) and experimental groups (4 and 8 mg OTC g(-1) feed, respectively) (N = 30). The results reveal a decrease of the natural haemolytic complement activity to the higher dose of OTC administered. By contrast, after 7 days of treatment, fish fed the lowest concentration of OTC were observed to have a statistically significant increase in the percentage of phagocytic cells, while the phagocytic capacity and respiratory burst increased after 14 days of treatment for both concentrations of OTC. No significant effects were observed after 21 days of treatment for both concentrations of OTC with regard to the values obtained in the control group. In head-kidney and gut, the dietary administration of OTC not caused statistically significant variation, only in C3 gene expression at 7 days in gut. The present results suggest that after dietary administration of OTC, the humoral and cellular innate immune parameters of gilthead seabream are depressed and stimulated respectively, and induce up- or down-regulation of the immune-relevant genes analysed depending on the concentration of OTC supplied in diet. (C) 2012 Elsevier B.V. All rights reserved.