Abstract
Oxytetracycline (OTC) is one of the most frequently used antibiotics in aquaculture, although negative side‐effects have been reported in some cases. The present study has determined the effect of the in vivo administration of OTC on several immune functions in turbot, Scophthalmus maximus (L.), such as kidney macrophage respiratory burst, phagocytosis and plasma bactericidal activity. Total plasma protein was also measured. The antibiotic was administered orally over 12 days, and this treatment was combined with OTC baths on days 1, 3 and 5. The drug did not suppress the immune functions analysed. Instead, the treatment produced a significant increase in the plasma bactericidal activity and total protein concentration. The effect of different doses of OTC on macrophage functions (i.e. respiratory burst activity and phagocytosis) and cell viability was also assayed in vitro . Although cell viability did not decrease after in vitro exposure, head kidney macrophage respiratory burst and phagocytosis were inhibited by the in vitro treatment, and were dose‐dependent.