Influence of oxytetracycline and oxolinic acid on the immune response of rainbow trout (Oncorhynchus mykiss)

Oxytetracycline and oxolinic acid were tested for possible immunomodulatory effects on fish. The aim of the study was to follow the kinetics of the immune response after vaccination with simultaneous antibiotic treatment. The fish were immunised with a commercial oil-based divalent (furunculosis/vibriosis) vaccine and were simultaneously given oral antibiotic treatment. The specific immune response was monitored by analysing the levels of specific antibodies with ELISA. As an indicator of the nonspecific immune response, the phagocytic activity of circulating leucocytes was measured by a chemiluminescence assay. Total circulating leucocyte counts and differentials were also monitored. The disease resistance was evaluated by challenge tests at the end of the experiment. The results indicate that both oxytetracycline and oxolinic acid significantly suppress antibody production as well as the level of circulating white cells, especially lymphocytes, in rainbow trout when administered in association with immunisation. The phagocytic activity of whole blood leucocytes was stimulated by oxolinic acid and slightly suppressed by oxytetra-cycline. The survival after challenge was not significantly affected by the antibiotics in the present tests.