Effect of sulphadiazine and trimethoprim on the immune response of rainbow trout (Oncorhynchus mykiss).

A combination of sulphadiazine and trimethoprim at a ratio of 5:1 (SDZ/TMP) was tested for possible immunomodulatory effects. The aim of the study was to follow the immune response after vaccination with simultaneous drug treatment. The fish were immunised with a commercial oil-based divalent (furunculosis/vibriosis) vaccine and were simultaneously given oral drug treatment. The specific immune response was monitored by analysing the levels of specific antibodies with ELISA. As indicators of the nonspecific immune response, the lysozyme activity of serum was measured and the phagocytic activity of circulating leucocytes was monitored 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 SDZ/TMP at a ratio of 5:1 does not interfere negatively with the immune response in rainbow trout after vaccination. A slight stimulation in the antibody response as well as in the chemiluminescence response of circulating granulocytes was recorded in fish treated with the drugs in connection with vaccination. The drugs did not significantly affect the survival after challenge.