Comparisons of nonspecific and specific immunomodulation by oxolinic acid, oxytetracycline and levamisole in salmonids.

Oxolinic acid, a promising drug for the treatment of bacterial fish disease agents, was tested for possible immunomodulatory effects on fish. Another antibiotic oxytetracycline, known to be immunosuppressive at higher treatment doses, and levamisole, a known immunostimulator for higher vertebrates, were also compared for causing changes in the nonspecific defense compartment and the specific immune system in rainbow trout. Groups of fish were immunized with Yersinia ruckeri O-antigen bacterin in combination with selected doses of the drugs. The nonspecific defense activity was measured by demonstrating neutrophil metabolic activity by the nitroblue tetrazolium assay, by counting engulfed bacterial cells for a phagocytic index and by counting leukocytes with adherent bacterial cells for the adherence index. The specific immune response was monitored by the passive hemolytic plaque assay demonstrating the numbers of antibody-producing cells. The results showed that oxolinic acid, used at recommended doses for the treatment of bacterial diseases, did not cause immunosuppression in either the nonspecific defense or specific immune system compartments, whereas tetracycline at 10 mg/kg caused reduced activity in both. Fish given levamisole injections before the antigen injection showed a stimulated nonspecific defense but a much reduced specific immune response.