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Summary Previous studies have shown that leucine metabolite β‐hydroxy‐β‐methylbutyrate (HMB) increases the immune function in animals as measured by cellular and humoral immune responses. In the present study the influence of feeding HMB on the nonspecific cellular and humoral defence mechanisms and protection against furunculosis in rainbow trout (Oncorhynchus mykiss) was examined. HMB was fed in a pelleted ration at either 0 (control), 10, 25 or 50 mg  kg−1 bw day−1 for 8 weeks. Blood and pronephros cells were taken at random from 10 fish in each group for the analyses. The respiratory burst activity (RBA) and potential killing activity (PKA) of phagocytes, lymphocyte proliferation stimulated by either concanavalin (ConA) or lipopolysaccharide (LPS), lysozyme activities and total immunoglobulin (Ig) levels in plasma were analysed at 0, 1, 2, 3, 4, 6 and 8 weeks. After 4 weeks of feeding HMB, a challenge test was performed by injection of Aeromonas salmonicida into the fish. HMB approximately doubled the respiratory burst activity and potential killing activity ability of polymorphonuclear (PMN) and morphonuclear (MN) cells (P < 0.01) and increased the mitogen‐stimulated lymphocyte proliferation (P < 0.01) when compared with the control group. HMB feeding also increased (P < 0.01) the lysozyme activity in plasma and total Ig levels in serum. During the 14‐day challenge test, mortality was decreased (P < 0.01) by up to 62% in HMB‐fed fish compared with the control group mortality. In conclusion, the non‐specific immune enhancement by HMB resulted in protection against furunculosis in the rainbow trout.

IntroductionImmune-potentiating functions of Lactobacillus plantarum strains in the common carp were evaluated.Material and MethodsFourteen days of feeding fish dry diet supplemented with the bacteria provided parameters of nonspecific humoral immunity (lysozyme, ceruloplasmin, γ-globulin, total protein levels, and serum bactericidal activity) and cellular immunity (pinocytosis, respiratory burst activity, and potential killing activity of organ phagocytes), as well as the proliferative response of organ lymphocytes stimulated with mitogens. The resistance of fish to infection with Aeromonas hydrophila was also determined.ResultsDietary supplementation with L. plantarum had a substantial influence on the activity of organ phagocytes, especially the potential killing activity of head kidney cells. A significant increase in the proliferative activity of LPS-stimulated B lymphocytes and in the levels of γ-globulins and total protein was observed. The supplemented diet conveyed higher resistance than the control diet as the cumulative fish mortalities after infection with A. hydrophila were 65% and 85%, respectively.ConclusionThe results indicate that dietary supplementation with L. plantarum stimulates the antibacterial resistance of common carp and may reinforce defence against bacterial infections, but further studies need to be conducted.

Immunostimulants activate nonspecific defence mechanisms, cell-mediated immunity and specific immune responses. Immunostimulants can be administered before, with, or after vaccines to amplify the specific immune response in generating elevations of humoral antibody levels and the number of antibody-secreting cells (ASC). Special applications of immunostimulants include assisting spray or other regimens to increase the topical uptake of vaccines. In the present study, the influence of dimerized lysozyme (KLP-602) and inosine pranobex (groprinosin) on the cellular and humoral immune response after in vitro and in vivo immunization of rainbow trout with Yersrnia ruckeri vaccine (Yersivax) was determined. Immunostimulants were administered by injection before, with, or after the vaccine applied by immersion. ELISPOT assays were used for the quantification of total and specific ASC after in vitro and in vivo immunization. Also, the specific immune response was determined 7, 14, 18,2 1,28,35 and 45 days after in vivo immunization. Immunostimulants administered before, with, or after the vaccine increased cell-mediated immunity and the specific immune response, compared with the control groups (vaccine only). The higher responses (adjuvant effect) after the application of dimerized lysozyme (KLP-602) were observed in each experimental study.

Influence of beta-1.3/1.6-glucan (Macrogard) on the innate immunity and protection against Aeromonas hydrophila in tench (Tinca tinca (L.)) was assessed. Macrogard was fed at doses of 0, 0.5, 1 and 2 g kg -1 of pellets for 1 month. The blood, spleen and head kidney from 10 fish of each group were separated and analysed for immunity parameters. Twenty tench from each group were infected with A. hydrophila. Macrogard at doses 1 and 2 g kg -1 of feed significantly (P \ 0.05) increased the phagocytic activity of macrophages and proliferative response of mitogens stimulated lymphocytes. The same doses significantly (P \ 0.05) increased lysozyme activity and Ig level in serum, compared to the control and dose 0.5 g kg -1 of feed. The challenge test showed that Macrogard reduced mortality of tench after experimental infection (5-35%).