Levamisole Enhances Non-Specific Immune Response of Cobia, Rachycentron canadum, Fingerlings

Abstract

Two experimental runs were undertaken to assess the effect of levamisole supplementation on the non-specific immunity of cobia fingerlings, as well as on its resistance to Photobacterium damselae subsp. piscicida infection. Levamisole was given to cobia fingerlings through feed incorporation at a dosage of 500 and 1000 mg/kg feeds. Control feed is without levamisole. Cobia fingerlings were fed to satiation with control and levamisole supplemented feeds for two weeks. One day after the last feeding, blood was collected from 10 fish samples from each replicate for the analysis of non-specific immune response, including potential killing and phagocytic activities of blood leukocytes and bactericidal and lysozyme activities of blood plasma. Challenge experiment using P. damselae subsp. piscicida was also undertaken after the immunostimulation. Five (Run Ⅰ) and 15 (Run Ⅱ) experimental and control fish were intraperitoneally injected with 10^7 CFU/fish, and mortality was monitored up to five days post-injection. Results show that levamisole supplementation has no significant effect on lysozyme activity of blood plasma. Bactericidal activity, on the other hand, is significantly higher (P＜0.05) in levamisole treated fish than the control fish. Potential killing activity of blood leukocytes also increased significantly (P＜0.05) in levamisole treated fish compared to the control. Similar results were obtained for phagocytic activity of blood luekocytes where higher index of phagocytosis was observed in cobia fingerlings fed with levamisole supplemented feeds. Dose-response was also observed in the activities of blood leukocytes with higher killing activities at 1000 mg/kg dose than at 500 mg/kg dose. Significantly lower (P＜0.05) cumulative mortality was observed in immunostimulated fish (16-30%) than control fish (70-93%) after experimental challenge with P. damselae subsp. piscicida. Results of this study showed that levamisole can enhance the non-specific immune response of cobia fingerlings, thus increasing its resistance to potential infection.