Effect of Laminaran [β(1,3)-D-Glucan] on the protective immunity of blue gourami,Trichogaster trichopterusagainstAeromonas hydrophila

The chemiluminescent (CL) response of head-kidney phagocytic cells isolated from blue gourami,Trichogaster trichopteruswhich were injected intra-peritoneally with laminaran [β(1,3)-D-Glucan] was studied. The phagocytes obtained from fish treated with doses of 5, 10, 20 and 40 mg kg−1had a higher CL response than that of the saline-treated fish. A dose of 20 mg kg−1significantly (P<0·05) enhanced the CL activity of fish phagocytes even at 22 days after the injection. However, in fish treated with 10 mg kg−1the peak response after 14 days was reduced significantly (P<0·05) at 22 days. Fish injected with doses of 5 mg and 40 mg kg−1showed a higher CL response initially but this was not sustained at 14 and 22 days respectively; a significantly reduced resistance to infection byAeromonas hydrophilawas also noted with these two concentrations. At eight days, the CL response in fish treated with laminaran at a dose of 10 mg kg−1plus formalin-killed bacteria was higher than that of the bacteria-injected fish. These fish also have a higher protection when challenged withA. hydrophilaat 14 days than those treated with laminaran or bacteria. However, fish injected with a combined dose of 20 mg kg−1laminaran and bacteria could sustain the high CL response and protective immunity at 22 days. The results suggest that laminaran [β(1,3)-D-Glucan] can significantly enhance phagocytic activity in the host. Furthermore, laminaran could enhance the resistance of fish againstA. hydrophilawhen injected together with the bacterial antigens. However this protection was not significantly (P>0·05) different at 29 days when compared with those injected with laminaran alone. Thus, a single intra-peritoneal injection of 20 mg kg−1laminaran is sufficient to protect the fish effectively from a virulent strain ofA. hydrophilafor up to 29 days.