Immunological parameters of rainbow trout (Oncorhynchus mykiss) with Flavobacterium psychrophilum gill infection

Flavobacterium psychrophilum, the etiological agent of bacterial coldwater disease and rainbow trout fry syndrome, causes great economic losses in salmonid aquaculture worldwide but the influence of this bacterium on non-specific immunity in fish and the development of disease is uncertain. In our studies rainbow trout were diagnosed with gill infections caused by the bacterium after experiencing mortality in one of the fish farm's tanks. For 6 weeks, we assessed the immunity parameters in affected fish. The activity of lysozyme and ceruloplasmin, as well as the level of total protein and immunoglobulins in fish serum were measured. Also, spleen phagocyte respiratory burst and potential killing activity, as well as head kidney lymphocyte proliferation stimulated by concanavalin A or lipopolysaccharide were determined. Almost all tested immunological parameters except for ceruloplasmin activity were reduced compared to the control group. F. psychrophilum seems to have mechanisms that allow it to evade the fish's immune system and suppress the basic non-specific humoral and cellular defense mechanisms of the infected fish.