Rainbow trout (10-20 g body weight) from a local fish farm were examined after low levels of mortality at a temperature of 10°C were observed. Dystrophic changes were observed only in the gills. Based on its unusual clinical presentation, this condition was termed atypical bacterial gill disease (ABGD). Only Flavobacterium psychrophilum was isolated from the gills. On the day the first symptoms of the disease appeared and on the third, seventh, and tenth days afterwards blood from 20 afflicted fish (diseased) and 20 unafflicted fish was drawn to examine the nonspecific humoral defense mechanism. In the present immunological studies a strong immunosuppression effect was observed in fish with symptoms of atypical bacterial gill disease. The results showed that serum lysozyme activity and Ig levels were statistically (P < 0.05) lower in fish with disease symptoms in comparison with control, disease-frie fish. The activity of serum ceruloplasmine was very high in fish with disease symptoms in comparison with the control fish. The results indicated that the suppression occurred as atypical bacterial gill disease was developing. All immunological parameters were statistically significantly lower after three, seven, and ten days of disease symptoms in comparison with the disease-free, control fish. This preliminary study indicated that ABGD with Flavobacterium psychrophilum has a strong suppression effect on the nonspecific humoral defense mechanisms.
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