BULLETIN OF THE EUROPEAN ASSOCIATION OF FISH PATHOLOGISTS
Abstract
Mortality episodes in 2-year-old Lena sturgeon (Acipenser baeri) occurred from the beginning of July until the end of September 2013 in a commercial fish farm in southern Russia. Water samples were collected to evaluate physical/chemical properties. High concentrations of sulphates (625.0-730.0 mg/L) and chloride (986.0-1003.0 mg/L) together with an increase of the mineralization rates (2.8-2.9 times the normal ratio) were detected. Fifteen fish were sampled: for haematological n=5, parasitological n=15, microbiological n=5, histological n=5 investigations. The changes in red blood cells consisted of erythrocyte agglutination, poikilocytosis, anisocytosis, and development of anaemia. The white blood cell changes were characterized by an increased number of banded neutrophils, monocytes and by a decrease of lymphocytes and segmented neutrophils. Sixty-seven different bacterial isolates were identified in water (32) and in liver, spleen and kidneys of the diseased fish (35) using standard microbiological methods. The bacteria were identified based on morphological and biochemical features by using conventional methods and MALDI-TOF mass-spectrometry. Microorganisms belonging to the genera Aeromonas, Vibrio and Edwardsiella were detected. Histological changes in spleen, liver and heart of fish appeared as congestion, haemorrhages, scattered necrotic areas and severe lymphocyte depletion. Ziehl-Neelsen acid-fast stain of the liver sections showed the presence of numerous bacteria exhibiting morphology and characteristics similar to those of mycobacteria. A long exposure of fish to unfavourable abiotic factors present in the water such as a high mineral and ion content might have caused an immune dysfunction and reduced the resistance of A. baerii sturgeon to multiple bacterial infections.
