Aeromonas hydrophila isolated from wild freshwater fish in Croatia

Aeromonas hydrophila was recovered from fish living in lake Vrana on the Cr oatian island of Cres. The occurrence of the bacterium in the fish was asse ssed and related to gross signs of disease and findings at necropsy as a po tential health hazard for fish. Isolated bacteria were subjected to morphol ogical, physiological, biochemical and antibiotic susceptibility tests. A t otal of 26 A. hydrophila isolates were obtained. There was a clear seasonal ity, as no isolates were recovered in the summer months. Most of the isolat es were sensitive to all the antimicrobials used in the study except novobi ocin and penicillin G. Affected fish manifested haemorrhages over the skin, in the liver, kidney and swim bladder, spleen infarcts, fatty liver, ascit ic fluid and swollen haemopoietic tissues. A. hydrophila does not appear to pose a major threat for the fish in the lake at present but under unfavour able and stressful conditions it could seriously compromise fish health.