Study of Bacterial Diseases in Farmed Rainbow Trout (Oncorhynchus mikyss) in Mazandaran Province, Iran

The possible bacterial infections were studied in six farmed rainbow trout in Mazandaran province at Haraz river region during spring, summer and autumn 2008. The external and internal clinical signs of affected fish were recorded and a number of 20 diseased fish from each fish farm were then obtained for bacteriology studies during each season. Clinically, the affected fish were lethargic showing blindness or exophthalmia, darkening of body, abdominal distension and prolaps of anal area. Internally, signs of hemorrhage in the intestine, pale and tattoo liver and accumulation of bloody fluids in the abdominal cavity were observable in dissection. From total 360 fish samples processed for routine bacteriological works, 149 samples were positive for Gram positive bacteria consisting of Streptococcus iniae (51 isolates), Lactococcus garviae (7 isolates), Staphylococcus areus and S. epidermidis (91 isolates). Also, 39 fish samples were positive for Gram negative bacteria consisting of Aeromonas hydrophila (20 isolates), Yersinia ruckeri (14 isolates) and Vibrio anguillarum (5 isolates). In antibiogram, all bacterial isolates were sensitive to enrofloxacine and flumequine while showing a variable sensitivity to peniciline and erythromycin. Total incidence of affected fish in the investigated fish farms to these bacterial infections were 52.22% and the highest incidence of infections were due to stapylococcosis (5.27%) and streptococcosis/lactococcosis (16.10%). Furthermore, the seasonal incidence of these bacterial infections during spring, summer and autumn were 40.83%, 6.66% and 49.61%, respectively. The results of this study show that the farmed rainbow trout of Haraz region are affected with different bacterial infections.