Detection and Identification of Bacterial Pathogens in Rainbow Trout (Oncorhynchus mykiss) Samples from Fish Farms in Iran

Rainbow trout (Oncorhynchus mykiss) infections are the most important risk factor causing losses in fish farming industry. The aim of the present study was to identify and determine the frequency of bacterial pathogens Streptococcus iniae, S. parauberis, Lactococcus garvieae, L. lactis, Flavobacterium branchiophilum, F. psychrophilum and Yersinia ruckeri in diseased rainbow trout samples collected from different fish farms in Zanjan, Iran. A total of 103 individual diseased rainbow trout including 412 organ samples of liver, kidney, gills and eye organs were collected from 17 fish farms. Overall, 92 (89.3%) out of 103 rainbow trout were positive for the presence of at least one tested pathogen. According to culture and biochemical tests, 199 (48.3%) bacterial isolates were detected. However, molecular analysis confirmed 92.4% (184/199) of isolates. The most common isolated pathogen was L. garvieae (56.3%, 112/199), followed by S. iniae (11.5%, 23/199), F. branchiophilum (9.5%, 19/199) and Y. ruckeri (9.1%, 18/199). Lactococcus lactis and F. psychrophilum were detected with lowest frequency (2.5%, 5/199). Considering the high frequency of bacterial infections in our study, it seems that appropriate surveillance and control measures are essential to prevent the emergence and transmission of bacterial pathogens in rainbow trout farms.