Comparison of in vitro and in vivo antibacterial efcacy for the control of Flavobacterium psychrophilum in rainbow trout ( Oncorhynchus mykiss) fry: the frst genotypical evidence in West Aegean region of Turkey

Abstract

The aim of this study was to investigate the presence of Flavobacterium psychrophilum and to examine the in vitro and invivo efcacy of enrofoxacin (ENR), doxycycline (DOX), and forfenicol (FFC) treatment in rainbow trout fry ( Oncorhynchus mykissWalbaum) against F. psychrophilum . A total of 220 naturally infected rainbow trout fry were examined and 26 bacterial isolates werephenotypically and genotypically identifed. Minimum inhibitory concentrations (MICs) of ENR, DOX, and FFC were determined. ENR was the most active, with an MIC for 90% of the isolates (MIC90) of 4 µg/mL. DOX was less active, with an MIC90 of 32 µg/mL forisolates. Approximately 25,000 naturally infected fry were divided as the control, ENR, DOX, and FFC group. ENR and FFC groups weregiven 10 mg kg 1 day 1 for 10 days, and DOX was given at 30 mg kg 1 day 1 on specifed days. Among the isolates, 12% were resistantto ENR, 38% were resistant to DOX, and 19% were resistant to FFC. Despite these in vitro results, ENR and DOX were not efectivein rainbow trout fry syndrome treatment. FFC reduced the mortality when compared with the other groups (P < 0.001). Tis studyindicated that in vivo and in vitro results are diferent from each other.