Comparison of in vitro and in vivo antibacterial efficacy for the control of Flavobacterium psychrophilum in rainbow trout (Oncorhynchus mykiss)fry: the first 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 in vivo efficacy of enrofloxacin (ENR), doxycycline (DOX), and florfenicol (FFC) treatment in rainbow trout fry (Oncorhynchus mykiss Walbaum) against F. Psychrophilum. A total of 220 naturally infected rainbow trout fry were examined and 26 bacterial isolates were phenotypically and genotypically identified. 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 for isolates. Approximately 25,000 naturally infected fry were divided as the control, ENR, DOX, and FFC group. ENR and FFC groups were given 10 mg kg-1 day-1 for 10 days, and DOX was given at 30 mg kg-1 day-1 on specified days. Among the isolates, 12% were resistant to ENR, 38% were resistant to DOX, and 19% were resistant to FFC. Despite these in vitro results, ENR and DOX were not effective in rainbow trout fry syndrome treatment. FFC reduced the mortality when compared with the other groups (P < 0.001). This study indicated that in vivo and in vitro results are different from each other.