Chemotherapy of Cytophaga/Flexibacter ‐like bacteria (CFLB) infections in fish: studies validating clinical efficacies of selected antimicrobials

In vitro and in vivo efficacies of commonly used chemotherapeutants were determined for Cytophaga johnsonae, Cytophaga psychrophila, Flexibacter colutnnaris and Flexibacter maritimus. Treatmeat of barramundi, Lutes calcarifer (Bloch), with oxolinic acid (OA) as a bath (50 ppm) or by mouth (10 mg kg −1 body weight) resulted in serum levels above the minimum inhibitory concentration (MIC) for F. columnaris and produced significant clinical efficacy ( P < 0.05). Amoxycillin (AM) was found to produce adequate serum levels against F. maritimus when used as a bath (200 ppm) or given orally (80 mg kg −1 body weight) to Atlantic salmon, Salmo salar L. and rainbow trout, Oncorhynchus mykiss (Walbaum), respectively. At these dose rates, this antibiotic was also clinically efficacious against F. maritimus ( P <0.05). Trimethoprim produced more than adequate serum levels for the control of F. maritimus when given as a bath (50 ppm) or orally (10 mg kg −1 body weight). Trimethoprim was significantly more protective than AM when tested in vivo ( P < 0.05). For C. johnsonae and C. psychrophila , the MIC values for OA and oxytetracycline were low, whereas that for TMP was high. Also, based on MIC values, C. psychrophila strains were more sensitive to AM and norfloxacin than C. johnsonae.