INITIAL DISEASE REPORT OF STREPTOCOCCUS-INIAE INFECTION IN HYBRID STRIPED (SUNSHINE) BASS AND SUCCESSFUL THERAPEUTIC INTERVENTION WITH THE FLUOROQUINOLONE ANTIBACTERIAL ENROFLOXACIN

Streptococcal infection (Streptococcus iniae) was diagnosed sequential ly in two tanks of hybrid striped bass Morone saxatilis male x M. chry sops female (Sunshine bass) grown in a commercial freshwater recircula tion facility in western Massachusetts. The pathogen was isolated in t he laboratory, biochemically and morphologically characterized, and an tibacterial sensitivities determined. Streptococcus iniae-induced lesi ons in the Sunshine bass were characterized at the gross and histopath ological levels. Initial treatment with oxytetracycline was unsuccessf ul. Based on sensitivity results, enrofloxacin-medicated feed was dose d at 10 mg/kg body weight for 10 d, while a subsequent trial was condu cted at 5 mg/kg body weight for 10 d. Mortality of fish subsided promp tly following initiation of enrofloxacin therapy, yielding a final mor tality in the initial tank of 10.83% (control tank 55.5%) and in the s econd tank of 16.97% (control tank 39.8%). Tissue enrofloxacin residue s, detected via a microbiologic bioassay, revealed greater quantities and longer duration of residues in various tissues from the 5-mg as co mpared to the 10-mg trial, potentially the result of adverse feed pala tability. Enrofloxacin appears to have excellent potential as an antib acterial agent for treating susceptible bacterial diseases of Sunshine bass.