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The fluoroquinolone antibacterial family is a relatively recent group of bactericidal compounds, generally characterized by efficacy against a wide spectrum of bacterial organisms and exhibiting minimal adverse effects in treated patients. The fluoroquinolones are widely prescribed in both human and veterinary medicine, though in veterinary medicine in the USA there are currently only two approved compounds, enrofloxacin (Baytril, Bayer Animal Health, Shawnee Mission, KS) and sarafloxacin (SaraFlox, Abbott Laboratories, North Chicago, IL), both with limited species and disease label approvals. Currently, there are no approved fluoroquinolone antibacterials to treat bacterial infectious diseases in cultured fish species. Enrofloxacin was administered to juvenile Atlantic salmon as a single bolus via intraarterial (i.a.), intraperitoneal (i.p.), intramuscular (i.m.), or oral gavage routes of administration. The drug was administered via the first three routes to achieve a dose of 10 mg/kg, and via oral gavage to achieve both 10 (p.o.-10) and 5 (p.o.-5) mg/kg doses. Two-compartment model kinetics were observed with elimination of half-lives (t1/2) of 130.6, 34.32, 84.98, 105.11, and 48.24 h, area under the drug concentration-time curves (AUC) of 84.3, 75.31, 55.61, 41.68, and 38.81 micrograms x h/mL, and bioavailabilities (F) of 100, 89.34, 65.97, 49.44, and 46.04% (i.a., i.p., i.m., p.o.-10, p.o.-5, respectively). All administration routes at 10 mg/kg were found to yield comparable drug concentration-time curves for multiple tissue, indicating no distinct advantage of using one route over another from a kinetics perspective. Finally, the 5 mg/kg dose (p.o.-5) yielded comparable multiple tissue drug concentration-time curves to the 10 mg/kg dose (p.o.-10), providing pharmacokinetic evidence to justify therapeutic efficacy trials with the lower dose.

Streptococcal infection (Streptococcus iniae) was diagnosed sequentially in two tanks of hybrid striped bass Morone saxatilis male ×M. chrysops female (Sunshine bass) grown in a commercial freshwater recirculation facility in western Massachusetts. The pathogen was isolated in the laboratory, biochemically and morphologically characterized, and antibacterial sensitivities determined. Streptococcus iniae‐induced lesions in the Sunshine bass were character at the gross and histopathological levels. Initial treatment with oxytetracycline was unsuccessful. Based on sensitivity results, enrofloxacin‐medicated feed was dosed at 10 mg/kg body weight for 10 d, while a subsequent trial was conducted at 5 mg/kg body weight for 10 d. Mortality of fish subsided promptly following initiation of enrofloxacin therapy, yielding a final mortality in the initial tank of 10.83% (control tank 55.5%) and in the second tank of 16.97% (control tank 39.8%). Tissue enrofloxacin residues, detected via a microbiologic bioassay, revealed greater quantities and longer duration of residues in various tissues from the 5‐mg as compared to the 10‐mg trial, potentially the result of adverse feed palatability. Enrofloxacin appears to have excellent potential as an antibacterial agent for treating susceptible bacterial diseases of Sunshine bass.

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