Successful therapy of furunculosis in Atlantic salmon, Salmo salar L., using the fluoroquinolone antimicrobial agent enrofloxacin

Abstract

Abstract An outbreak of furunculosis was diagnosed in an Atlantic salmon, Salmo salar L., commercial sea‐pen farm and treated with the fluoroquinolone antimicrobial enrofloxacin. The affected post‐smolt, adult fish and broodstock readily consumed pelleted feed impregnated with enrofloxacin at concentrations of 1000, 1300 and 2000ppm, respectively, daily for 10 days. Mortalities of infected post‐smolts were reduced from an average of 5.8% per month for the 2 months preceding therapy to 0.6% per month for the 2 months following therapy. Infected adult fish had a 2 month mean mortality rate of 2.6% prior to enrofloxacin treatment and 0.3% over the 2 months subsequent to therapy. Apparently noninfected (nonclinical) fish in the same farm exhibited an average monthly mortality of 0.4% over the 4‐month study period. Sea water temperatures gradually rose from less than 2°C in March to approximately 12°C in September. Mean antimicrobial tissue concentrations over the 10 complete days of medication were 4.58, 7.32, 4.58, 8.89 and 3.40ppm for muscle, skin plus fat, liver, kidney and gill, respectively. Within 24h of consuming medicated feed, tissue concentrations of antimicrobial were nearly 66% of the average concentrations for the medication period. Half‐lives of elimination for the antimicrobial residues were approximately 2 days for liver, 3 days for kidney, 6 days for gill, 11 days for muscle and 22 days for skin plus fat. The producers reported a successful harvest after a 180‐day medication‐free period.