SUCCESSFUL THERAPY OF FURUNCULOSIS IN ATLANTIC SALMON, SALMO-SALAR L USING THE FLUOROQUINOLONE ANTIMICROBIAL AGENT ENROFLOXACIN

An outbreak of furunculosis was diagnosed in an Atlantic salmon, Salmo salar L., commercial sea-pen farm and treated with the fluoroquinolon e antimicrobial enrofloxacin. The affected post-smolt, adult fish and broodstock readily consumed pelleted feed impregnated with enrofloxaci n at concentrations of 1000, 1300 and 2000 ppm, respectively, daily fo r 10 days. Mortalities of infected post-smolts were reduced from an av erage 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 morta lity of 0.4% over the 4-month study period. Sea water temperatures gra dually rose from less than 2-degrees-C in March to approximately 12-de grees-C in September. Mean antimicrobial tissue concentrations over th e 10 complete days of medication were 4.58, 7.32, 4.58. 8-89 and 3.40 ppm for muscle, skin plus fat, liver, kidney and gill, respectively. W ithin 24 h of consuming medicated feed. tissue concentrations of antim icrobial were nearly 66% of the average concentrations for the medicat ion period. Half-lives of elimination for the antimicrobial residues w ere 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 repo rted a successful harvest after a 180-day medication-free period.