Efficacy of orally administered florfenicol and oxolinic acid for the treatment of vibriosis in cod ( Gadus morhua)

Abstract

This study was performed to determine the efficacy of orally administered oxolinic acid and florfenicol in the treatment of experimentally induced vibriosis in cod Gadus morhua. The Vibrio anguillarum strain HI-610 was used. This strain has minimum inhibitory concentration (MIC) values of 0.016 mg/l against oxolinic acid and 0.5 mg/l against florfenicol. Ten groups of 40 fish each were challenged by bath, 8.5×10 6 cells/ml for 1 h. Three days following challenge, medication with oxolinic acid or florfenicol was introduced in eight of the groups. The dosages used were 10 or 20 mg/kg day for both antibacterials and administered at days 1, 2, 4, 6, 8 and 10 following initiation of treatment for oxolinic acid and daily for 10 consecutive days for florfenicol. Among challenged unmedicated fish, the mortality started at day 3 post-challenge reaching a final cumulative mortality of 87.5% at day 22 when the experiment was terminated. In the medicated groups, the majority of deaths occurred from days 3 to 5 post-challenge reaching final cumulative mortalities of 34% and 28%, respectively, for the fish treated with 10 and 20 mg/kg of oxolinic acid and 31% and 20%, respectively, for the fish treated with 10 and 20 mg/kg of florfenicol. Survival of medicated fish in all groups were significant ( p<0.005) greater than survival of challenged unmedicated fish. No significant difference ( p>0.1) in survival was however found between groups with parallel treatment or between groups given different drugs, dosages or medication regimens. Twenty-four hours following last medication, fish ( n=5) given a daily dosage of 10 mg/kg of florfenicol had mean plasma and muscle concentrations of 5.0±1.6 mg/l and 4.6±0.9 mg/kg, respectively. Corresponding values for fish given 20 mg/kg day of florfenicol were 6.5±1.3 mg/l (plasma) and 7.0±2.7 mg/kg (muscle). The plasma and muscle concentrations for fish treated with oxolinic acid were 0.8±0.2 mg/l and 1.9±0.4 mg/kg, respectively, when administered a dosage of 10 mg/kg day and 1.4±0.6 mg/l and 3.4±1.1 mg/kg, respectively, for the fish given a dosage of 20 mg/kg day.