| dc.description.abstract | Streptococcus agalactiae is one of the most important pathogens affecting farm-raised tilapia worldwide. The oral therapy with antibiotics is the main therapeutic measure applied in outbreaks. Several antimicrobial agents have been used for the treatment of streptococcosis, being the florfenicol one of the most widely used drug. In Brazil, reports of treatment failures and recurrent mortality in herds treated with FLO have been done with the recommended dose of 10 mg/Kg of LW. Despite to being licensed in different countries, there arent studies about the therapeutic efficacy and optimal dosage of FLO for the treatment of SA infections in Nile tilapia (Oreochromis niloticus L.). The aim of this work was to evaluate the therapeutic efficiency and optimal dose of FLO for the treatment of SA infection in Nile tilapia (Oreochromis niloticus). The S. agalactiae strain SA 95-10, previously isolated from diseased tilapia in Brazil was used in this study. The minimum inhibitory concentration (MIC) of FLO for the SA 95-10 was determined according to VET04-A2 guidelines of CLSI. For the evaluation of therapeutic efficiency, Nile tilapia juveniles were experimentally infected with SA 95-10 and orally medicated with FLO in doses of 10, 20, 40 and 60mg/Kg of LW for 10 consecutive days and observed for a period of 20 days post-treatment. The MIC of FLO for SA 95-10 was 1 g/mL. The doses of 20 and 40mg/Kg were effective in the control of mortalities during the treatment period, however, mortality rates of 90% and 60% were observed at the observation period. After the end of experimental period 44.4% of the fish were shown to be carriers of bacteria. SA 95-10 was submitted to an, in vitro evaluation of persistence behavior induced by FLO, being exposed to 100 times the MIC value for FLO. The strain remained alive and viable after 12 hours of exposition, decreasing the bacterial counting from 108 CFU/mL to 106 CFU/mL. To evaluate the transmission of bacterium from carriers to healthy animals, a cohabitation assay was performed. Two groups of fish were challenged with the SA 95-10, medicated with FLO at 20 and 40 mg/Kg of LW for 10 consecutive days, and subsequently transferred to aquaria containing healthy animals. Mortalities of 40 to 50% and were observed in cohabitated animals. This might indicate that the ability of transmission and virulence is kept in FLO induced persistent S. agalactiae cells in carrier fish. The FLO was shown unable to control SA infection in Nile tilapia. It controls the disease during the treatment, however, mortalities are immediately verified when the administration ends. This is the first report of the persistence phenomenon induced by antibiotics in fish pathogenic S. agalactiae. Future studies should be carried out to determine the physiological and molecular mechanisms that allow the bacterium to resist to FLO | |
