Recurrent Streptoccoccus agalactiae infection in Nile tilapia (Oreochromis niloticus) treated with florfenicol

Streptococcus agalactiae is a highly pathogenic bacterium of farmed tilapia. Antimicrobial therapy is the principal control measure applied during outbreaks. Several reports have documented treatment failure and recurrent disease in herds treated with FLO. The present study aimed to: evaluate the profile of florfenicol resistance of Streptococcus agalactiae to calculate the provisional epidemiological cut-off value (ECV); address the therapeutic efficiency of FLO in treating streptococcosis in Nile tilapia; and determine the persister phenotype induced by FLO. Evaluation by MIC found 100 strains to possess sensitivity to FLO. The ECV was 8 μg/mL for 94% of the tested strains classified as wild-type. Therapeutic efficiency of FLO under doses of 10, 20, 40 and 60 mg/Kg for 10 days was tested. Dosages of 20 and 40 mg/Kg of LW were able to control mortality during therapy period, however, they did not cure the fish and mortality was observed post-treatment. A persistence assay was carried out and the strain SA95-10 remained viable for 10 days when exposed to 100 times its MIC value. In conclusion, the ECV for FLO, recommended dosage to control Streptococcus agalactiae mortality and the persister phenotype are described here for the first time.