Streptococcus agalactiae (GBS), stands as the primary bacterial pathogen affecting cultured Nile tilapia (Oreochromis niloticus) globally, leading to significant mortalities throughout the farming cycle. Due to the indiscriminate use of antimicrobials and to promote viable alternatives for their replacement, this study investigated the therapeutic efficacy of tylosin against Streptococcus agalactiae serotype Ib.
To evaluate the potential treatment, fish were divided into three groups (20 fish/each). G1: negative control (fed with commercial food without medication and not inoculated with the bacteria); G2: positive control (fed with commercial food without medication and inoculated with 2.3 x 106 UFC/ml S. agalactiae); G3: treated group (fed with medicated feed – 10mg/ kg tylosin and inoculated with 2.3 x 106 UFC/ml S. agalactiae). Clinical signs, behavior, and daily mortality of fish were recorded four times a day until mortalities stopped.
The post-bacterial challenge observation period was 21 days. The negative control group showed 100% survival throughout the experimental period. In the groups inoculated with the bacteria, mortality was recorded from day 5 to day 15. Animals that died presented anorexia, lethargy, erratic swimming, melanosis, exophthalmos, and ocular opacity in both groups. significant difference (p = 0.0480) was observed in the survival of the untreated and Tylosin -treated groups (10mg/kg) as illustrated in Figure 1.
Treatment with tylosin significantly increased the survival of fish infected with S. agalactiae during the evaluation period. Survival curves showed different trajectories over the course of the days. The effect of the treatment was sufficient to alter the mortality rate compared to the group not treated with the antimicrobial. The group treated with tylosin had a 19% higher survival rate than the group not treated with the antimicrobial. The study demonstrates the effectiveness of tylosin in treating Streptococcus agalactiae infections, suggesting it is a viable alternative to other antimicrobials.
Acknowledgements : The authors thank São Paulo Research Foundation (FAPESP) for financial support 2021/ 08152-0 and National Council for Scientific Research (CNPq), 305092/2023-5 for the productivity grant (Fabiana Pilarski).