Evaluation of a Florfenicol Formulation for Treating Streptococcosis and Francisellosis in Nile Tilapia (Oreochromis niloticus): A Study of Safety, Withdrawal Period and Efficacy

This study evaluates the efficacy of a new florfenicol-based drug, both in vitro and in vivo, in Nile tilapia (<i>Oreochromis niloticus</i>) against pathogens commonly found in fish farming and its withdrawal period. The drug's efficacy was tested using prophylactic, metaphylactic, and therapeutic approaches against <i>Streptococcus agalactiae</i> (serotypes Ib and III) and <i>Francisella orientalis</i>. The minimum inhibitory concentration of florfenicol was 4, 5, and 128 μg/mL for the different pathogens. Fish mortality was lower in the prophylactic treatment compared to the metaphylactic and therapeutic treatments for <i>S. agalactiae</i>. No difference in mortality was observed for <i>F. orientalis</i> across treatments. Mild to moderate lesions suggestive of intoxication were observed, mainly in the liver of fish treated with overdoses or exposed to low temperatures. Florfenicol reduced mortality rates, especially with early treatment (metaphylactic), in fish experimentally challenged with the pathogens. Moreover, prophylactic antimicrobial use is not recommended, as it promotes the selection of multidrug-resistant bacterial strains. Additionally, the residual concentration of the drug in muscle tissue lasted for a shorter period than that recommended by the manufacturer, and at lower concentrations than required by national and international legislation.