Efficacy of Oxytetracycline in the treatment of Streptococus agalactiae in tilapia at different breeding temperatures

Abstract

Streptococcosis is one of the main causes of morbidity and mortality in Brazilian tilapia farming and oxytetracycline is one of the main antibiotics that have been used for decades in its treatment. There are studies that determine the differences in the pharmacokinetics of oxytetracycline in Nile Tilapia according to breeding temperatures. However, there are no studies evaluating the impact of these changes in breeding temperature on the clinical efficacy of oxytetracycline at usual doses. The objective of this study was to evaluate the effect of changes in breeding temperature on mortality rates, carrier animals and cure in Streptococcus agalactiae infections. The pharmacokinetic / pharmacodynamic index (PK/PD) used for the determination of efficacy was the area below the plasma concentration of oxytetracycline versus time (AUC)/minimum inhibitory concentration (MIC) curve. A Monte Carlo simulation of the AUC followed by the PK/PD modeling was used to estimate the rates of cured, carrion and dead animals. Changes were observed in the frequencies of dead individuals and estimated carriers related to variations in breeding temperature. The estimated mortality rates were 58% and 12% and the rates of asymptomatic carriers were 11% and 19%, respectively, according to the breeding temperatures of 25° C and 30° C. The probability of obtaining bacteriology cure after treatment increased for treatments performed at 30° C breeding temperature. It was observed that small increments in the CIM determined large changes in the rates of cured, carcass and dead animals. This demonstrates the need to incorporate in the therapeutic protocol bacteriological isolation, MIC determination and optimization of therapeutic doses in order to avoid therapeutic failures and consequently potentiate the development of resistance.