Evaluation of resistance against Streptococcus agalactiae in four farmed strains of Nile tilapia (Oreochromis niloticus)

Abstract

Nile tilapia (Oreochromis niloticus) has always been considered more resistant to diseases compared to other fish species. However, in recent decades, this fish species has also been susceptible to many bacterial, fungal, parasitic, and nutritional diseases. In Brazil, streptococcosis is one of the main diseases that affect tilapia and has caused many economic losses. A resistance test was conducted with 93 tilapia of four varieties (UFLA strain and three commercial varieties: SC1, SC2, and SC3) to evaluate resistance to experimental inoculation with Streptococcus agalactiae. Each variety was distributed in five aquariums, with nine specimens of the same variety. Among the five aquariums, four housed fish were intraperitoneally inoculated with 107 CFU/fish of S. agalactiae and one aquarium harbored fish from the control group, which received 0.1 mL of sterile BHI (Brain Heart Infusion). Significant differences were observed in the survival rates accumulated at the end of the experiment (day 15), with values of 22, 36, 36, and 15% for SC1, SC2, SC3, and UFLA, respectively. The UFLA strain presented lower survival than other strains, which did not differ among the 15 days of challenge. These results demonstrate that there are animals with a better response to the resistance test than others.

Frequency of pathogens in routine bacteriological diagnosis in fish and their antimicrobial resistance

“…To attenuate this situation, which is critical in the One Health prism, many studies have been conducted to find an alternative to the use of antimicrobials, such as probiotics, genetic selection for disease resistance, and phytogenic and immunostimulants (Kavitha, Raja, & Perumal, 2018;Mian et al, 2020;Suphoronski et al, 2019;Vianna et al, 2020); but for the most part, the use of antibiotics is still prevalent in aquaculture. Another way to decrease the use of antimicrobials is to implement biosecurity measures, using reliable sources of stocks, immediate detection of pathogens with appropriate diagnostic tools, correct disinfection, and mainly perform the screening of pathogens to block entry routes (Bera et al, 2018) In this study, we analyzed the data on the resistance to antimicrobials of bacteria isolated from aquaculture fish for bacterial diagnostics in the period between January 01, 2017 and October 31, 2020.…”