Emerging Fish Pathogens Lactococcus Petauri and L. Garvieae in Nile Tilapia (oreochromis Niloticus) Farmed in Brazil

Lactococcosis in fish has been associated with Lactococcus garvieae and the recently described L. petauri. However, the relevance of these emerging fish pathogens to Nile tilapia still requires thorough understanding. This study investigated lactococcosis outbreaks in Nile tilapia on Brazilian farms and characterized the isolates through multilocus sequence typing (MLST), virulence to Nile tilapia, and antimicrobial susceptibility. Lactococcosis outbreaks were monitored from 2019 to 2022 throughout Brazil. The outbreaks occurred mainly during warmer months, and co-infections were observed in four farms, whereas concurrent bacterial infections were identified in all farms. Since the sequence of the 16S rRNA was not capable of differentiating between L. petauri and L. garvieae, Lactococcus spp. isolates were identified at the species level using the gyrB gene sequence. In total, 30 isolates were classified as L. petauri and two as L. garvieae. According to the MLST, all L. petauri isolates were grouped in the sequence type (ST) 24, except for one isolate which belonged to the newly described ST47. A new ST was also described for the L. garvieae isolates identified, ST46. The L. petauri ST24 and ST47 were charac-terized as singletons, whereas L. garvieae ST46 was grouped with ST16 and ST17 and formed CC17. For the challenge trial, an L. petauri ST24 isolate was chosen considering that this MLST lineage was the most frequently observed. Fish were challenged by intraperitoneal injection and different bacterial doses were used (106 to 102 CFU per fish). The infection in the challenged fish progressed very rapidly, and within 48 h post-challenge clinical signs and the first mortalities were observed. The pathogenicity of L. petauri to Nile tilapia was confirmed. The estimated LD50 was 5.74 x 103 CFU 15 days post-challenge. Provisional epidemiological cutoff values were determined for L. petauri for six antimicrobial agents from different drug classes. All isolates were characterized as wild type (WT) for neomycin and oxytetracycline, whereas 96.67% of the isolates were char-acterized as WT for amoxicillin, erythromycin, and florfenicol, and 83.33% were WT for norfloxacin. The L. petauri ST24 was observed in different regions of the country, illustrating a rapid expansion of this bacterial lineage.