Isolation characterization, virulence potential of Weissella ceti responsible for weissellosis outbreak in rainbow trout (Oncorhynchus mykiss) cultured in Mexico.

Weissella ceti, a Gram-positive nonmotile bacterium, is currently an emerging pathogen within rainbow trout (Oncorhynchus mykiss) farms in China, Brazil, the United States, and Japan. This study is the first to isolate, identify, and characterize W. ceti isolates from rainbow trout farmed in Mexico. In late 2015, a severe disease outbreak caused a 60% mortality rate among 20,000 fish. The diseased rainbow trout (100-300 g average) exhibited severe cachexia, body darkening, abdominal distension, exophthalmia, haemorrhages, and corneal opacity. Internally, diseased fish had pale gills; multifocal, disseminated whitish spots on the liver; haemorrhages in the swim bladder, ovary, and on the parietal surface of the muscle; and hearts with pseudo-membrane formation. Histologically, lesions were characterized by corneal oedema, degenerative and necrotic hepatitis, and meningitis. A brain (W-1) and kidney (W-2) isolate were identified as W. ceti through polyphasic taxonomy, which included phenotypic characterization and 16S rRNA sequencing. RAPD and ERIC-PCR analyses demonstrated genetic homogeneity among the Mexican isolates. Virulence tests in rainbow trout through intraperitoneal W. ceti injections at concentrations of 1 × 104 , 1 × 105 , and 1 × 106 CFU per fish resulted in cumulative mortality rates of 25%, 62.5%, and 87.5%, respectively, as well as the same clinical signs of hemorrhagic septicaemia as were recorded for the natural outbreak. The present report is the first to confirm the presence of W. ceti in Mexico, thus extending the known geographical distribution of this pathogen across the Americas.