Mortalities of up to 60% farmed rainbow trout (Oncorhynchus mykiss) were recorded in six floating cages farms in the Lake Titicaca during the wet season (December to April) of 2018. Fish presented uni or bilateral exophthalmia, ocular hemorrhages and melanosis. Necropsy showed severe hemorrhage in swim bladder, brain, liver and muscle, splenomegaly, and gut congestion. Histology revealed retrobulbar hemorrhage with macrophage infiltration, multifocal necrosis in several tissues, meningitis, splenitis, epicarditis, and congestion. After microbiological culture in blood agar, flat and circular colonies with smooth edges and a zone of α-hemolysis were observed. The presence of Weissella ceti was confirmed through biochemical and molecular analysis (16S rRNA gene sequence analysis), without differences between Peruvian isolates. Worldwide, W. ceti isolates from rainbow trout were similar using 16S rRNA gene sequence, however, showed discrepancies between those isolates from different hosts. Finally, Koch's postulate was confirmed with experimental infections using rainbow trouts (~9 cm) infected intraperitoneally (0.1 mL of 104 CFU). After 21 days, clinical signs and lesions similar to the recorded in natural outbreaks were observed, cumulative mortality rate was 90% and bacteria was successfully recovered from both moribund and survivor fish. This is the first report and description of weissellosis caused by Weissella ceti isolated from rainbow trout cultured in Lake Titicaca, Peru and it is the first report of the primary isolation of this bacterium on TSA and TYES media instead of TSAB medium.
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