Diagnosis of Weissellosis in Cultured Rainbow Trout in Costa Rica Further Evidence Emergence of Weissella tructae as Important Pathogen of Salmonids in Latin America.

Weissellosis, caused by the Gram-positive (GP) lactic acid bacterium Weissella tructae, is an emerging disease of farmed rainbow trout (Oncorhynchus mykiss) reported in Asia, South and North America. In this study, W. tructae was isolated from diseased cultured trout in Costa Rica on four facilities with mortalities seen predominantly in sub-adult (100-700 g) and adult fish (>2 kg) during April and September 2024 (rainy season). Losses occurred at seasonally high temperatures (>18°C) with clinical signs and gross changes including high morbidity and mortality, lethargy, abnormal swimming, exophthalmia, and hemorrhagic lesions in external and internal tissues consistent with bacterial sepsis. Whole-genome sequencing and phylogenomic analysis of isolates from Costa Rica (n = 4), Peru (n = 3), and Colombia (n = 2) revealed >99.99% genetic identity to each other and isolates from Brazil, Mexico, Colombia, and the United States. Western blot analysis using antibodies against W. tructae isolates from the Eastern United States, showed the isolates are antigenically similar to those from Costa Rica. These results provide evidence that the same pathogenic strain is causing outbreaks from the United States to Peru, suggesting a monovalent vaccine could protect trout farms across the Americas. To better define the role of temperature in weissellosis outbreaks, laboratory-controlled challenges were performed at 13 and 18°C using intracelomic (ICe) injection and cohabitation models. Rainbow trout ICe shedders were cohabitated with naïve fish (cohabitants) for 21 days. Injected trout maintained at 13°C experienced mortalities of 40%; however, ~89% (16/18 fish) remained infected at the end of the challenge. No mortality occurred in cohabitants at 13°C, and only 1/30 survivors (~3%) showed evidence of infection. In contrast, mortality in injected shedders and cohabitants exceeded 96% (29/30 injected fish) and 16% (5/30 cohabitants) at 18°C, respectively. Persistent infection was observed in 100% (1/1 injected) and 4% (1/25 cohabitant) of fish surviving challenge at 18°C, confirming a significant role for temperature in the pathogenesis of weissellosis.