Diagnosis of Weissellosis in cultured rainbow trout in Costa Rica further evidence emergence of Weissella tructae as important pathogen of salmonids in Latin America

Authors: SOTO, ESTEBAN - University Of California, Davis; ELIZONDO, CAROLINA - National University Of Costa Rica; YAZDI, ZEINAB - University Of California, Davis; JACOBSEN, KIM LI - University Of California, Davis; YUNIS-AGUINAGA, JEFFERSON - Universidad Cientifica Del Sur Peru; BARATO, PAOLA - Veterinary Pathology Corporation, Corpavet®; DAMME, MONICA - Veterinary Pathology Corporation, Corpavet®; FUENTES, DENNIS - Royal Trouts Of Costa Rica Sa; AGUILAR, FERNANDO - National University Of Costa Rica; VIQUEZ-RUIZ, EUNICE - National University Of Costa Rica; PEREIRA, FRANCINE - National University Of Costa Rica; MEDINA-MORILLO, MARCO - Institute Of The Sea Of Peru (IMARPE); ROJAS, GUSTAVO - National University Of Costa Rica; BARQUERO-CALVO, ELIAS - National University Of Costa Rica; Iwanowicz, Luke; Welch, Timothy

Submitted to: Transboundary and Emerging Diseases
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/13/2026
Publication Date: 6/17/2026
Citation: Soto, E., Elizondo, C., Yazdi, Z., Jacobsen, K., Yunis-Aguinaga, J., Barato, P., Damme, M., Fuentes, D., Aguilar, F., Viquez-Ruiz, E., Pereira, F., Medina-Morillo, M., Rojas, G., Barquero-Calvo, E., Iwanowicz, L.R., Welch, T.J. 2026. Diagnosis of Weissellosis in cultured rainbow trout in Costa Rica further evidence emergence of Weissella tructae as important pathogen of salmonids in Latin America. Transboundary and Emerging Diseases. 2026(1). Article 4690289. https://doi.org/10.1155/tbed/4690289.
DOI: https://doi.org/10.1155/tbed/4690289

Interpretive Summary: Weisselosis is a deadly bacterial disease of rainbow trout that is spreading rapidly through commercial trout farms across the Americas. This study reveals Costa Rica as the latest affected country and demonstrates that the same strain of the pathogen is causing outbreaks from the United States to Peru. This research also provides experimental evidence demonstrating that water temperature is an important predisposing factor for this disease as mortality rates climb sharply at temperatures of 18degree C. This result likely explains why outbreaks occur during the warmer months of the year at sites affected by this disease. The genetic uniformity among these strains suggests that a single vaccine may be able to protect farms across the entire hemisphere, greatly simplifying the development of a broadly effective vaccine.

Technical Abstract: Weissellosis, caused by the Gram-positive lactic acid bacterium Weissella tructae (formerly Weissella ceti), is an emerging disease of farmed rainbow trout (Oncorhynchus mykiss). First diagnosed in China in in 2007, weissellosis has since been reported at rainbow trout production facilities in Brazil, United States, Colombia, Mexico, Peru, and Vietnam. In this study, W. tructae was isolated from diseased cultured trout in Costa Rica. Outbreaks of weissellosis were diagnosed at three facilities with mortalities occurring predominantly in sub-adult and adult animals during April and September 2024, losses were associated with seasonally high water temperatures (>18C). Clinical signs and gross changes in affected groups included high morbidity and mortality, lethargy, abnormal swimming, exophthalmia, and hemorrhagic lesions in external and internal tissues consistent with bacterial sepsis. Isolates recovered were compared to those previously associated with weisellosis outbreaks with similar clinical signs and gross pathology in Peru and Colombia in the last 7 years. Whole-genome sequencing and phylogenomic analysis of isolates from Costa Rica, Peru, and Colombia revealed high genetic similarity (>99% identity) to each other and to strains previously recovered from Brazil, Mexico, Colombia, and the United States. Western blot analysis, using antibodies raised against W. tructae strains isolated in the Eastern United States, showed that isolates from Costa Rica are also antigenicity similar to the US isolates. These results provide evidence that the same strain of this pathogen is causing outbreaks from the United States to Peru suggests the possibility that a monovalent vaccine could protect trout farms across the American continent. To better define the role of temperature in weissellosis outbreaks, laboratory-controlled challenges were performed at both 13 and 18 degrees C using an injection challenge and cohabitation model of infection. Rainbow trout, maintained at the treatment temperatures, were challenged by intracoelomic (ICe) injection and used to expose uninfected tankmates, fish were monitored for 21 days. At 13 degrees C, injected trout experienced mortalities of 40%; however, most ~89% (16/18 fish) of the injected survivors remained persistently infected at the end of the challenge. No mortality occurred in cohabitant fish exposed at this temperature, and only 1/30 survivors in this group (~3%) showed evidence of infection. In contrast, trout exposed via injection and cohabitation at 18 degrees C presented mortalities of >96% (29/30 fish) and ~17% (5/30 fish), with persistent infection in 100% (1/1) and 4% (1/25) survivors for each group, confirming a significant role for temperature in the pathogenesis and transmission of weissellosis. Due to the growth of salmonid aquaculture in Latin America, and the emergence of this and other fish pathogens, further development of preventative methods such as vaccines, immunostimulants, probiotics and the implementation of better aquaculture practices are warranted for the growth of this industry.