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Title: Mortality associated with Weissellosis (Weissella sp.) in USA farmed rainbow trout: Potential for control by vaccination

Author
item Welch, Timothy - Tim
item GOOD, CHRISTOPHER - Freshwater Institute

Submitted to: Aquaculture
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/16/2013
Publication Date: 2/1/2013
Citation: Welch, T.J., Good, C.M. 2013. Mortality associated with Weissellosis (Weissella sp.) in USA farmed rainbow trout: Potential for control by vaccination. Aquaculture. 388-391:122-127.

Interpretive Summary: Emerging pathogens are a significant threat to Aquaculture in the United States. In August of 2011, the authors identified a novel gram positive bacterial pathogen in farmed rainbow trout in North Carolina. The pathogen was demonstrated to be causing a severe outbreak resulting in a high level of sustained mortality (approximately 2000 fish/day) among 0.5-1.0 kilogram fish. Molecular analysis demonstrated that the pathogen is similar to bacterial isolates found associated with recent disease outbreaks in farmed rainbow trout in both China and Brazil and thus this represents the first report of this emerging pathogen in the US. We also report the development and validation of an efficacious Weissella sp. vaccine. This vaccine was additionally shown to be effective when mixed with another vaccine that is currently used in North Carolina thus demonstrating that these vaccines can be delivered together minimizing the increased production costs and negative effects caused by multiple injections. Early detection of this pathogen and rapid development and implementation of this vaccine will aid control efforts and reduce the likelihood of further pathogen dissemination in the US.

Technical Abstract: Recent reports indicate that novel Weissella sp. bacteria have been associated with disease outbreaks in cultured rainbow trout (Oncorhynchus mykiss) at commercial farms in China and Brazil. In the summer of 2011 a severe disease outbreak displaying similar clinical signs occurred at a commercial rainbow trout farm in western North Carolina. Observed signs included dark skin coloration, lethargic swimming, bilateral exophthalmia, corneal opacity, ocular hemorrhage, occasional corneal rupture, and in some cases cerebral hemorrhage. Bacteria isolated from moribund fish were identified to the genus level as Weissella sp. by 16S rRNA gene sequence analysis and were 99% identical to the sequences of isolates collected from the Chinese and Brazilian outbreaks. Laboratory-based disease challenge experiments using the isolated pathogen replicated both the disease signs and induction of mortality in in exposed healthy rainbow trout. Aqueous vaccine formulations containing formalin-inactivated whole-cell Weissella sp. antigens conferred significant protection against experimental infection when both the vaccine and the pathogen were delivered by injection (relative percent survival [RPS] of 87.5% and 85% at 38 and 72 days after vaccination, respectively). The Weissella sp. vaccine was equally effective when combined with a commercially available Yersinia ruckeri vaccine, and this bivalent formulation did not alter the efficacy of the Y. ruckeri component of the vaccine. This is the first identification of this emerging rainbow trout disease in the United States, and the spread of this pathogen might pose a significant threat to the rainbow trout aquaculture industry. Our results also suggest that a bivalent Weissella/Y. ruckeri vaccine could be used as an effective and economical means for controlling this pathogen.