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

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. Mortality was most severe in larger fish approaching market size. 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 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 72days 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, which we have termed Weissellosis, in the United States, and the spread of this pathogen might pose a significant threat to the domestic 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.