Location: Cool and Cold Water Aquaculture ResearchTitle: qPCR screening for Yersinia ruckeri clonal complex 1 against a background of putatively avirulent strains in Norwegian aquaculture
|RIBORG, ANDREAS - Norwegian Veterinary Institute
|GULLA, SNORRE - Norwegian Veterinary Institute
|STRAND, DAVID - Norwegian Veterinary Institute
|WIIK-NIELSEN, JANNICKE - Norwegian Veterinary Institute
|RONNESETH, ANITA - University Of Bergen
|Welch, Timothy - Tim
|SPILSBERG, BJØRN - Norwegian Veterinary Institute
|COLQUHOUN, DUNCAN - Norwegian Veterinary Institute
Submitted to: Journal of Fish Diseases
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/6/2022
Publication Date: 6/1/2022
Citation: Riborg, A., Gulla, S., Strand, D., Wiik-Nielsen, J., Ronneseth, A., Welch, T.J., Spilsberg, B., Colquhoun, D.J. 2022. qPCR screening for Yersinia ruckeri clonal complex 1 against a background of putatively avirulent strains in Norwegian aquaculture. Journal of Fish Diseases. https://doi.org/10.1111/jfd.13656.
Interpretive Summary: Yersinia ruckeri is an important pathogen in salmon aquaculture causing a devastating disease known as yersiniosis. The recent discovery of non-virulent strains of this pathogen has severely complicated its surveillance since all existing detection methods are unable to distinguish between virulent and non-virulent strains. In this paper we describe the development and validation of a PCR assay that distinguishes between avirulent strains and the highly virulent CC1 group of Y. ruckeri that causes most outbreaks of yersiniosis in farmed Norwegian Atlantic salmon. Laboratory and field trials validated the specificity and utility of the assays and verified the widespread prevalence of avirulent Y. ruckeri strains in Norwegian salmon farms. In addition, the assay confirmed the effect of thermal delousing on shedding of Y. ruckeri from fish with subclinical infection suggesting that this stressful louse management procedure could be the cause of acute outbreaks. These improved assays will be useful for non-lethal screening for sub-clinically infected salmon populations and will better inform the need for disease control interventions.
Technical Abstract: Although a number of genetically diverse Yersinia ruckeri strains are present in Norwegian aquaculture environments, most if not all outbreaks of yersiniosis in Atlantic salmon in Norway are associated with a single specific genetic lineage of serotype O1, termed clonal complex 1. To investigate the presence of virulent and putatively avirulent strains in Norwegian salmon farms, PCR assays specific for Y. ruckeri (species level) and Y. ruckeri clonal complex 1 were developed. Following extensive screening of water and biofilm, the widespread prevalence of putatively avirulent Y. ruckeri strains was confirmed in freshwater salmon hatcheries, while Y. ruckeri clonal complex 1 was found in fewer farms. Notably, both PCR-assays also identified Y. ruckeri DNA originating from yersiniosis vaccines several weeks post-vaccination, severely limiting their usefulness for screening in such cases. Moreover, field studies and laboratory trials confirmed that stressful management procedures may result in increased shedding of Y. ruckeri by sub-clinically infected fish. Analysis of seawater sampled throughout thermal delousing proved effective for detection of Y. ruckeri in sub-clinically infected populations.