ARS | Publication request: Independent emergence of biotype 2 Yersinia ruckeri in the United States and Europe

Submitted to: Applied and Environmental Microbiology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: April 14, 2011
Publication Date: May 1, 2011
Citation: Welch, T.J., Verner-Jeffreys, D.W., Dalsgaard, I., Wiklund, T., Evenhuis, J., Garcia Cabrera, J.A., Hinshaw, J.M., Drennan, J.D., Lapatra, S.E. 2011. Independent emergence of biotype 2 Yersinia ruckeri in the United States and Europe. Applied and Environmental Microbiology. 2011 May;77(10):3493-9

Interpretive Summary: Yersinia ruckeri infection in salmonid aquaculture has been successfully controlled by immersion vaccination for over three decades. Recently, atypical strains of Y. ruckeri have been isolated from disease outbreaks in previously vaccinated fish in the Southeastern United States (US) and throughout Europe. These novel strains are highly related to the typical disease-causing strains of Y. ruckeri and have been classified as Y. ruckeri biotype 2 (BT2). Y. ruckeri biotype 2 strains pose an emerging threat to aquaculture. We identified the mutations responsible for the BT2 phenotype. Four distinct BT2-causing mutations were identified among US and European isolates of BT2 Y. ruckeri each in a gene encoding a critical component of the flagellar motility apparatus. Our findings demonstrate that the BT2 phenotype has arisen independently several times in the US and Europe and that BT2 strains have also been disseminated by means of cross-border movement of strains. Further dissemination, or emergence, of BT2 Y. ruckeri to different areas within the US could cause serious problems for US trout producers. A comprehensive understanding of the genetic basis of vaccine resistance in BT2 Y. ruckeri is critical for understanding Y. ruckeri vaccine failure and for developing and evaluating improved vaccines or other management practices aimed at BT2 isolates.

Technical Abstract: Biotype 2 (BT2) variants of the bacterium Yersinia ruckeri are an increasing disease problem in United States (US) and European aquaculture and have been characterized as serovar 1 isolates that lack peritrichous flagella and secreted phospholipase activity. The emergence of this biotype has been associated with an increased frequency of enteric redmouth disease (ERM) outbreaks in previously vaccinated salmonid fish. In this study, four independent specific natural mutations that cause the loss of both motility and secreted lipase activity were identified in BT2 strains from the US, United Kingdom (UK) and mainland Europe. Each of these was a unique mutation in either filR, flhA or flhB; all of which are genes predicted to encode essential components of the flagellar secretion apparatus. Our results demonstrate the existence of independent mutations leading to the BT2 phenotype, thus this phenotype has emerged separately at least four times. In addition, BT2 strains from the UK were shown to have the same mutant allele found in US BT2 strains suggesting a common origin of this BT2 lineage. This differentiation of distinct BT2 lineages is of critical importance for the development and validation of alternative vaccines or other treatment strategies intended for the control of BT2 strains.