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Title: Characterisation of geographically and temporally diverse Yersinia ruckeri isolates: evidence that UK and mainland European biotype 2 isolates represent different clonal groups

Author
item WHEELER, R - CEFAS WEYMOUTH, UK
item DAVIES, R - UNIV. OF GLASGOW SCOTLAND
item DALSGAARD, I - UNIV OF DENMARK
item GARCIA, J - UNIV COMPLUTENSE MADRID
item Welch, Timothy - Tim
item WAQLEY, S - CEFAS WEYMOUTH UK
item BATEMAN, K - CEFAS WEYMOUTH UK
item VERNER-JEFFREYS, D - CEFAS WEYMOUTH UK

Submitted to: Diseases of Aquatic Organisms
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/29/2009
Publication Date: 2/9/2009
Citation: Wheeler, R.W., Davies, R.L., Dalsgaard, I., Garcia, J., Welch, T.J., Waqley, S., Bateman, K.S., Verner-Jeffreys, D.W. 2009. Characterization of geographically and temporally diverse Yersinia ruckeri isolates: evidence that UK and mainland European biotype 2 isolates represent different clonal groups. Diseases of Aquatic Organisms. 84:25-33.

Interpretive Summary: Yesinia ruckeri infection in salmonid aquaculture has been successfully controlled by immersion vaccination for over three decades. However, recently strains of this pathogen have emerged that appear to cause outbreaks of infection in vaccinated fish. This study reports the development of a pulsed field gel electrophoresis (PFGE) method for the genetic characterization of Yesinia ruckeri strains. The method was used to characterize a geographically and temporally diverse collection of isolates, and PFGE typing was compared to other traditional typing schemes. Vaccine resistant strains of Yesinia ruckeri in the United Kingdom were slow to form a distinct genetic subgroup that differs from vaccine resistant strains isolated from other regions within Europe, suggesting that they represent different clones that have emerged independently. Interestingly, PFGE analysis of a vaccine resistant strain from the United States suggests that there has been exchange of these strains between the UK and US in the past. This work will lead to a better understanding of Yesinia ruckeri vaccine failure and will aid in the production of vaccines against these newly emerging strains of this important salmonid pathogen.

Technical Abstract: There have been increased reports of outbreaks of Enteric Redmouth Disease (ERM) caused by Yersinia ruckeri in previously-vaccinated salmonids in Europe, with some of these outbreaks attributed to emergent non-motile, Tween 80 negative, biotype 2 isolates. To gain information about their likely origins and relationships, a geographically and temporally diverse collection of isolates were characterised by serotyping, biotyping, pulsed field gel electrophoresis (PFGE) and outer membrane protein (OMP) profiling. A total of forty-four pulsotypes were identified from 159 isolates by PFGE, using the restriction enzyme Not1. Serotype O1 isolates responsible for ERM in rainbow trout in both the US and Europe, and including biotype 2 isolates, represented a distinct subgroup of similar pulsotypes. Biotype 2 isolates, responsible for outbreaks of disease in rainbow trout in the UK, Denmark and Spain, had different pulsotypes, suggesting they represented different clones that may have emerged separately. Danish biotype 2 strains recovered since 1995 were indistinguishable by PFGE from the dominant biotype 1 clone responsible for the majority of outbreaks in Denmark and the rest of mainland Europe. In contrast, US biotype 2 strain YRNC10 had an identical pulsotype and OMP profile to UK biotype 2 isolates, suggesting there has been exchange of these strains between the UK and US in the past. UK Atlantic salmon isolates were genetically and serologically diverse, with twelve distinct pulsotypes identified among 32 isolates. Overall, the results are consistent with the suggestion that worldwide immunization of salmonids against Y. ruckeri for more then 30 years is driving Y. ruckeri strain replacement.