Studies on the pathogenicity of Yersinia ruckeri biotype 2 to rainbow trout (Oncorhynchus mykiss, Walbaum)
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Recently outbreaks of enteric redmouth disease in previously vaccinated salmonids was attributed to biotype 2 (non-motile, lipase negative) isolates of Yersinia ruckeri. Currently, isolates of biotype 2 are the most widely encountered form in Europe and North America. Biochemical analysis of over 90 clinical isolates revealed that there are potentially eight novel phenotypes associated with disease. Cell surface characteristics of biotype 2 isolates displayed a unique O antigen structure. A multilocus sequence typing scheme was developed using 4 „housekeeping‟ gene loci in order to understand the phylogeny and population structure of Y. ruckeri. The scheme highlighted that although there is low species diversity, this is characteristic of a small population size which is not limited to fish species or geographical region. The study suggests that biotype 2 isolates have arisen due to changes in the population structure in the natural environment not as a direct mutation caused by vaccination. Analysis of extracellular products revealed that Y. ruckeri is a poor producer of excreted compounds. However, it was observed that these compounds are homogenous to all serotypes and biotypes. Antigenic characterisation highlighted that the O antigen is the serospecific antigen between biotypes although there was marked cross reaction with whole cell proteins. In this context, vaccination studies revealed that the O antigen is the dominant immunogenic molecule involved in protection against the disease. Virulence studies demonstrated that all serogroups and biotypes were virulent to rainbow trout. Y. ruckeri was able to stimulate the innate immune response in order to survive and proliferate in vaccinated rainbow trout.