Recovery of a new biogroup of Yersinia ruckeri from diseased rainbow trout (Oncorhynchus mykiss, Walbaum)

Cultures of a new biogroup of Yersinia ruckeri, the causal agent of enteric redmouth (ERM), were recovered in England from diseased rainbow-trout (Oncorhynchus mykiss, Walbaum), which had been previously vaccinated with a commercial ERM vaccine. The bacterial isolates were confirmed as Y. ruckeri by the results of sequencing the 16S rRNA, but differed from the characteristics of the taxon by positivity for the Voges Proskauer reaction and a general lack of motility, and could not be equated with any of the existing serovars. Cultures were pathogenic in laboratory-based infectivity experiments with 100% mortalities occurring in juvenile rainbow trout (average weight = 10 g) within 4-days of intraperitoneal or intramuscular injection with 10⁵ cells/fish. Protection against disease was achieved using a formalin-inactivated whole vaccine prepared against a representative isolate.