There has been discussion about the taxonomic position
of the aetiological agent of ERM. Strong agglutination
with Salmonella enterica subsp arizonae O group 26,
and a weak reaction with O group 29 was reported
. In addition, biochemical similarities with enterics,
notably Enterobacter liquefaciens, Serratia marcescens
subsp. kiliensis as well as Salmonella enterica subsp. arizonae
were mentioned. Serological cross-reactions
were also recorded with Hafnia alvei. Nevertheless,
a new species, i.e. Yersinia ruckeri was described
although there was only a 30-31% DNA homology with
Y. enterocolitica and Y. pseudotuberculosis. This
compares to DNA homologies of 24-28% and 31% with
Serratia marcescens and Serratia liquefaciens, respectively
. Indeed, it has been suggested that the causal
agent of ERM should belong in a new genus of the
Enterobacteriaceae. A complication developed
when a new non-motile form of the pathogen was
recovered from rainbow trout. By 16S rRNA sequencing
and a homology of 100%, the organisms were linked to
Y. ruckeri but regarded as a new biogroup. Similar
non-motile variants were also recovered from previously
vaccinated rainbow trout in Spain