First description of Yersinia ruckeri serotype O2 in Spain

Yersinia ruckeri was isolated from diseased unvaccinated rainbow trout obtained from a farm in which vaccination against enteric redmouth disease (ERM) has been systematically performed for more than 10 years. Affected fish presented typical signs of yersiniosis including exophthalmia and eye haemorrhages. Serological characterization of the isolate demonstrated that it belongs to the serotype O2b of this bacterial pathogen, which was never before described in Spain. Enteric redmouth disease (ERM) or yersiniosis, caused by Yersinia ruckeri, is a threatening infectious disease which has produced significant losses in the aquaculture farming industry worldwide (Furones et al., 1993; Stevenson, 1997). In Spain, the first description of the ERM occurred in 1985 (Cruz et al., 1986) and since that date, Y. ruckeri has been regularly detected, producing significant losses in rainbow trout (Toranzo et al., 1990). A great serological variability was demonstrated for this bacterial pathogen. In previous works we proposed a O-serotyping scheme for Y. ruckeri composed by four serotypes (O1 to O4), being defined two subtypes in serotype O1 and three in serotype O2 (Romalde et al., 1993). Serotypes O1a (formerly serovar I or “Hagerman” type) and O2b (formerly serovar II) are the most virulent, but whereas serotype O1 has a worldwide distribution, serotype O2b is restricted to North America and Baltic Countries (Romalde et al., 1993; Stevenson, 1997). ERM is the first fish disease for which an effective vaccine was developed and licensed (Newmann, 1993; Stevenson, 1997). Despite of the serological heterogeneity mentioned above, most commercial vaccines used in rainbow trout (Oncorhynchus mykiss) cultures include only serotype O1 strains in their formulations. However, high success rates were obtained regardless of the bacterin employed or the route of administration (Newmann, 1993), because of the highest incidence of this serotype. Four episodes of low mortality (0.1-0.2 %) of rainbow trout (approximate size of 4.5-5.5 cm) were detected in a Spanish farm during the autum season of 2002. External signs of afNotes Bull. Eur. Ass. Fish Pathol., 23(3) 2003, 136 fected fish include bilateral exophthalmia and haemorrhagic iris. Microbiological analysis of internal organs and exophthalmic eyes rendered in all cases the growth in pure culture of motile Gram-negative bacteria. Biochemical characterization of representative isolates from each episode of mortality was performed by conventional tube and plate tests procedures previously described (Romalde, 1992; Furones et al., 1993). The isolates were found to be fermentative, oxidase negative, and positive for decarboxylation of lysine and ornithine, Voges-Proskauer, and utilization of glucose, mannitol and sorbitol (Table 1). These characteristics allowed their presumptive identification as Yersinia ruckeri (Romalde, 1992; Furones et al., 1993). The utilization of the miniaturized system API 20E rendered for all the isolates the same profile 5105500, with no inconsistencies between these results and those obtained in the conventional tests. Although according to the API database the profile obtained should be considered as Hafnia alvei (very good identification), it can also correspond to Y. ruckeri. In fact, in a wide study including 53 Y. ruckeri strains with different host and geographical origin (Romalde & Toranzo, 1991) this API profile was described as belonging to this bacterial species. In addition, the possibility of misidentification of Y. ruckeri as H. alvei by using this miniaturized system was also indicated by these authors. It is interesting to point out that in other fish of the same tanks Flavobacterium psychrophilum was detected, but no coinfection of the two pathogens was observed in any of the fish analyzed. No isolation of pathogens was achieved for apparently healthy fish. Confirmative serological identification was carried out employing antisera raised against serotypes O1a and O2b of Y. ruckeri obtained t s e T t l u s e R t s e T t l u s e R m a r G r e u a k s o r P s e g o V + e p a h S d o r n o i t a z i l i t u e t a r t i C y t i l i t o M + s i s y l o r d y h n i t a l e G e s a l a t a C + e s a e r U e s a d i x O n o i t c a e r F / O : m o r f d i c A F e s o c u l G + e s a d i s o t c a l a g ß + l o t i n n a M + e s a l o r d y h i d e n i n i g r A l o t i s o n I e s a l y x o b r a c e d e n i s y L + l o t i b r o S + e s a l y x o b r a c e d e n i h t i n r O + e s o n m a h R e l o d n I e s o r c u S e d i h p l u s n e g o r d y H e s o i b i l e M e s a n i m a e d n a h p o t p y r T n i l a d g y m A n o i t c u d e r e t a r t i N +