Tenacibaculum sp. associated with winter ulcers in sea-reared Atlantic salmon Salmo salar
2011
Coldwater-associated ulcers, i.e. winter ulcers, in seawater-reared Atlantic salmonSalmo salar L. have been reported in Norway since the late 1980s, and Moritella viscosa has beenestablished as an important factor in the pathogenesis of this condition. As routine histopathologicalexamination of winter ulcer cases in our laboratory revealed frequent presence in ulcers of long, slenderrods clearly different from M. viscosa, a closer study focusing on these bacteria was conducted.Field cases of winter ulcers during 2 sampling periods, 1996 and 2004–2005, were investigated andlong, slender rods were observed by histopathological examination in 70 and 62.5% of the ulcersexamined, respectively, whereas cultivation on marine agar resulted in the isolation of yellowpigmentedcolonies with long rods from 3 and 13% of the ulcers only. The isolates could be separatedinto 2 groups, both identified as belonging to the genus Tenacibaculum based on phenotypic characterizationand 16S rRNA sequencing. Bath challenge for 7 h confirmed the ability of Group 1 bacteriumto produce skin and cornea ulcers. In fish already suffering from M. viscosa-induced ulcers,co-infection with the Group 1 bacterium was established within 1 h. Ulcers from field cases of winterulcers and from the transmission experiments tested positive by immunohistochemistry with polyclonalantiserum against the Group 1 bacterium but not the Group 2 bacterium. Our results stronglyindicate the importance of the Group 1 bacterium in the pathogenesis of winter ulcers in Norway. Thebacterium is difficult to isolate and is therefore likely to be underdiagnosed based on cultivation only.
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