Real-time PCR detection of Moritella viscosa, the likely causal agent of winter-ulcer in Atlantic salmon Salmo salar and rainbow trout Oncorhynchus mykiss

We report the development of a real-time PCR protocol for specific detection of Moritella viscosa. This bacterium is considered to be the main aetiological agent in development of winter-ulcer, a disease severely affecting salmonid aquaculture in Norway. From a newly elaborated draft version of the genome of M viscosa, the tonB gene sequence was selected as a suitable basis for the development of the real-time PCR assay. The real-time PCR demonstrated the presence of M. viscosa DNA sequences in 88.1 % of samples collected from 35 outbreaks of winter-ulcer in Norwegian fish farms. In contrast, standard culturing on blood agar identified M viscosa in only 39.7 % of fish. While the culturing method revealed a similar prevalence (26 to 27 %) of M viscosa in kidney and ulcer samples, substantially more ulcer (81.5 %) than kidney (49.7 %) samples were shown positive by real-time PCR.