Tenacibaculosis in Norwegian Atlantic salmon (Salmo salar) cage-farmed in cold sea water is primarily associated with Tenacibaculum finnmarkense genomovar finnmarkense

Skin conditions associated with Tenacibaculum spp. constitute a significant threat to the health and welfare of sea-farmed Atlantic salmon (Salmo salar L.) in Norway. Fifteen presumptive tenacibaculosis outbreaks distributed along the Norwegian coast during the late winter and spring of 2018 were investigated. Bacteriological culture confirmed the presence of Tenacibaculum spp. Seventy-six isolates cultured from individual fish were selected and subjected to whole-genome sequencing and MALDI-TOF MS analysis. Average nucleotide identity and MALDI-TOF analyses confirmed the presence of T. finnmarkense and T. dicentrarchi, with further division of T. finnmarkense into genomovars (gv.) finnmarkense and ulcerans. Core genome multilocus sequence typing (cgMLST) and single-nucleotide polymorphism (SNP) analyses identified the presence of a genetically conserved cluster of gv. finnmarkense isolates against a background of relatively genetically diverse gv. finnmarkense and gv. ulcerans isolates in 13 of the 15 studied cases. This clustering strongly suggests a link between T. finnmarkense gv. finnmarkense and development of clinical tenacibaculosis in sea-farmed Norwegian salmon in the late winter and spring. Analysis of 25 Tenacibaculum isolates collected during the spring of 2019 from similar cases identified a similar distribution of genotypes. Low water temperatures were common to all cases, and most incidences involved relatively small fish shortly after sea transfer, suggesting that these fish are particularly predisposed to Tenacibaculum infection.

Automated summary

Skin conditions associated with Tenacibaculum spp. pose a significant threat to sea-farmed Atlantic salmon in Norway. In this study, 15 suspected outbreaks were investigated, confirming the presence of Tenacibaculum spp. Through genetic analysis, T. finnmarkense and T. dicentrarchi were identified, with further differentiation of T. finnmarkense into genovars finnmarkense and ulcerans. The analysis revealed a genetically conserved cluster of T. finnmarkense gv. finnmarkense isolates, suggesting a link between this genotype and the development of clinical tenacibaculosis in sea-farmed Norwegian salmon. Similar genotypic distribution was observed in isolates collected in 2019, indicating a predisposition of small fish shortly after sea transfer to Tenacibaculum infection.