Erwan Lagadec

Since 2014, Atlantic salmon ( Salmo salar L.) displaying clinical signs of red skin disease (RSD), including haemorrhagic and ulcerative skin lesions, have been repeatedly observed in Swedish rivers. Although the disease has since been reported in other countries, including Norway, Denmark, Ireland and the UK, no pathogen has so far been conclusively associated with RSD. In this study, the presence of 17 fish pathogens was investigated through qPCR in 18 returning Atlantic salmon with clinical signs of the disease in rivers in Sweden and Norway between 2019 and 2021. Several potential pathogens were repeatedly detected, including a protozoan ( Ichthyobodo spp.), an oomycete ( Saprolegnia spp.) and several bacteria ( Yersinia ruckeri , Candidatus Branchiomonas cysticola, Aeromonas spp.). Cultivation on different media from ulcers and internal organs revealed high concentrations of rod‐shaped bacteria typical of Aeromonadaceae . Multilocus phylogenetic analysis of different clones and single gene phylogenies of sequences obtained from the fish revealed concurrent isolation of several bacterial strains belonging to the species A. bestiarum , A. piscicola and A. sobria . While these bacterial infections may be secondary, these findings are significant for future studies on RSD and should guide the investigation of future outbreaks. However, the involvement of Aeromonas spp. as putative primary etiological agents of the disease cannot be ruled out and needs to be assessed by challenge experiments.