RSD-MLO infection in Atlantic salmon

Red skin disease (RSD) is affecting an increasing number of adult Atlantic salmon ascending rivers in Scandinavia and the British Isles during summer. RSD causes hemorrhagic skin changes, likely leading to secondary infections. The cause of this pathology is unknown. However, screening of samples from RSD-affected salmon with qPCR directed at RMS-MLO (RMS= red mark syndrome, MLO=Midichloria-like organism) in rainbow trout, resulted in semi-positive results: Amplification efficiency was low and melting temperature differed relative to RMS control samples. This indicates that there was no RMS-MLO in the samples, but possibly a related MLO-type bacterium. In addition, amplification with RMS-MLO qPCR was only observed with skin samples from RSD-affected sites, not from apparently healthy skin from the same individuals. This further implicates “RSD-MLO” as the possible causative agent of RSD. The proposed research project aims to investigate these findings by attempting to transfer RSD-MLO from infected salmon tissue to healthy juvenile salmons by injection.