First report of Red Mark Syndrome (RMS) in farmed rainbow trout in Slovenia

Red mark syndrome (RMS) is an inflammatory skin disorder spreading in farmed adult rainbow trout (Oncorhynchus mykiss), firstly reported in the UK and USA , then signalled in Finland, Austria, Switzerland , Italy , Turkey, Iran and Chile. The RMS skin lesions evolve in terms of size, elevation and reddening, also showing a progressive scale loss. The possible involvement of an infectious agent has been confirmed by cohabitation trials, whereas the positivity of affected tissues for a Rickettsia-like organisms (RLO)-related DNA or a Midichloria-like organisms (MLO)-related DNA was demonstrated. Recently, intracytoplasmic oval or short rod shaped micro-organisms, resembling Rickettsiales, were observed by transmission electron microscopic (TEM) within macrophages, fibroblasts and erythrocytes in the skin of RMS affected rainbow trout in Italy; therefore, the authors indicated them as possible causative agents of RMS. Being evident from field observations that the disease is slowly spreading in Europe, here we report for the first time the description of an RMS episode in rainbow trout farmed in Slovenia.