Abstract
Red mark syndrome ( RMS ) and US strawberry disease ( US SD ) are skin disorders affecting rainbow trout farmed in Europe and USA . The disease etiology has not yet been established. In spite of specific investigations, identifying Rickettsia ‐like organism ( RLO )‐ and Midichloria ‐like organism ( MLO )‐related DNA in affected individuals, these pathogens have never been observed. We performed histological, ultrastructural and biomolecular analysis on skin and spleen samples of trout with RMS . Examination by TEM revealed the presence of intracytoplasmic microorganisms resembling Rickettsiales within macrophages, fibroblasts and erythrocytes. The microorganisms were oval or short rod shaped (400–800 nm in length and 100–200 nm in width) and often showed a cell wall similar to Gram‐negative bacteria. PCR analysis for Rickettsiales supported these findings: 53% of affected trout were positive by both PCR and TEM The primers Ri FC fw‐Ri FC rev were used to anneal both the RLO 16S DNA sequence and the MLO 16S DNA sequence. For this reason, and in agreement with previous studies confirming the presence of Rickettsiales‐related DNA in trout with RMS , we assume that TEM detected microorganisms morphologically consistent with bacteria belonging to Rickettsiales order and could be considered as possible causative agents of RMS .