Red mark syndrome – Current state of knowledge

Skin conditions, especially those of unknown aetiology, are often reported in rainbow trout (Oncorhynchus mykiss) aquaculture and are a true challenge for the trout industry. Red mark syndrome (RMS) is a skin condition affecting cultured rainbow trout, which first appeared in the United States of America (USA) in the late-1960s, then referred to as strawberry disease. It later appeared in Europe in the 1980s and was reported in the United Kingdom (UK) in late 2003. Since then, it has quickly spread to the rest of Europe and more recently has been reported in the Middle East, the Americas and Asia. Due to the downgrading of affected fish, RMS can cause significant economic loss to the rainbow trout industry. Several papers have described the histology associated with RMS in detail, and there is a good case definition for the disease. Although the aetiology has not yet been confirmed, two pathogens have been associated with the disease: a Rickettsia-like organism (RLO), now identified as a Midichloria–like organism (MLO), and Flavobacterium psychrophilum, however, an association between the latter and RMS has not been supported in subsequent studies. Although the MLO has yet to be isolated, ultrastructural features resembling Rickettsiales order organisms have been observed in affected tissues and a strong positive correlation has been found between the MLO and RMS-affected fish using a variety of techniques. More recently papers have described a reproducible challenge model based on cohabitation, which has been used to investigate the disease dynamics of RMS under controlled conditions. This review aims to examine the current state of knowledge relating to RMS and to highlight where there are gaps in this knowledge.