Red mark syndrome is an infectious disease, which is causing increasing problems for rainbow trout aquaculture in Europe. Affected fish typically display large, red, inflammatory lesions, but in most cases the disease is neither accompanied by mortality nor any substantial loss of appetite. The problem for the fish farmer is that RMS affects fish of market size. This leads to down-grading, and the fish farmer is left with the options of selling cheap or keeping the fish until the symptoms disappear, which may take months.
The cause of the disease has not been firmly established, but a 16S rDNA sequence from an undescribed bacterium closely related to Midichloria mitochondrii has been identified in RMS lesions on several occasions, and is thus the most likely candidate for causing the disease. For now we call this bacterium MLO (Midichloria-like organism).
At the EURL for fish diseases we are leading a 2½ year project on RMS. We are now two years into the project (March 2018) and have developed a stable continuous cohabitation model for red mark syndrome. This model has provided further evidence that MLO is at least partly involved in the disease, as well as information on the process leading to development of lesions.
In addition to experiments on our in house RMS cohabitation model we are sampling a number of RMS-affected fish farms. The first symptoms were recognized in Denmark around the year 2010. A questionnaire in 2015 showed that approximately one third of Danish fish farmers experienced RMS symptoms on their farm.
We welcome collaboratory research on the RMS cohabitation model.