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Background  In Norway, x-cell parasites associated with disease in farmed salmonids have been known as a rare phenomenon for two decades. These parasites cause systemic infections in farmed rainbow trout ( Oncorhynchus mykiss ) and Atlantic salmon ( Salmo salar ), but have so far not been characterized and described.    Methods  The x-cells from several cases of diseased fish were studied using light and electron microscopy, and by phylogenetic analysis based on small subunit ribosomal RNA ( SSU  rRNA) gene sequences.    Results  We describe here the x-cell parasite as a new species in a new genus,  Salmoxcellia vastator  n. gen., n. sp. Phylogenetic analyses placed  Salmoxcellia  n. gen. together with  Gadixcellia  among the xcelliids, a group of perkinsozoan alveolates. The new genus and species were found to have vacuolate plasmodial x-cells filled with lipid droplets, and an electron-dense alveolar pellicle. Electron-dense cytoplasmic inclusions, which are characteristic of the other xcelliid genera  Xcellia  and  Gadixcellia , are lacking in  Salmoxcellia  n. gen. These x-cell plasmodia divide by plasmotomy and occur as aggregates in the host tissues, particularly in blood-rich tissues such as those of the kidney, red musculature, heart and liver. Host reaction and the refractive lipid droplets in the x-cells result in  S. vastator  n. gen., n. sp. aggregates appearing as white patches in the tissues.    Conclusions  We describe a new genus and species of xcelliid protist parasites from two very important farmed fish species and provide molecular methods for detection. The new parasite is associated with disease, but more importantly it has a spoiling effect on farmed salmonid fillets, rendering them unsuitable for sale. Consequently, this parasite represents a threat to the aquaculture industry.    Graphical abstract

A novel protist parasite, Salmoxcellia vastator n. gen., n. sp. (Xcelliidae, Perkinsozoa), infecting farmed salmonids in Norway