Piscirickettsiosis in Farmed Turbot, Scophthalmus maximus.

Piscirickettsia salmonis is a well-established intracellular pathogen of farmed Atlantic salmon around the world. The bacterium has also been detected from a number of other hosts including lumpfish (Cyclopterus lumpus), seabass (Dicentrarchus labrax) and rainbow trout (Oncorhynchus mykiss). In February 2025, increased mortality was observed in a population of turbot, Scophthalmus maximus, farmed in Norway. Macroscopic signs of disease included stomach mucosa bleedings. Piscirickettsiosis was diagnosed based on histopathological findings including rickettsia-like inclusions immunostaining positive for Piscirickettsia salmonis in internal organs and gills, qPCR and in situ hybridisation (ISH) using RNAscope. 16S-23S internal transcribed sequences (ITS) retrieved directly from infected tissues clustered together with previous Norwegian, Scottish and Canadian isolates from Atlantic salmon. To the best of our knowledge, this represents the first case of Piscirickettsia salmonis infection reported from turbot.