First Molecular Confirmation of Loma salmonae in Farmed Rainbow Trout (Oncorhynchus mykiss) in Korea, with Histopathological Characterization

Loma salmonae is a gill-infecting microsporidian long recognized in Oncorhynchus species across North America, Europe, and Japan (masu/yamame salmon), but molecular confirmation in rainbow trout, or in any other salmonid host, in Korea has not previously been reported. Here, we report the first molecularly confirmed detection of L. salmonae infection in farmed rainbow trout (Oncorhynchus mykiss) in Korea, characterized by histopathology and molecular analysis, including SSU rRNA sequencing, and compared with previously characterized isolates from North America and Chile. L. salmonae was detected at two of five farms sampled, with an apparent prevalence of 11.0% (8/73) based on stereomicroscopic detection of gill xenomas. The detected infections were not associated with clinically apparent disease in the sampled fish; histopathology revealed limited inflammatory change and slight gill damage. SSU rRNA sequences showed 0.00% divergence from reference isolates within the 416 bp region analyzed, supporting species-level identity. Although the limited, non-random sampling of five farms in this study precludes extrapolation to estimate regional prevalence or the broader distribution of L. salmonae in Korean rainbow trout aquaculture, these findings establish its presence for the first time in Korea and provide a valuable foundation for broader, randomly sampled surveillance using sensitive molecular methods.