First report of puffy skin disease in farmed rainbow trout Oncorhynchus mykiss from Switzerland

Puffy skin disease (PSD) is an economically important syndrome in rainbow trout, Oncorhynchus mykiss (Walbaum, 1792). Because of its horizontal transmission, an infectious aetiology was hypothesised. However, aetiology remains unknown so far. PSD was first reported in 2002 in fish farms in England. Since then, cases have increased significantly, with additional reports in still water fisheries. Typically, the prevalence and severity of PSD increase during the summer and autumn months (13°C – 15°C). In February 2022, rainbow trout from a mountainous fish farm in Switzerland showed lethargy, inappetence, white to grey patches on the skin and slightly increased mortality. Twenty-one rainbow trout were submitted for diagnostics to the Institute for Fish and Wildlife Health, University of Bern, Switzerland. On post-mortem examination, 7/21 had up to 75% to 90% of the skin surface affected with bilateral, focally extensive white to grey, rough patches. The scales on the affected areas were raised, and the skin thickened. The histologic assessment revealed focally extensive, severe, epidermal hyperplasia with spongiosis and hydropic degeneration, severe scale pocket oedema with mild infiltration by degenerated neutrophils and lymphocytes, necrosis, and cellular debris displacing the scales. These findings are consistent with PSD. This is the first report of PSD in Switzerland. Here, contrary to previous reports, the disease occurred in rainbow trout exposed to 6-7°C water during the winter.