Field surveillance has revealed a high prevalence of PRV-3 in clinically healthy salmonids in Italy

Piscine orthoreovirus genotype 3 (PRV-3) has been reported in several European countries, and in Italy it was detected only once in 2017 without any associated disease outbreak. This study aimed to investigate the presence of PRV-3 in the area with the highest salmonids production in Italy. Samples were collected between September 2023 and May 2024 from three regions of northeastern Italy. Pooled samples were screened by a generic PRV real-time RT-PCR, and positive samples were subsequently characterized as PRV-3. The geographic distribution of PRV-3 was visualised and risk factors analysis was performed. Of the 267 tested specimens, 37.1 % yielded positive results, corresponding to 43.6 % positive farms in the investigated area. Phylogenetic analyses confirmed that the Italian PRV-3 strains clustered within group 3b. The highest positivity was observed in Salmo trutta (49.1 %), followed by Oncorhynchus mykiss (38.7 %) and Salmo marmoratus (24.3 %). No signs of diseases or history of mortality were observed from the positive batches of fish. Apparently healthy wild fish also tested positive. These results suggest that PRV-3 is widespread in both farmed and wild salmonid populations in the investigated area and that it does not cause specific health issues under the current Italian farming conditions and wild environment. • PRV-3 was detected in 43.6 % of tested salmonid farms in northeastern Italy. • S. trutta showed the highest positivity, followed by O. mykiss and S. marmoratus. • All strains were phylogenetically close to European PRV-3. • No clinical signs or mortality were observed, suggesting a subclinical infection. • First evidence of endemic PRV-3 in both wild and farmed salmonids in Italy.