First Field Evidence of Aerosolised Atlantic Salmon Viruses SGPV, ISAV-HPR0, and IPNV in Recirculating Aquaculture Systems (RAS): Implications for Pathogen Transmission and Biosecurity Risk

Recirculating aquaculture systems (RAS) for Atlantic salmon ( Salmo salar L.) are closed-containment systems (CCS) offering biosecure, water-efficient farming conditions, yet pathogen transmission remains a critical concern. While horizontal transmission in water is well-documented in RAS, the potential for aerosol-mediated transmission remains underexplored. The current study was conducted at two commercial Faroese Atlantic salmon RAS smolt farms. At Smolt farm 1, aerosolised pathogens were monitored using two aerosol samplers (Coriolis®+ and Coriolis® Compact, Bertin Technologies SAS, France), along with water and fish swab samples, to evaluate pathogen dynamics. A sequential infection pattern was observed, beginning with salmon gill pox virus (SGPV), followed by non-virulent infectious salmon anaemia virus (ISAV-HPR0), piscine orthoreovirus-1 (PRV-1), and sporadic detections of infectious pancreatic necrosis virus (IPNV) and Flavobacterium psychrophilum . All pathogens were detected in aerosol samples with the highest detection rates and pathogen loads at the biofilter room compared to the local tank degassers. Detection trends for SGPV and ISAV-HPR0 in aerosols reflected those in fish and water samples. Coriolis®+ performed marginally better compared to Coriolis® Compact in reflecting the infection dynamics. Viable IPNV was not isolated from initial aerosol samples at Smolt farm 1, though bacterial culture identified relevant colonies for Atlantic salmon RAS. Targeted aerosol sampling for IPNV at Smolt farm 2 post IPNV outbreak produced IPNV-specific cytopathic effects in cell lines from Coriolis® Compact aerosol samples, marking the first field-based evidence of viable aerosolised IPNV from a RAS. This study highlights water as a non-invasive surveillance tool, with enhanced detection in anaesthetic water, and demonstrates the potential for aerosol-mediated pathogen spread, underscoring the need for strict biosecurity in Atlantic salmon RAS.