Interplay between disinfection and the enigmatic diplomonad parasite Spironucleus salmonicida in Atlantic salmon.

The diplomonad parasite Spironucleus salmonicida causes spironucleosis, a systemic infection characterised by internal haemorrhaging, splenomegaly, and granulomatous lesions in multiple organs. Prevention is the primary control strategy, particularly in recirculating aquaculture systems (RAS). This study evaluated the effectiveness of common water-disinfection methods against the parasite. In situ experiments assessed parasite survival following exposure to low- and medium-pressure ultraviolet (UV) radiation at different doses. A subsequent in vivo trial simulated a biosecurity breach in RAS using Atlantic salmon smolts. Two loop-disinfection strategies were tested: continuous UV treatment (70-80 mJ/cm²) and ozone (300-350 mV oxidation-reduction potential), alongside a no-disinfection control. The parasite was introduced via make-up water over three days, and fish were monitored for four weeks. In situ, medium‑pressure UV doses ≥ 50 mJ/cm² immediately inactivated the parasite, whereas lower doses achieved inactivation within 48 h. In vivo, no fish developed disease, and the parasite was not detected in any group. Disinfection did not affect mucosal integrity, although ozone increased expression of oxidative stress markers in the gills. UV and ozone also altered microbial community composition. Overall, S. salmonicida was sensitive to medium-pressure UV, and under the tested conditions did not persist or cause disease in RAS.