Introduction
The diplomonad parasite Spironucleus salmonicida is the causative agent of spironucleosis, a severe systemic disease in Atlantic salmon characterised by haemorrhage, organ enlargement, and granulomatous lesions. Although outbreaks are sporadic, their impact can be devastating, and no approved treatment is currently available, making prevention through biosecurity the primary control strategy. In land-based aquaculture, especially recirculating aquaculture systems (RAS), water disinfection represents a critical barrier against pathogen introduction and spread. Ultraviolet (UV) irradiation and ozonation are widely applied disinfection technologies, yet limited data exist regarding their effectiveness against S. salmonicida or their broader effects on fish health and the microbial ecology of RAS. This study investigated the susceptibility of S. salmonicida to UV irradiation under controlled conditions and assessed the outcomes of different RAS loop-disinfection strategies following a simulated biosecurity breach involving Atlantic salmon smolts.
Materials and Methods
The study combined in situ UV exposure experiments with an in vivo RAS challenge trial. For the in situ experiments, cultured S. salmonicida trophozoites isolated from diseased Atlantic salmon were exposed to increasing doses of low���pressure (LP) and medium���pressure (MP) UV irradiation under controlled brackish water conditions. Parasite viability was assessed through motility observations and vital staining, with follow-up incubation to evaluate regrowth.
The in vivo trial was conducted using nine independent experimental RAS units stocked with Atlantic salmon smolts. Three treatments were applied in triplicate: no RAS loop disinfection (control), continuous LP UV disinfection, and continuous ozone treatment. After a two-week acclimation period, a simulated biosecurity breach was performed by introducing cultured S. salmonicida into the make-up water for three consecutive days. Fish were monitored for four weeks post-breach. Growth performance, gross pathology, and infection status were evaluated. Parasite presence was assessed using RT���qPCR across multiple tissues, peritoneal fluid, tank walls, and biofilter media. Gill, skin, and gut tissues were examined histologically and transcriptionally to assess mucosal integrity, inflammatory responses, and oxidative stress. Additionally, microbial communities on tank walls and biofilter media were characterised using full-length 16S rRNA gene sequencing.
Results
In situ, MP UV irradiation rapidly inactivated S. salmonicida, achieving ≥5���log reductions in viability at doses of 50 mJ/cm2 and above, with no evidence of regrowth. LP UV irradiation was less effective, requiring substantially higher doses for comparable reductions, and parasite recovery was observed at lower doses.
In the in vivo RAS trial, no fish developed clinical spironucleosis, and S. salmonicida was not detected in any tissue, environmental sample, or culture attempts, irrespective of disinfection treatment. Growth performance and condition factor did not differ among treatments. Histological evaluation showed no treatment-related impairment of gill or skin mucosal integrity. However, ozone treatment induced a pronounced upregulation of oxidative stress–related genes in the gills and skin, while UV treatment had minimal molecular impact. Water quality was improved by both disinfection strategies, with higher UV transmittance and lower turbidity compared to the control.
Microbial analyses revealed distinct communities between biofilter media and tank walls, with disinfection exerting selective pressures on community composition. UV and ozone treatments promoted different microbial taxa, although overall microbial diversity remained stable across treatments.
Discussion
The findings demonstrate that S. salmonicida is highly sensitive to MP UV irradiation under controlled conditions, supporting the potential of UV-based disinfection as a biosecurity measure in salmon RAS. The failure of the simulated waterborne biosecurity breach to establish infection suggests that transmission dynamics are more complex than previously assumed and may involve additional stressors or vectors. While both UV and ozone improved water quality without compromising fish growth or mucosal integrity, ozone elicited measurable oxidative stress responses, indicating the need for careful operational control. Overall, UV disinfection appears to offer an effective and biologically mild approach for strengthening RAS biosecurity against S. salmonicida, while highlighting the importance of considering host, pathogen, and environmental interactions.
Acknowledgement
This study was financed by the Norwegian Seafood Research Fund (SpiroFri #901831; PathoRAS #901826) and the Research Council of Norway (CandRAS #331892).