Introduction
Recirculating aquaculture systems (RAS) are increasingly being adopted for land-based Atlantic salmon farming in Norway. While RAS provides relatively higher biosecurity compared to flow-through systems, disease outbreaks can still occur. One such disease is spironucleosis , which may be introduced into the system via the water intake. In Northern Norway, outbreaks of spironucleosis in farmed salmon remain a persistent problem, leading to mass mortality , compromised welfare and significant economic losses.
The diplomonad parasite Spironucleus salmonicida ( Figure 1) is the causative agent of these systemic infections, which are characterised by internal haemorrhaging , splenomegaly, and granulomatous lesions in the liver and spleen. Environmental conditions significantly influence the severity and distribution of the disease . Currently, no known treatment for spironucleosis exists, making prevention the primary means of control. Anecdotal reports from the industry suggest that the parasite may be sensitive to UV disinfection.
This study investigated the use of UV and ozone as water disinfection methods within the RAS loop to prevent spironucleosis outbreaks following a biosecurity breach in Atlantic salmon RAS.
Materials and methods
In situ and in vivo trials were conducted to evaluate the effects of disinfection on the viability and potential proliferation of Spironucleus salmonicida in RAS.
In the in situ trial, the survival of S. salmonicida flagellates following UV treatment was assessed under controlled laboratory conditions. The flagellates were exposed to two types of UV lamps: low-pressure (dose range: 10–200 mJ /cm²) and medium-pressure (dose range: 10–75 mJ /cm²). Parasite survival was evaluated immediately after UV exposure and again at 24 and 48 hours post-treatment.
In the in vivo trial, brackish water-adapted Atlantic salmon smolts were reared in nine individual RAS units. Three units received UV disinfection in the RAS loop at a continuous dose of 100 mJ/cm². Another three units were disinfected using ozone, targeting an oxidation-reduction potential (ORP) of 320–350 mV. The remaining three units, which received no RAS loop disinfection, served as controls.
The parasite (approximately 10⁵ cells/mL) was introduced via the make-up water over three consecutive days at a concentration of 1% (v/v) of the total daily water volume (approximately 20 L/day). Disease progression was monitored over a four-week period, including assessments of water quality parameters, collection of swab samples for microbial profiling, and tissue sampling for histology and qPCR analysis.
Results
No parasites survived treatment with medium-pressure UV lamps at a dose of 50 mJ /cm² or higher at any time point. At lower doses (10 and 25 mJ /cm²), the parasites were not viable over time and were unable to reproduce, despite appearing alive under the microscope at 0 and 24 hours post-treatment. However, they were significantly weakened and all died within 48 hours.
Water quality in the nine experimental RAS units remained within favourable thresholds for Atlantic salmon smolts. UV transmittance was highest in the UV-treated units and lowest in the control group.
All fish tested negative for the presence of the parasite. Molecular markers associated with innate immunity and stress response were differentially regulated in the foregut, hindgut, skin, and gills. Histological indicators of skin and gill health changed significantly over time, particularly in the control group, although inter-treatment differences were minimal.
RAS loop disinfection influenced the microbial communities in the system, including those on tank walls and biomedia . Microbial composition evolved over time in all treatment groups. Certain members of Alphaproteobacteria were enriched in the ozone-treated group but not in the UV-treated group, relative to the control.
Conclusion
Medium-pressure UV treatment at 50 mJ /cm² effectively inactivated Spironucleus salmonicida . The in vivo biosecurity breach of the parasite in RAS did not cause infection, and water quality remained within optimal ranges across all RAS units. Disinfection methods influenced microbial community composition, particularly with ozone, which enriched certain Alphaproteobacteria.
Acknowledgement
The study was financed by the Norwegian Seafood Research Fund (SpiroFri #901831; PathoRAS #901826) and The Research Council of Norway (CandRAS #331892 ).