Spironucleus salmonicida is a parasitic flagellate that affects salmonid fish and poses significant challenges for aquaculture in recirculating aquaculture systems (RAS). Outbreaks of spironucleosis in farmed Atlantic salmon (Salmo salar) are a recurrent issue, causing massive mortality and economic losses, such as those reported in northern Norway (Xu et al., 2022).
A study published by scientists from Nofima examines recent findings on the efficacy of ultraviolet (UV)-based disinfection strategies for combating Spironucleus salmonicida under controlled laboratory conditions in a recirculating aquaculture system (RAS).
The Salmonid Killer
Spironucleus salmonicida, also known as the “salmonid killer,” causes systemic spironucleosis in farmed salmonids such as Atlantic salmon, Chinook salmon, and Arctic char. This disease can lead to significant economic losses due to high mortality rates and reduced fish quality during harvest (Helsø, 2024).
The clinical signs of systemic spironucleosis in salmonids can vary but may include loss of appetite and weight, weakness, spiral swimming, and death. In chronic stages, symptoms such as exophthalmia (pop-eye) and fluid- or pus-filled lumps (abscesses) under the skin, especially near the tail base, may occur. At this stage, the flagellates are mainly found in the abscesses rather than in the bloodstream.
Other reported symptoms include internal hemorrhages, splenomegaly, and granulomatous lesions in the liver and spleen. Infection with S. salmonicida can occur in various host sites, including the circulatory system.
The disease can be recurrent in Atlantic salmon aquaculture, with outbreaks causing mortality rates of up to 60% in a single tank. Such outbreaks often result in downgraded fish or their destruction due to muscular abscesses. Outbreaks of systemic spironucleosis in Atlantic salmon have mainly been observed in northern Norway. However, it has also been reported in Chinook salmon in British Columbia, Canada.
Treatment and Prevention
Currently, no treatments are available for systemic spironucleosis. The primary control measure is the removal of infected fish. Prevention remains the best way to avoid outbreaks.
Background
No approved pharmacological treatment exists for systemic spironucleosis caused by S. salmonicida. Therefore, removing or culling infected fish groups is essential to prevent the parasite’s spread.
Prevention plays a crucial role in controlling this disease, involving strict hygiene measures in rearing facilities, especially hatcheries where infections are more likely. It also includes monitoring water quality.
Treating incoming water with UV light is a well-established practice in salmonid aquaculture in Norway. However, the optimal UV dose required to effectively inactivate Spironucleus salmonicida remains unclear. This study aimed to identify effective doses and compare the performance of low-pressure (LP) and medium-pressure (MP) UV lamps.
Methods
Experimental Setup
- Parasites were cultured in low-salinity water (0.3% NaCl) to simulate aquaculture conditions.
- UV exposure was applied using LP and MP UV lamps at various doses ranging from 10 to 200 mJ/cm².
- Viability was assessed immediately after treatment and at intervals of 24, 48, and up to 120 hours using microscopy and staining methods.
Evaluation Metrics
- Logarithmic reduction of live parasites.
- Observations of motility and reproductive potential over time.
Results
Low-Pressure UV Lamps
- Immediate inactivation was limited even at high doses (200 mJ/cm²).
- Parasites exposed to doses ≥50 mJ/cm² were non-viable within 24 hours, with complete mortality observed at 120 hours for doses ≥75 mJ/cm².
Medium-Pressure UV Lamps
- MP lamps demonstrated superior efficacy:
- Immediate inactivation occurred at doses ≥50 mJ/cm².
- At 25 mJ/cm², no viable parasites were detected 24 hours after treatment.
- Even at 10 mJ/cm², parasites could not reproduce and were non-viable at 48 hours.
Recommendations
For MP UV Lamps:
- A minimum dose of 50 mJ/cm² is recommended for effective inactivation under conditions of low organic load and high water transparency.
- Adjustments should be made based on water quality parameters, such as organic load and UV transmittance, which may vary seasonally.
For LP UV Lamps:
- Higher doses (≥75 mJ/cm²) are required for effective inactivation, making them less efficient compared to MP UV lamps.
Implications for the Salmon Farming Industry
This study highlights the importance of selecting appropriate UV technology and dosing based on water conditions. MP UV lamps offer significant advantages, including efficacy at lower doses, reduced energy costs, and improved pathogen control. These findings provide valuable insights for aquaculture facilities aiming to enhance biosecurity measures and mitigate the impact of parasitic infections.
Ongoing research on S. salmonicida is critical, as further studies are needed to better understand the disease and develop more effective treatment and control strategies. The information presented is based on the studies available to date.
Contact
Lill Heidi Johansen
Research Director
Nofima
Reference (open access)
Lill-Heidi Johansen, Hanne Brenne. 2024. Effekten av ulike desinfeksjonsstrategier på Spironucleus salmonicida. Nofima, Rapport 25/2024. Utgitt Juli 2024
Others references
Helsø, Maiken. 2024. Miljøoverlevelse hos Spironucleus salmonicida – parasitten som forårsaker systemisk spironukleose hos oppdrettslaks. Master thesis. University of Bergen.
Xu, F., Kurt, Z., Ástvaldsson, Á., Andersson, J. O., & Svärd, S. G. (2022). A chromosome-scale reference genome for Spironucleus salmonicida. Scientific Data, 9(1), 1-7. https://doi.org/10.1038/s41597-022-01703-w
