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 several organs. Prevention remains the primary strategy for controlling the disease, especially in production systems such as recirculating aquaculture systems (RAS). This study evaluated the effectiveness of established water disinfection methods against the parasite. In situ experiments examined parasite survival following exposure to low and medium pressure ultraviolet (UV) radiation at varying doses. A follow-up in vivo trial simulated a biosecurity breach in RAS using Atlantic salmon smolts. Two loop disinfection strategies were tested: one group received continuous UV treatment at 70-80 mJ/cm2, while another was treated with ozone at an oxidation reduction potential of 300 to 350 mV. A third group without loop disinfection served as the control. The parasite was introduced via make-up water over three days, and fish were monitored for four weeks. In situ results showed that medium pressure UV at doses of 50 mJ/cm2 or higher fully inactivated the parasite, while lower doses left parasites weakened and non-viable within 48 hours. In the in vivo trial, no fish in any group showed signs of disease, and the parasite was not detected. Some parameters related to mucosal integrity of the gills and skin changed during the trial, but these changes were not affected by UV or ozone. Gene expression analysis indicated that ozone could increase the expression of oxidative stress markers in the gills by the end of the trial. Disinfection also influenced the microbial community in the RAS: UV treatment was associated with an enrichment of Verrucomicrobiota and Saccharimonadales, whereas ozone treatment favoured the presence of Rhizobiales and Acidimicrobiales. Overall, the findings show that S. salmonicida is sensitive to medium pressure UV under in situ conditions, and that under the conditions tested, the parasite did not persist or cause disease in RAS, regardless of whether loop disinfection was applied.