Evaluation of ozone and peracetic acid use during a Yersinia ruckeri challenge in Atlantic salmon (Salmo salar) freshwater recirculating aquaculture systems

Abstract

Yersinia ruckeri (Y. ruckeri) the causative agent of Enteric red-mouth disease (ERM), a serious septicemic bacterial disease of salmonid fish, poses a significant challenge in salmon production. With the emergence of recirculating aquaculture system (RAS) in Norway, new biological challenges in relation to infectious disease outbreaks have arisen. There is a gap in knowledge on how potential disinfection methods in the rearing water could help reduce or eliminate the risk of infectious disease outbreaks in RAS. Ozone and peracetic acid (PAA) are two disinfectants used in aquaculture. Both are reported to inactivate a range of pathogens, including Y. ruckeri. There are however limitations to the use of disinfectants in RAS as the re-use of water leads to accumulation of substances added to the water, and an ideal disinfectant need to balance pathogen control, fish health and welfare and biofilter performance.

This study investigates the use of continuous ozone and semi-continuous PAA addition in RAS as a tool for preventing a disease outbreak of Y. ruckeri in Atlantic salmon parr. Nine replicated RAS units were stocked with Atlantic salmon parr (N=1800, ±19 g) and were treated with PAA (50.88 ml PAA product dose of 0.05% of make-up water daily), ozone (ORP: 500mV through protein skinner) or no treatment (control), in triplicates (n=3). To induce a ERM outbreak, Y. ruckeri was introduced through the make-up water. Health and welfare of fish were followed for 28 days after treatment started and 20 days after pathogen challenge, where water quality was monitored to see the effect of disinfectants on different water quality parameters.

Although Y. ruckeri introduced to the systems did not lead to significant mortalities in any treatment group, and only mild clinical signs of the disease were observed, the pathogen was detected in the spleen of fish across all treatment groups. A trend could be seen towards higher infection prevalence in the control group, but no significant difference among the treatments (control: 33.3% ± 30.6, PAA: 13.3% ± 11.5, ozone: 6.7% ± 11.5). Sub-clinical pathologies were observed toward end of trial, where mild cases of hemorrhage in liver and eyes, and enlarged spleen were recorded in some fish across treatments. Presence of enlarged spleen was the only pathology where statistically significant differences were present among groups, as the prevalence was higher in control group, followed by PAA and ozone. However, histological evaluation of spleen did not show inter-treatment differences.

Furthermore, the results indicate that the tested disinfection methods did not severely affect health and welfare of the fish as no major alterations were observed regarding growth, external welfare, stress levels (glucose, lactate, and hematocrit) or histopathological changes in gills or spleen between treatments. Regarding water quality, lower turbidity levels were measured in PAA and ozone treatments. Nitrite levels were also significantly higher in PAA treatment than the control, which may indicate that nitrite-oxidating bacterial community in the biofilter were negatively affected by this compound. The result of this study may be used in further research, with the goal of improving biosecurity measures by developing effective disinfection protocols in RAS to limit disease outbreaks.