Peracetic acid as a potential treatment for amoebic gill disease (AGD) in Atlantic salmon - Stage 1

Peragill is an initiative that ultimately aims to develop an alternative treatment for the currently available therapies for amoebic gill disease (AGD) that have several practical and environmental issues. Peracetic acid (PAA) is a potent oxidant with a broad spectrum of antimicrobial activity and decays into relatively safe residuals, thus, has been widely recognised as a sustainable disinfectant in aquaculture.

Earlier reports on PAA underscore its potential to address the challenges of the current AGD treatments. Hence, this project aimed to establish its credentials as a chemotherapeutant for AGD. Stage 1 documented the impacts of PAA exposure on the health and welfare of salmon, its degradation kinetics and its antiparasitic activity against the Paramoeba perurans, the causative agent of AGD.

There were 3 in vivo exposure experiments performed where salmon were exposed to varying levels of PAA. Experiment 1 was designed to evaluate whether previous exposure history might desensitise the responses upon re-exposure. Salmon were exposed to different nominal concentrations (0, 0.6, and 2.4 ppm) of PAA for 5 min, followed by a re-exposure to the same concentrations for 30 min 2 weeks later.

Experiment 2 explored how a stressful episode before exposure might interfere with the adaptive responses to PAA. Fish were subjected to crowding stress prior to PAA exposure at 4.8 ppm for 30 min. And lastly, Experiment 3 investigated the impacts of repeated exposures to PAA. Salmon were exposed to 10 ppm PAA either for 15 min to 30 min every 3 weeks, with 3 exposures in total.

Growth performance was not affected in all exposure trials. Behavioural changes such as agitation, erratic swimming, increased ventilation and loss of balance during exposure were only observed in experiment 3. No significant mortality was recorded in all experiments, and exposed fish recovered quickly after exposure as evidenced by unaffected feeding patterns.

Though there were external welfare changes (e.g. skin damage, fin damage) following exposure, the degree of alterations was not dramatically high. Histological analyses of gills and skin revealed that despite the presence of some pathologies in PAA-exposed fish, mucosal barriers can still be categorised as healthy.

Repeated exposure, however, may compromise the barrier status of the gills as observed in experiment 3. PAA could trigger oxidative stress. In addition, classical players of systemic stress responses were activated by PAA exposure. The adaptive responses were robust and, in most cases, the level returned to basal concentrations hours after exposure.

Crowding stress prior to exposure could interfere with the normal systemic stress and antioxidant responses to PAA. Metabolic profiling revealed that PAA concentrations in experiments 1 and 2 did not substantially alter the plasma metabolomes. Recurrent exposures, however, have a significant impact.

Metabolites that were differentially affected by PAA exposure were known to be involved in protecting the cells from oxidative stress damage, suggesting that salmon were able to mount a strong protective response against PAA-induced oxidative stress. Transcriptomic profiling of the mucosal tissues (i.e., skin and gills) demonstrated that PAA could trigger a strong immunological response as several differentially expressed genes following PAA exposure have known roles in immunity.

Skin transcriptome was more responsive than the gills at lower PAA dose. However, the opposite trend was identified at a higher dose. The developed gill explant culture could be used as a model to compare mucosal responses to oxidants (i.e., PAA vs. H2O2). PAA exhibited amoebicidal activity against P. perurans.

Viability of the amoeba can be reduced by 50 % following exposure to 4.8 ppm PAA and higher. Toxicity of PAA towards the amoeba was influenced by different factors (i.e., density, temperature, light, culture age) at varying degrees. Toxic effect of PAA against the amoeba is rendered by disruption of the cell membrane.

The decay of PAA was affected by several factors including light, fish density and salinity. It was demonstrated that PAA degrades significantly faster compared with H2O2 in seawater. Taken together, the results indicate that PAA is safe for use in salmon, with promising potential as a chemotherapeutant for AGD with low environmental risk.