In Vivo Efficacy of Purified Quillaja Saponin Extracts in Protecting against Piscirickettsia salmonis Infections in Atlantic Salmon (Salmo salar)

SIMPLE SUMMARY: The global population is growing at higher rates and food production must grow at a similar rate. Aquaculture is one the most important sources of protein for human consumption, and its sustainable growth is of vital importance. The Chilean salmon industry exported 751,000 metric tons of fish in 2022; however, many diseases affected the production of these products. Piscirickettsia salmonis is the main bacteria infecting salmon in Chile, causing 10% of losses. Currently, no successful treatment exists against this bacterium. Quillaja saponins are secondary metabolites with biological applications, and have shown antiviral, antifungal, and antibacterial properties, as well as acting as immunomodulators. Here, we tested the effect of quillaja extracts in protecting against P. salmonis infection. These products were applied as a supplement in the feed, and displayed in different model trials protective effect against the infection. All treated groups showed a higher survival percentage in comparison to the control group. In addition, quillaja extracts were able to increase the immune responses measured throughout different cytokines. Finally, in a pilot-scale trial, it was observed that fish fed with quillaja saponins exhibited a reduction in mortality and a reduction in the need for antibiotic treatment. These results represent evidence that quillaja saponins are a potential natural non-pharmacological strategy to prevent intracellular infections. ABSTRACT: Piscirickettsiosis, the main infectious disease affecting salmon farming in Chile, still has no efficient control measures. Piscirickettsia salmonis is a facultative intracellular bacterium that can survive and replicate within the host macrophages, evading the immune response. Triterpenic saponins obtained from the Quillaja saponaria tree have been widely studied, and have been shown to be immunomodulatory agents, suitable for feed and vaccine applications for veterinary and human uses. The impact of the oral administration of two extracts of Quillaja saponins on the infection of P. salmonis in Salmo salar and the corresponding gene expressions of immunomarkers were studied under three in vivo models. In the intraperitoneal challenge model, the group fed with Quillaja extracts showed lower mortality (29.1% treated vs. 37.5% control). Similar results were obtained in the cohabitation model trial (36.3% vs. 60.0%). In the commercial pilot trial, the results showed a significant reduction of 71.3% in mortality caused by P. salmonis (0.51% vs. 1.78%) and antibiotic use (reduction of 66.6% compared to untreated control). Also, Quillaja extracts significantly modulated the expression of IFN-II and CD8. These results represent evidence supporting the future use of purified Quillaja extracts as a natural non-pharmacological strategy for the prevention and control of P. salmonis infections in salmon.