Effects of a phytogenic diet on growth, haemato-immunological parameters, expression of immune- and stress-related genes, and resistance of Oncorhynchus mykiss to Lactococcus garvieae infection

Phytogenic additives have been implicated in aquafeed owing to their proven growth-stimulating, antimicrobial potential, antioxidant, and immune-enhancing efficacies. Herein, a short-term feeding experiment (21 days) was designed to explore the effects of a phytogenic additive, PASSIF MOOD® (PM, a mixture of Valeriana officinalis and Passiflora incarnata extracts), on growth, haemato-biochemical variables, immunity, and mRNA expression responses of rainbow trout fingerlings. After feeding, fish were experimentally infected with Lactococcus garvieae and monitored for an additional 20 days. Six groups were fed a commercial diet supplied with different PM levels as 0, 470, 940, 1410, 1880, and 2350 mg/kg and labeled as PM0 (control), PM1, PM2, PM3, PM4, and PM5 groups, respectively. Results revealed significant improvements in the growth of PM-supplied groups compared to the control. Dietary PM significantly increased RBC count, hemoglobin, and hematocrit values compared to those fed on a control diet. Stress indicators such as blood glucose and cortisol levels were significantly decreased in all PM groups than in controls. Moreover, there were significant increases in myeloperoxidase, respiratory burst, and lysozyme activities in PM groups than the control. The highest expression levels of the lysozyme II gene were found in the PM3, PM4, and PM5 groups. Moreover, the mRNA transcripts of il-10 and IgM heavy chain genes were significantly increased in all PM groups compared to the controls. Alternatively, the expression of tnf-α and hsp70 genes were significantly decreased in all PM groups compared to the controls. After challenge, the survival rates in PM0, PM1, PM2, PM3, PM4, and PM5 groups were 37.34%, 84.0%, 85.34%, 68.0%, 64.0%, and 36.0%, respectively, suggesting enhancement of the fish resistance in PM1, PM2, PM3, and PM4 groups compared to control. The results suggest the growth-promoting, antistress, and immunomodulatory roles of the tested phytogenic additive.