Determination of the immunostimulant effect of propolis extracted with olive oil in trout (Oncorhynchus mykiss)

In this current study, a 21-day feeding trial was carried out to determine the immunostimulant effect of propolis extracted with olive oil in rainbow trout (Oncorhynchus mykiss). Juvenile rainbow trout with an initial body weight of 5.21 ± 0.13 g were supplemented with a diet containing different amounts of propolis extracted with olive oil. The doses utilized in the study were 0.1%, 0.3%, 0.5%, and propolis-free as the control. Treatment fish were randomly sampled every 7th, 14th and 21st day of the study. Hematological parameters were examined, and the results showed that only the 21st day (p<0.05) demonstrated a significant difference between the groups in terms of red blood cells (RBC), hemoglobin (HGB), hematocrit (HCT), mean corpuscular hemoglobin (MCH), and mean corpuscular hemoglobin concentration (MCHC). The results of histopathological analysis of the intestine, kidney, spleen and liver were similar among all treatment groups (p>0.05). Oxidative radical production showed no significant difference in all treatment groups. The results of gene expression analyses performed on fish intestine, kidney, and spleen tissues showed that the expression levels of IFN, IL-1, TNF, and TGF- cytokines varied according to tissues and at different doses in comparison to the control group. The result of the control test using Yersinia ruckeri bacteria revealed that the survival rate of P01 group increased most dramatically compared with the control group (p<0.05). The outcome of this study indicates that propolis extracted with olive oil improved the immune system and control of fish infection against Y. ruckeri.