Anti-inflammatory, immunostimulant and antimicrobial properties of tannic acid in the diet of Oreochromis niloticus infected with Aeromonas hydrophila

The study aimed to assess the impact of dietary supplementation with tannic acid on the growth, health, and survival of Oreochromis niloticus following exposure to Aeromonas hydrophila. A total of 320 fish were divided into 16 tanks and assigned to four treatment groups: feed with 0.2 % tannic acid (TA_0.2 %), 0.4 % tannic acid (TA_0.4 %), 0.8 % tannic acid (TA_0.8 %), or no tannic acid (Control_0%), with each treatment replicated four times, over a 50-day period. At the end of the 50-day period, biological samples were collected from the fish, which were then intraperitoneally injected with A. hydrophila. No significant differences in growth performance were detected between treatments. As expected, levels of total leukocytes, lymphocytes, monocytes, hemoglobin, and mean corpuscular hemoglobin concentration (MCHC) were notably higher in the blood of the fish after infection, regardless of the treatment received. During both the pre- and post-infection periods, monocytes were more abundant in the TA_0.2 % and TA_0.8 % treatments compared to the TA_0.4 % treatment. Additionally, there was a significant interaction between the factors affecting thrombocytes, neutrophils, basophils, hemoglobin, and MCHC. Thrombocytosis and neutrophilia were significantly greater in the TA_0.8 % treatment pre-infection than in the post-infection and control group. Conversely, a higher number of basophils were observed in the post-infection period in the TA0_.8 % treatment group compared to the pre-infection period. Total plasma protein levels decreased significantly in the post-infection period, regardless of tannic acid supplementation levels, while immunoglobulin levels increased after exposure to A. hydrophila. Histological analyses revealed a significant increase in the perimeter and number of intestinal villi in the TA_0.4 % treatment group before infection. The number of goblet cells also increased in the control group (0 %), TA_0.4 %, and TA_0.8 % before infection. In splenic tissue, the TA_0.4 % treatment resulted in a reduction in eosinophilic and mononuclear infiltrates, as well as decreased congestion and vacuolation. Hemosiderin levels were lower in the TA_0.4 % and TA_0.2 % treatment groups. In the liver, lymphocytic infiltrates were reduced in the TA_0.2 % and TA_0.4 % treatment groups, and portal vein congestion was decreased in the TA_0.2 % post-infection and TA_0.4 % pre-infection groups. Post-infection survival rates were significantly higher (p < 0.05) in the TA_0.4 % treatment group (91 %) compared to the TA_0.8 % treatment group (85 %) and the control group (71 %). The results of the present study show that tannic acid has a positive effect on the immune system of Nile tilapia. This is supported by improvements in innate immunity in the blood, as well as the antimicrobial and anti-inflammatory effects seen in histological analyses. Therefore, it is suggested to use a 0.4 % tannic acid dose for dietary supplementation of Nile tilapia, along with further studies on the potential benefits of this food additive for tilapia.