Immunoprotective Effects of Mori Cortex Radicis Water Extract on Major Aquatic Pathogen (Aeromonas veronii) in Crucian Carp

Mori Cortex Radicis (MCR), which is abundant in resources and low in cost, is a Chinese herbal medicine with antitussive, anti-inflammatory, analgesic, and hypoglycemic effects; however, its application in the prevention and control of aquatic pathogens remains understudied. In this study, a MCR water extract (MCR-WE) was prepared, and its contents of polysaccharides, polyphenols, and proteins were found to be 0.63%, 1.17%, and 2.79%, respectively. LC-MS metabolomics revealed that L(+)-Arginine, 9,12,13-Todea, Citric acid, 1-Deoxynojirimycin, and 4-Guanidinobutanoic acid were the most abundant compounds. Subsequently, by feeding the MCR-WE to crucian carp (Carassius auratus) and challenging them with Aeromonas veronii, it was found that the MCR-WE enhanced the activities of immune factors (AKP, ACP, LZM) and the phagocytic activity of leukocytes (p < 0.05). Furthermore, the MCR-WE improved the survival rate of crucian carp (p < 0.05), reduced the bacterial load in the kidneys (p < 0.05), decreased the mRNA expression of inflammatory factors (IL-6, IL-1β, TNF-α), and lowered the expression levels of antioxidant-related factors (CAT, GSH-Px, SOD, MDA) and the mRNAs of oxidative stress pathway factors (Nrf2, HO-1, Keap1) (p < 0.05). Histopathological sections and immunofluorescence assays showed that the MCR-WE maintained the structural integrity of internal organs and reduced renal cell apoptosis and DNA damage. Therefore, MCR-WE is rich in immunologically active substances, can activate the immune response of crucian carp, reduce fish mortality, exert anti-inflammatory and antioxidant activities, and maintain the structural and functional integrity of internal organs. Thus, the MCR-WE holds promise as a therapeutic agent against A. veronii infection in fish.