Effects of feed supplementation with fermented Broussonetia papyrifera leaves on non-specific immune responses, resistance to Vibrio parahaemolyticus, growth, intestinal histology, and microbiota of white shrimp (Penaeus vannamei)

White shrimp (Penaeus vannamei) is a globally significant aquaculture species, yet the industry often suffers economic losses due to pathogens, water quality fluctuations, and climate change. Aquaculture technology frequently incorporates probiotics or herbal supplements to enhance shrimp immunity and growth performance. Broussonetia papyrifera leaf has long been used as a medicinal herb. However, due to the large molecular size of their active compounds, absorption by organisms is limited, making microbial fermentation necessary before use in feed. This study investigated the effects of fermented B. papyrifera leaf (FBL) on white shrimp immunity, resistance to Vibrio parahaemolyticus, and growth performance. In vivo trial confirmed that FBL is non-toxic to shrimp hemocytes and demonstrated that FBL concentrations between 50 and 1000 μg/mL significantly enhanced respiratory burst, phenoloxidase, and phagocytic activity (p < 0.05). There are three experiments in the in vivo trials. Firstly, a 56-day feeding trial was conducted using five dietary treatments: control, FBL0.5 (0.5 g/kg), FBL1 (1 g/kg), FBL5 (5 g/kg), and FBL10 (10 g/kg). The results indicated that the FBL5 group exhibited notably improved growth performance, as well as enhanced intestinal structure and microbial composition. Following, a 28-day in vivo immune trial, shrimp were fed the same FBL diets to evaluate their effects on non-specific immune responses. The results showed that dietary supplementation with FBL significantly modulated immune responses and upregulated immune-related gene expressions, particularly on Days 7 and 14 (p < 0.05). Finally, a challenge test revealed that the FBL5 group significantly reduced mortality after V. parahaemolyticus infection compared to the control group (p < 0.05). In summary, this study demonstrates that supplementing 5 g FBL per kilogram of feed can bolster immune responses, improve growth performance, and enhance resistance to Vibrio infections in P. vannamei.