In recent years, viral and bacterial diseases have posed serious challenges to the sustainable development of Macrobrachium rosenbergii (giant freshwater prawn) aquaculture, resulting in considerable economic losses across China. Among the bacterial pathogens, Aeromonas caviae has emerged as a notable opportunistic agent capable of causing large-scale mortality in various aquatic species. In this study, a highly virulent strain of A. caviae (designated GMRS4) was isolated from diseased M. rosenbergii exhibiting mass mortality in Yangzhou, Jiangsu Province. The isolate, a Gram-negative bacillus, was identified as the causative agent based on morphological, molecular, and histopathological analyses. Pathogenicity was confirmed through experimental infection, with the strain displaying marked virulence, evidenced by an LD₅₀ of 1.91 × 10⁶ CFU/mL at 96 h. Whole-genome sequencing of GMRS4 revealed 4078 coding sequences, including a suite of virulence-associated genes encoding extracellular enzymes (DNase, hemolysin, caseinase, and lecithinase) and toxins (serine protease, elastase, and flagellin). Antimicrobial susceptibility testing indicated resistance to several antibiotics, particularly those in the penicillin and sulfonamide classes, while maintaining sensitivity to quinolones. Genomic analysis further revealed multiple antibiotic resistance genes and virulence genes, offering insights into the pathogenic mechanisms and resistance profile of the strain. This study underscores the threat posed by A. caviae to freshwater prawn aquaculture and provides a genetic basis for developing targeted disease management strategies.

The giant freshwater prawn (Macrobrachium rosenbergii) is a commercially pivotal crustacean species in global freshwater aquaculture, while vibriosis caused by non-O1 Vibrio cholerae has become the primary bottleneck restricting its sustainable intensive farming. Vitamin D₃ (VD₃) is a biologically active fat-soluble nutrient involved in mineral metabolism and immune regulation in animals, yet its effects on the physiological responses of M. rosenbergii under a pathogenic bacterial challenge remain largely unelucidated. In this study, six experimental diets were formulated with graded dietary VD₃ concentrations (0.10, 0.18, 0.28, 0.48, 0.68, and 0.88 mg/kg) to feed juvenile M. rosenbergii (initial body weight: 2.06 ± 0.07 g) for 70 d, followed by a 72 h non-O1 Vibrio cholerae challenge test. The results showed that dietary VD₃ supplementation alleviated the non-O1 Vibrio cholerae-induced histopathological damage in the intestine and hepatopancreas of prawns. VD₃ supplementation significantly modulated antioxidant enzyme activities in a tissue-specific manner: SOD activity increased in the intestine, while CAT and GSH-PX activities increased in the hepatopancreas. However, the MDA, T-AOC, and GSH-PX (intestine) or SOD (hepatopancreas) showed no significant responses, indicating that VD₃’s antioxidant effects were selective rather than comprehensive. Meanwhile, an appropriate amount of VD₃ markedly upregulated the mRNA expression of multiple immune-related genes (pattern recognition receptors, antimicrobial peptides and immune effectors) in a tissue-specific pattern. However, some antioxidant indices and immune-related genes showed no significant responses. Furthermore, VD₃ supplementation significantly modulated certain muscle physicochemical parameters of prawns, including altered redness, cooking loss, and textural characteristics. Based on the evaluated physiological indicators, 0.48–0.68 mg/kg VD₃ appeared to produce the most favorable overall response. These findings suggest that dietary VD₃ may modulate the physiological responses associated with host defense during a bacterial challenge in M. rosenbergii, potentially through modulation of antioxidant capacity and immune-related gene expression, while benefiting the flesh quality. VD₃ is thus a potential nutritional approach for improving health status during a bacterial challenge in freshwater crustacean farming.