Histopathological and Molecular Insights into Grass Carp Kidney Responses to Co-Infection with Aeromonas hydrophila and Aeromonas veronii

Grass carp (Ctenopharyngodon idella), a key species in freshwater aquaculture, is particularly vulnerable to opportunistic pathogens, including Aeromonas hydrophila and Aeromonas veronii. While the pathogenic mechanisms of individual infections have been extensively characterized, the host immune responses during co-infection remain poorly understood. This research explored the renal pathological alterations and transcriptomic shifts in grass carp subjected to simultaneous infection by A. hydrophila and A. veronii. Mortality occurred as early as 24 h post-challenge, ultimately reaching a cumulative death rate of 65%. Quantitative analysis of renal bacterial burden revealed a marked increase in colonization at 3 days post-infection (dpi). The histopathological assessment showed progressive kidney damage, including tubular collapse, epithelial necrosis, interstitial edema, and widespread epithelial desquamation, with the most severe lesions observed at 5 dpi and partial signs of recovery by 7 dpi. A total of 1106 and 472 genes were found to be differentially expressed at 1 and 5 dpi, respectively, based on transcriptome profiling. The functional enrichment analysis indicated that the differentially expressed genes (DEGs) were mainly involved in the complement and coagulation cascade pathways. Notably, the immune-related genes exhibited a biphasic trend, with predominant downregulation at 1 dpi followed by marked upregulation by 5 dpi, indicating dynamic changes in immune modulation during co-infection. These results provide new insights into host responses during dual bacterial infections in fish and may inform disease prevention strategies in aquaculture.