Synergistic effects of co-infection with Flavobacterium psychrophilum and Aeromonas salmonicida on mortality, pathophysiology, and immune responses in rainbow trout (Oncorhynchus mykiss)

Bacterial co-infections pose a significant threat to aquaculture, often leading to more severe outcomes compared than mono-infections. Understanding the underlying mechanisms is essential to preventing and controlling diseases. In this study, we investigated the effects of mono-infection and co-infection with Flavobacterium psychrophilum (Fp) and Aeromonas salmonicida (As) on rainbow trout (Oncorhynchus mykiss). Results indicated that the mortality rate in the co-infection group was significantly higher than that in the mono-infected groups, with more severe histopathological damage observed in the liver and muscle tissues. On day 7 post-infection, the antioxidant enzyme (SOD and CAT) activity in the liver of the co-infection group peaked and subsequently declined. Transcriptomic analysis revealed that Fp mono-infection primarily activated the Toll-like receptor signaling pathway, whereas As mono-infection activated the MAPK signaling pathway. In contrast, co-infection (Fp + As) showed significant enrichment of DEGs in the p53 pathway, suggesting a unique apoptotic response potentially contributing to host defense through the elimination of compromised cells. This study demonstrates that co-infection with F. psychrophilum and A. salmonicida synergistically exacerbates disease progression in rainbow trout. These findings provide novel insights into co-infection risks, elucidate key immunopathological pathways, and establish a scientific basis for developing targeted disease interventions in aquaculture.