Co-infection dynamics of Aeromonas jandaei and Edwardsiella tarda in Oscar fish (Astronotus ocellatus): Effects on pathogenicity, oxidative stress and immune responses.

Co-infections involving multiple bacterial pathogens represent an emerging challenge in aquaculture, yet their effects on host physiology and immune responses remain poorly understood. The present study investigated the pathogenicity and host responses associated with experimental co-infection of Aeromonas jandaei and Edwardsiella tarda in Oscar fish (Astronotus ocellatus). Following determination of the median lethal dose (LD50) for each pathogen, fish were challenged with single infections or a co-infection and evaluated for mortality, clinical signs, biochemical responses, and immune-related gene expression in the spleen and kidney. Co-infection resulted in the highest cumulative mortality (87.9%), compared with fish infected with A. jandaei (68.5%) or E. tarda (56.5%) alone, and produced more severe clinical manifestations, including lethargy, anorexia, abnormal swimming behaviour, and internal haemorrhaging. Biochemical analyses revealed significant elevations in glutamate pyruvate transaminase (GPT) and glutamate oxaloacetate transaminase (GOT) activities, with GPT reaching 43.52 ± 5.51 U/mg tissue in the co-infected group on 7th day post infection. In contrast, superoxide dismutase (SOD) activity declined markedly in infected fish, indicating increased oxidative stress, while catalase (CAT) activity increased significantly during the later stages of infection. Immune gene expression analysis demonstrated significant modulation of immunoglobulin M heavy chain (IgM-H), interleukin-8 (IL-8), major histocompatibility complex-IIα (MHC-IIα), and chemokine-1 (CC-1). In the spleen, MHC-IIα expression was strongly induced, reaching 15.7 ± 0.25 fold in the E. tarda infected group and 12.0 ± 0.52 fold in the A. jandaei infected group on 7th day post infection, whereas IgM-H expression peaked at 4.6 ± 0.61 fold in A. jandaei infected fish. In the kidney, co-infection elicited a pronounced inflammatory response, characterized by elevated IL-8 (3.7 ± 0.29 fold) and CC-1 (2.5 ± 0.37 fold) expression on 7th day post infection, indicating enhanced immune cell recruitment and activation. Co-infection altered the magnitude and pattern of immune gene expression, suggesting complex pathogen-host interactions and immune modulation. The findings demonstrate that simultaneous infection exacerbates disease severity, enhances oxidative stress, and significantly influences host immune responses, highlighting the importance of considering polymicrobial infections in disease management strategies for ornamental aquaculture.