Hypoxia increases susceptibility of grass carp to Aeromonas hydrophila infection by inducing oxidative stress and impairing antimicrobial defense.

Hypoxia is a major environmental stressor that threatens fish health in intensive aquaculture, particularly when it co-occurs with bacterial infection. To clarify how hypoxia influences host responses to Aeromonas hydrophila, we established a combined stress model in grass carp using hypoxia (2.0 mg L-1 dissolved oxygen) and bacterial challenge. Survival, hepatic bacterial load, histopathology, hematological indices, antioxidant and immune enzyme activities, and the expression of hypoxia- and immune-related genes were evaluated systematically. The median lethal dose (LD50) of A. hydrophila was 2.41 × 108 CFU mL-1. Compared with normoxic infection, hypoxia markedly worsened disease outcome, as shown by a higher hepatic bacterial burden and more severe lesions in the gill and liver. Combined stress also reduced the proportions of neutrophils and monocytes, suggesting impaired cellular defense. In parallel, SOD, CAT, and GSH-PX activities increased, whereas ACP and LZM activities declined, together with elevated MDA levels, indicating pronounced oxidative stress and weakened innate immune function. Gene-expression analysis showed that hypoxia altered both oxygen-sensing and immune signaling pathways. In the liver, hif-1α expression was markedly induced under hypoxic conditions, whereas inflammatory cytokines, especially il-6 and il-10, were strongly upregulated. Notably, hepcidin expression was significantly increased and showed a pattern consistent with the induction of il-6, suggesting a potential association with the IL-6-hepcidin regulatory pathway under combined stress. However, this response may represent a compensatory mechanism that is insufficient to counteract the overall impairment of immune defense under hypoxic conditions.Overall, hypoxia enhanced the pathogenic effects of A. hydrophila by intensifying oxidative stress, impairing immune defense, and altering immune and antimicrobial regulatory responses. These findings improve our understanding of the interaction between environmental hypoxia and bacterial infection in grass carp and provide a basis for disease prevention in aquaculture.