Selective Systemic Responses During Aeromonas salmonicida Subsp. Salmonicida Infection in Northern Snakehead (Channa argus).

Northern snakehead (Channa argus) is a bimodal air-breathing fish in which erythrocyte-associated parameters remain relatively stable during physiological challenges such as aerial exposure. However, whether this characteristic is maintained during systemic infection remains unclear. In this study, temporal host responses were characterized following experimental Aeromonas salmonicida subsp. salmonicida infection by integrating haematological, leukocyte morphological, biochemical, and multi-tissue transcriptional analyses. Most erythrocyte-associated indices, including RBC, HGB, HCT, MCV, MCH, and RDW, showed no significant temporal changes, whereas MON, GRA, and PLT exhibited significant responses. Morphologically abnormal leukocytes increased from approximately 7% in healthy fish to 56% at 6 days post-infection and remained above 50% through 8 days, indicating persistent leukocyte-associated cellular alterations during infection. Biochemical responses were also selective, with changes primarily in protein- and metabolism-associated parameters. Transcriptional responses differed among peripheral blood, head kidney, spleen, and liver, including sustained casp7 suppression in the head kidney and distinct inflammatory and apoptosis-associated responses across tissues. Two-way ANOVA further identified significant tissue × time interactions for casp8, tnfα, and casp3. Collectively, A. salmonicida infection in northern snakehead was characterized by comparatively limited temporal variation in most erythrocyte-associated indices, alongside selective leukocyte, biochemical, and tissue-dependent transcriptional responses. These findings highlight a contrasting blood response characterized by persistent leukocyte alterations despite comparatively limited temporal variation in most erythrocyte-associated indices during bacterial infection in northern snakehead.