Hematology-centered characterization of systemic physiological alterations during viral haemorrhagic septicemia virus (VHSV) infection in olive flounder (Paralichthys olivaceus).

Viral haemorrhagic septicemia virus (VHSV) is a major viral pathogen that causes severe haemorrhagic disease and hematological disturbances in olive flounder (Paralichthys olivaceus). However, integrated hematology-centered analyses of infection-associated blood alterations remain limited. In the present study, temporal changes in viral replication, hematological parameters, blood cell morphology, biochemical responses, and gene expression were analyzed following VHSV infection in olive flounder. VHSV infection caused dose-dependent mortality, reaching 100% cumulative mortality at 106-107 50% Tissue Culture Infectious Dose. Viral replication exceeded 106 copies/mg in major target tissues, including the spleen, heart, and kidney, whereas circulating blood maintained comparatively low viral loads (∼103 copies/μL) without a significant correlation with heart or kidney viral loads. Hematological analysis revealed sustained lymphocyte suppression from 1 to 11 days post-infection (dpi), whereas monocyte and granulocyte populations significantly increased at 15 dpi. Red blood cell counts remained significantly reduced throughout the infection period, accompanied by progressive decreases in hematocrit and hemoglobin levels. Severe leukocyte abnormalities, including cytoplasmic vacuolization, nuclear abnormalities, and binucleation, were observed at 15 dpi. Gene expression analysis revealed tissue-specific transcriptional responses characterized by transient early upregulation of FTH1, FTMT, HBB, and TF levels in blood, delayed increases in FTMT expression in heart tissue, and significant induction of HIF1A and STAT3 in kidney-associated responses. Overall, these findings demonstrate that VHSV infection induces stage-dependent hematological, morphological, biochemical, and transcriptional alterations in olive flounder and highlight the value of blood-based analyses as indicators of systemic host physiological responses during viral infection.