Abstract
In olive flounder (Paralichthys olivaceus) suffering from emaciation disease, the intestinal myxozoan Enteromyxum leei is considered a major causative agent. This disease causes severe economic losses in East Asian aquaculture, and even though the pathological outcomes have been well described, the molecular mechanisms underlying host immune imbalance are unclear. We performed RNA sequencing of posterior intestinal tissue from infected and control fish, yielding high-quality datasets and 2666 differentially expressed genes (1589 downregulated, 1077 upregulated). Enrichment analyses revealed a significant modulation of immune processes, particularly cytokine activity, chemokine signaling, apoptosis regulation, and lymphocyte trafficking. Kyoto Encyclopedia of Genes and Genomes analysis identified six immune-related pathways that were the most affected: Toll-like receptor, NOD-like receptor, intestinal immune network for IgA production, C-type lectin receptor, RIG-I-like receptor, and cytosolic DNA sensing. Network mapping highlighted nine hub genes, including cxcl8a, pik3r1, mapk10, and itpr1b, which were shared across multiple pathways and validated by qRT-PCR. Our results demonstrate that E. leei disrupts intestinal immune homeostasis by suppressing chemokine-driven inflammation and adaptive responses while simultaneously enhancing nucleic acid-sensing and stress pathways. This dual modulation provides new insights into the intestinal immune dysregulation underlying enteromyxosis and establishes a molecular basis for future diagnostic and preventive strategies in olive flounder aquaculture.