Immune response of Rhinogobio ventralis to Ichthyophthirius multifiliis infection: Insights from histopathological and real-time gene expression analyses.

Ichthyophthirius multifiliis is a parasite that poses a considerable threat to aquaculture and the ornamental fish industry, but with limited effective treatment options available. This study employed RT-qPCR to detect and analyze the expression changes of partial toll-like receptor (TLR) genes (TLR1 and TLR21), adapter protein and signal transduction molecule genes (MyD88, TRIF, NF-κB, IRAK4, and IRF3), and cytokines (IL-6, IL-8, IL-13, CXC-α and CXCR1), as well as complement C3, in the skin, gill, fin, liver, head kidney and spleen of Rhinogobio ventralis under different infection conditions. Additionally, tissue sections and scanning electron microscopy were utilized to observe the pathological changes in the gills and fins of R. ventralis after infection with I. multifiliis. The expression patterns of TLR-related DEGs (differentially expressed genes) in diseased wild fish were analyzed, revealing upregulation of TLR1, TLR21, MyD88, NF-κB, IRAK4, TRIF, IRF3, IL-6, IL-8, IL-13, CXC-α, CXCR1, and C3 genes in various tissues, indicating that these genes may be involved in the immune response of R. ventralis to I. multifiliis infection. To further analyze the gene expression of sampled from the field, an artificial infection model of R. ventralis was established under laboratory conditions, with additional sampling from the skin and fins. These genes continued to show varying degrees of upregulation, but the results were not entirely consistent with those from Wudongde samples, which may be due to the more complex environment in the wild or differences in the degree of I. multifiliis infection in wild fish. The infection of I. multifiliis caused severe damage to the gills and fins of R. ventralis, characterized by extensive secretions on the gill and fin surfaces, with the presence of attached I. multifiliis trophonts, including damage and loss of gill filaments, swollen gill lamellae, and deformed gill plates, as well as cell proliferation and necrosis of gill epithelial cells. This study sheds light on the role of the TLR signaling pathway in resisting I. multifiliis infection and its associated histopathological changes in R. ventralis, providing valuable insights for the prevention and treatment of I. multifiliis infection in R. ventralis.