Abstract：The Ichthyophthirius multifiliis has high incidence and high mortality, and lacks effective treatment. It poses a great threat to the aquaculture breeding industry. To observe the infection of Ichthyophthirius multifiliis in rainbow trout (Oncorhynchus mykiss), the gill and skin of rainbow trout (treatment group and control group) were observed by tissue section and scanning electron microscope, and the dynamic expression of Toll-like recepotor (TLR) signaling pathway TLR2, TLR3, TLR4 genes and cytokines tumor necrosis factor-α (TNF-α), interleukin-1β (IL-1β), interferon-γ (IFN-γ) genes in gill, skin, liver, head kidney, spleen and intestine were detected by qPCR method. The results showed that the structure of skin and gill were seriously damaged by I. multifiliis. There were a large number of irregular secretions on the skin surface. There were gaps between epidermis and dermis, and the trophozoites of different sizes were parasitic in the gaps. The gill filament was damaged and shed, the gill lamellae were deformed and twisted, and the gill filament bone was swollen and deformed. Compared with the control group, TLR2 gene expression in the treatment group was significantly higher in gill and skin by 17.19 and 30.57 times (P<0.05), and significantly lower in spleen by 0.65 times than the control group, with no significant difference in other tissues. The expression of TLR3 gene in gill, skin, spleen and intestine was significantly increased (P<0.05) and were 3.8, 14.09, 1.66 and 1.89 times higher than those in the control group, respectively, and there was no significant difference in liver and head kidney (P>0.05); TLR4 gene expression was significantly higher in the skin by 2.24 times and lower in the liver and gill by 0.36 and 0.22 times than in the control group (P<0.05). There was no significant difference in other tissues (P>0.05). Compared with the control group, the treatment group expression of TNF-α in gill, skin and head kidney increased significantly, IL-1β and IFN-γ were significantly increased in all tissues in this study. The expression of TNF-α in skin was the highest, it was 8.22 times higher than the control group. The expression of IL-1β in head kidney was the highest, it was 14.6 times higher than the control group. The expression of IFN-γ in intestine was the highest, it was 5.1 times higher than the control group. TLR signaling pathway was involved in the defense mechanism of rainbow trout after being infected with I. multifiliis, and the gene expression in this pathway was tissue specific. After rainbow trout were infected by I. multifiliis, the organism established a systemic immune mechanism and produced multiple immune responses. This study revealed the pathological changes and the role of TLR signaling pathway after rainbow trout infection, and provides a theoretical reference for the prevention and treatment of rainbow trout infestation with I. multifiliis.
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