Toll-like receptors are PRRs with a unique and essential physiological function in host immune systems activated by pathogen-associated molecular patterns (Medzhitov, 2001). Expression profiles of two PRRs—TLR3 and TLR7—were examined in the target tissues of H5N6-infected chickens. As shown in Figure 3A, in contrast to mock-infected chickens, their expression level of TLR3 in the brain and lung was significantly elevated when induced by both viruses, with a fold increase of 2.80–78.56 in the brain or lung. In the spleen, the expression level of TLR3 was downregulated in response to SW8 infection, yet upregulated following infection with ZH283. In the bursa of Fabricius, the expression level of TLR3 was markedly downregulated when induced by both viruses. The expression level of TLR7 was upregulated in the lung when induced by SW8 or ZH283, by 1.79- and 19.41-fold, respectively. However, the expression level of TLR7 in the brain, spleen, and bursa of Fabricius showed different expression patterns when induced by the viruses; TLR7 expression level was downregulated when induced by both viruses compared to the control, with a fold change of 0.003–0.78 in all tested tissues except lung tissue. In particular, TLR7 expression remained low and was no longer visible in the bursa of Fabricius when triggered by both viruses. Notably, the expression levels of TLR3 and TLR7 in target tissues induced by ZH283 were generally greater than those induced by SW8 (Figures 3A,B).