The differences in physiological and immunological parameters and pathological damage to organ tissues exposed to chronic heat stress provide the basis for evaluating heat resistance of different chicken breeds (white recessive rock [WRR] and The Lingshan [LS]). Ninety broilers of each breed were divided equally into a chronic heat stress group and a no heat stress group. The effects of chronic heat stress on the physiological and immunological parameters of broilers were analyzed using flow cytometry, ELISA, RT-qPCR, etc. Under heat stress conditions: (1) H and H/L values were significantly increased (P < 0.01) in the 2 breeds, and were higher in the WRR broilers than in the LS broilers at a late stage (P < 0.05). Although the corticosterone levels were also significantly increased (P < 0.01) in both breeds, they were lower in the 49 d WRR broilers than in the LS broilers (P < 0.01). The number of leukocytes were significantly increased in the 49 d WRR broilers (P < 0.01), whereas the number of CD3+, CD8+ cells, and erythrocytes were significantly reduced (P < 0.05). A significantly (P < 0.01) lower number of CD3+, CD4+ T-lymphocytes, and CD4+/CD8+ were present in WRR compared to that in the LS broilers. (2) The HSP70 transcript was significantly increased in the WRR broilers (P < 0.01), and was higher than the level in the LS broilers. The expression level of HSP70 protein was significantly (P < 0.05) increased in WRR broilers. (3) The WRR broilers developed cardiac and leg muscle inflammatory cellular hyperplasia and local inflammatory lesions, as well as cerebral meningitis and inflammatory hyperplasia of the brain tissue. The LS broilers developed mild cerebral inflammatory hyperplasia and mild inflammatory cellular proliferation in the leg muscle. In conclusion, under heat stress conditions, the relative physiological and immunological parameters were worse in the WRR broilers than in the LS broilers. The WRR broilers showed poor heat tolerance as evidenced from the expression of HSP70 and the extent of histopathological damages.

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