Objective To study the effect and pathological mechanisms of the neuro-immune response to viral encephalomyelitis caused by virulence reversion following the intrathalamic neurovirulence test for poliomyelitis vaccine in rhesus macaques. Methods Stock solutions (≥7000 lgCCID₅₀/ L) of inactivated polio vaccines (Vero cells) of type I,type II, and type III Sabin strains and 10-1 dilution of each type of polio vaccine were given to macaques, which were subjected to a intrathalamic neurovirulence test. Using immunohistochemical method, the pathological changes caused by polio, as determined by the distribution of CD155 and CD4⁺receptor T lymphocytes, CD20⁺B lymphocytes, and CD68+microglia, were detected. Results Lesions were observed on the virulence-reverted polio cases. Inflammatory cell infiltration, neuronal degeneration and necrosis, satellite phenomena, perivascular cuffing, and glial cell proliferation were observed in the spinal cord. The inflammatory cells in the perivascular cuffing and proliferative glial nodules were mainly CD4⁺T lymphocytes, CD20⁺B lymphocytes, and CD68⁺ microglia. There was no significant difference in the distribution of the poliovirus receptor CD155 in the neurons and glial cells of monkeys with and without polio, and no expression was observed in their vascular endothelial cells. Conclusions Polio caused by virulence reversion of the intrathalamic neurovirulence test is viral encephalomyelitis.