Effects of virulence and infection routes of rabies viruses on pathogenicity in mice

【Objective】This study investigated the differences in clinical symptoms and the expression of related cytokines in mice infected with rabies virus（RABV）through 2 infection routes：intracerebral infection and intramuscular infection，in order to clarify the neurotropic mechanisms of RABV infection and provide theoretical basis for the scientific prevention，control，and treatment of rabies.【Method】Kunming mice were infected with RABV strains：the attenuated rRC-HL strain，standard virulent CVS-24 strain，and street virus GX074 strain，via intramuscular injection and intracerebral injection. The DMEM group（DMEM injected with the same volume）and the blank group（no processing）were set up at the same time. The clinical symptoms and disease course of the mice were observed after RABV intramuscular injection and intracerebral injection. Brain samples from moribund mice were collected. Viral protein expression in the mice brain was detected using Western blotting，real-time fluorescence quantitative PCR was used to detect the expression changes of viral gene and related cytokine gene in mouse brain tissue.【Result】The rRC-HL strain caused mild clinical symptoms in mice following intracerebral infection. However，after 7 d of infection，clinical symptoms gradually disappeared，the body weight of mice gradually recovered，and no fatalities were observed. Intramuscular infection did not induce neurological clinical symptoms in the mice. Both the CVS-24 and GX074 strains，regardless of whether the infection was intracerebral or intramuscular，caused disease in the mice，with a 100% mortality rate. However，the clinical symptoms and time to death differed between the 2 infection routes. In the intramuscular infection group，the onset of disease and time to death were later compared to the intracerebral infection group. Early symptoms included unilateral hind limb incoordination in movement，which later progressed to bilateral hind limb paralysis. In the intracerebral infection group， CVS-24 strain-infected mice showed paralysis，while GX074 strain-infected mice displayed a rabid，hyperactive phenotype. Intracerebral infection with the rRC-HL strain led to the up-regulation of cytokine genes such as IL-1β，IL-12，TNF-α， IFN-γ，MCP-1，STAT1 and RIG-1 in the brain tissue of mice. However，no obvious changes in cytokine gene expression were observed following intramuscular infection. Both intracerebral and intramuscular infections with the CVS-24 and GX074 strains caused up-regulation of cytokine genes including IL-1β，IL-6，IL-12，TNF-α，IFN-γ，MCP-1，STAT1 and RIG-1.【Conclusion】The virulence of the virus and the infection route both jointly affect the pathogenicity of RABV in mice. The differences in cytokine levels in brain tissue of the hosts are the main reason for the variation in clinical symptoms observed after RABV infection.