Encephalitis in mice inoculated intranasally with an influenza virus strain originated from a water bird.

Five-week-old ddY mice were inoculated intranasally with a low virulent (4e) or highly virulent (24a5b) avian influenza virus strain originated from a water bird. None of mice in the 4e group showed clinical signs and brain lesions. Of the 24a5b group, two mice died and one mouse was killed at a moribund state at day 7 post-inoculation (PI). Four mice of the 24a5b group necropsied at day 5 or 7 PI had mild to severe encephalitis in the brain stem and the cerebellar white matter. Influenza virus antigen was detected in neurons, glial cells and vascular endothelium in the lesions. The distribution of the lesions seems to indicate the transneuronal invasion of the virus via cranial nerve fibers into the brain.