Age dependent susceptibility to Reovirus type 3 encephalitis: role of viral and host factors.

Reovirus type 3 inoculated intracerebrally or subcutaneously into newborn mice induced an acute necrotizing and uniformly fatal encephalitis. Subsequently, between the eighth and tenth day of age, reovirus type 3 infection changed to a nonlethal infection. This pattern of virulence was directly correlated with the ability of the virus to replicate in the brain and histologically was accompanied by a gradual diminution in the intensity and extent of encephalitis such that histological abnormalities were absent in the brains of virus-injected adult animals. Some animals injected at ages 10 to 18 days developed brainstem and diencephalic lesions, indicating a nonuniform resistance of neurons to viral infection with aging. Neither a changing immune response pattern nor a modification or loss of viral receptors appears to explain either the susceptibility of newborn or the resistance of adult animals to reovirus type 3 encephalitis. Splenic mononuclear cells from mice younger (but not older) than 7 days also permitted reovirus type 3 replication in vitro. Thus, the age dependent resistance to reovirus type 3 encephalitis appears related to an intrinsic resistance to viral replication by neuronal cells, a resistance that may occur simultaneously in nonneuronal cell populations.