Type 3 reovirus neuroinvasion after intramuscular inoculation: direct invasion of nerve terminals and age-dependent pathogenesis.

Neonatal but not adult mice are vulnerable to reovirus invasion of the central nervous system after peripheral inoculation. After hindlimb injection, type 3 reovirus travels via the sciatic nerve to replicate in spinal cord motor neurons before spread to the brain and development of lethal encephalitis. Here we provide ultrastructural evidence for direct reovirus invasion of unmyelinated neonatal motor nerve terminals within 2 h and replication in spinal cord motor neurons within 14 h after hindlimb injection of 1-day-old mice. In adult mice, resistance to reovirus lethality after intracranial (IC) injection correlates with the restriction of virus growth in cortical neurons. We found that neuroinvasion also is age dependent after intramuscular injection. Virus lethality and CNS infection decreased sharply during the first postnatal week, while lethality after IC injection continued for 2 additional weeks. Mice inoculated at 7 days of age with high virus doses suffered paralysis of the injected limb, but significant brain infection was not lethal. These results suggest that reovirus invasion of the neonatal CNS is restricted by several progressive age-dependent mechanisms.