Prolonged intracerebral infection with poliovirus in asymptomatic mice

A heretofore unrecognized prolonged asymptomatic infection has been identified in mice intracerebrally inoculated with Lansing strain type II poliovirus. Virus was detected by infectivity assay and nucleic acid hybridization and was found in brains as long as 77 days after injection. Viral RNA replication occurred, indicating that persisting virus was not an inactive residuum of the infecting dose. Virus was sometimes found by infectivity even when viral RNA could not be demonstrated by hybridization, but detection by nucleic acid hybridization alone never occurred. Among 56 animals–including mice that died of infection, mice with prolonged infection, and survivors from which virus was not recovered–no animal had a serum neutralization titer above 1:2 in a plaque reduction assay Moreover, brain homogenates of 30 survivors without virus did not have neutralization titers above 1:4 in the plaque reduction assay. These findings were the same as in mock‐infected mice.