Horizontal transmission. Two or three noninoculated
weanling voles were exposed in the same cage to four voles
which had been inoculated with 103.5 ID50 of virus. Exposed
voles were removed after 2 to 3 weeks of exposure and
replaced with other noninoculated animals. After exposure,
voles were kept an additional 5 weeks in individual cages
before being sacrificed, and lung tissues were examined for
viral antigen.