Abstract
A method for titrating the infectivity of a Flacherie virus of the silkworm has been described, with special interest to determine the minimum number of virus particles required to cause infection in a larva of the silkworm. Partially purified virus particles were serially diluted and inoculated per os to newly hatched larvae of the silkworm (of a Japanese race H4×H4). Most larvae inoculated died between 6 and 12 days after the virus inoculation. Contamination experiment showed that sick larvae due to contamination with feces of the inoculated larvae died always later than the 13th day after the inoculation. Therefore, virus titer (LD50) has been calculated based on the mortality ratios determined by the 12th day after the inoculation. The obtained mortality ratios of the virus-infected larvae groups bore a close resemblance to the theoretical curve (derived from Poisson's formula) on the assumption that a single virus particle is sufficient to initiate infection in a larva.
