Infection of the one-day old fertile hen's egg with rabies virus. III. Some observations on high egg passage (HEP) strains.

The previous reports from this laboratory (Yoshino et al., 1956 a, 1956 b) indicated that both mouse passaged CVS and 7-day egg passaged Flury strains of rabies virus could infect the 1-day egg resulting in high viral concentrations in the embryo which were comparable to those usually reached in the infected mouse brain, and that these strains after adapted to the 1-day egg by serial passage could be satisfactorily titrated in 1-day eggs. This new titration method was especially of use for rabies virus, since the 1-day egg LD titration could be performed with more ease and shorter time than the conventional mouse infectivity test which had been the only method of titration available for this virus. Meanwhile, Koprowski et al. (1954) found that 7-day egg passage of the Flury strain eventually produced a high egg passage (HEP) line which was unable to kill adult mice. The HEP Flury virus could be titrated by inoculation into baby mice, but there has been no other titration method for this particular virus routinely applicable in laboratory. Yet the use of baby mice has the disadvantage of occasional appearance of cannibalism, and therefore attempts were made here to adapt the HEP Flury virus to the 1-day egg with the expectation that it would become able to be titrated in 1-day eggs. When this attempt was started, however, a surprising fact was discovered that the HEP Flury virus, without any preliminary passage in 1-day eggs, was already as highly pathogenic to the 1-day egg as if it had received many previous 1-day egg passages. In the meantime, Takamatsu (1956) also passed Nishigahara strain in 7-day eggs for several years and reproduced the results of Koprowski et al. (1954), providing another HEP rabies virus. On examination of the pathogenicity for the 1-day egg of the HEP Nishigahara strain, we again found the same fact as revealed with the HEP Flury virus, i. e, it possessed high pathogenicity for the 1-day egg before any 1-day egg passages were tried. Owing to this unexpectedly high pathogenicity for the 1-day egg of these HEP rabies viruses, they could easily be titrated in 1-day eggs, and as a consequence a way was opened for easy access to the biological properties. Detailed experi-