Replication of arboviruses in mouse organ cultures. II. Multiplication of virulent and avirulent Semliki Forest and western equine encephalitis viruses in mouse organ cultures.

The titre of WEE virus produced by organ cultures of brain, muscle and spleen decreased as tissue was taken from older mice. The multiplication of SFV and SFA in muscle organ cultures of mice remained nearly constant, whereas in spleen organ cultures, titers of both viruses decreased with increasing age of mice. There was a striking difference in the rate of decline of the yield of brain organ culture infected with virulent and avirulent SF virus strains. Organ cultures made from the brains of mice over 16 days old were totally resistant to SFA infection, whereas similar cultures from mice up to 42 days of age supported SFV multiplication. These results show that the avirulence of this strain of SF virus is due to age related intrinsic brain insusceptibility.