Disappearance of scrapie virus from tissues of the mouse

Examination of newborn mice, inoculated intraperitoneally with high doses of scrapie virus, revealed that the virus could not be reisolated from their tissues after about 1 week following inoculation, until almost 1 year later. The inoculum was rapidly removed and was not detectable, although the animals became latently infected. Homogenization of whole inoculated newborn animals showed that only about 3% of virus could be recovered by the 2nd day postinoculation (p.i.). During the first 6 days p.i. the half-life of titratable scrapie was about 15 h. A further study of the rate of disappearance of clarified scrapie virus from blood after intravenous inoculation showed an even more rapid disappearance, with a half-life of 5.16 min. Prior treatment of the recipient mice with either carbon black or silica to block the reticuloendothelial system (RES) did not affect the rate of disappearance. It was concluded that the mouse possesses a very efficient means of scrapie virus removal from the blood which is not dependent upon an active RES. However, after 1 h the rate of disappearance changed dramatically; the residual virus level was very stable, with no significant drop during the next 21 h. This finding was compatible with the possibility that two forms of scrapie virus, with different removal rates, coexisted in the inoculum. Silica treatment caused a shortened scrapie incubation period.