During experiments on the purification of scrapie infectivity, changes were found in the dynamics of scrapie titration. After exposure to detergent, infectivity estimates by both endpoint titration and incubation period were altered. The addition of detergent to the diluent used in titration resulted in at least a 100-fold increase in the infectivity estimate. This suggests that the amount of scrapie in a sample, as measured by serial dilution and titration, may be underestimated to different extents, depending on the biochemical milieu of the inoculum. Membrane fractions treated with detergents before dilution exhibited longer incubation periods than untreated fractions for the same number of infectious units of scrapie. This demonstrates that detergent treatments and possibly other biochemical manipulations can cause changes in the response of the host to the inoculum that are not detectable if incubation periods alone are used to estimate scrapie ‘titre’.


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