Measurement of the scrapie agent using an incubation time interval assay

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Abstract
The titer of the scrapie agent was determined by measurements of time intervals from inoculation to onset of illness and from inoculation to death. Both intervals were found to be inversely proportional to the size of the dose injected intracerebrally into random‐bred weanling Syrian hamsters. The logarithms of the time intervals minus a time factor were linear functions of the logarithm of the inoculum size. The time factors were determined by regression analysis in order to maximize these linear relationships. An equation relating the titer of the inoculum to the dilution of the sample and the length of the time intervals was developed. This equation facilitates the use of a computerized data base. Validation of these relationships was provided by comparing samples for which the agent was measured both by end‐point titration and by time interval assay. Agreement between the two methods was generally within ±0.5 log10 median infective dose units. No differences between the molecular properties of the agents from hamster and murine sources were observed using primarily the incubation time interval method with the former and end‐point titration with the latter. The advantages of this new approach based on time interval measurements are considerable with respect to time and resources.