Second passage transmission studies of CH1641 sheep isolate inoculated i.c. to Tg(OvPrP4) mice, compared to murine scrapie strains (C506M3, 87V, 79A and Chandler), “CH1641-like” natural isolates and other natural isolates, including BSE.

Incubation periods are expressed in days as the mean +/− standard error (SE) of the mean. The incubation periods of first passage experiments already reported elsewhere (data from [6], [18], [21]) are recalled in order to facilitate interpretation of the second passage data. In the case of murine scrapie strain in accordance with [21], only approximations of mean incubation periods in days are reported. Attack rates are given to indicate transmission efficacy (number of PrPsc(IHC)/PrPres(WB) positive mice/total number of inoculated mice). Compared to first passage experiment, the unexpected prolonged survival of ovine scrapie O111 with large SE may result from the PrPc expression level in this mouse model as described in atypical scrapie transmission studies published previously [33].