Table 1. 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.

Prion sources into Tg(OvPrP4) mice	Mean survival periods at 1st passage	Attack rate	Mean survival periods at 2nd passage	Attack rate
Ovine scrapie strain
CH1641	245+/−17 [26]	12/12	220+/−31	12/12
Murine scrapie strains
C506M3	350 [21]	4/8	333+/−26	12/12
Chandler	450 [21]	11/21	396+/−100	9/11
79A	540 [21]	6/15	342+/−20	12/12
87V	460 [21]	9/16	258+/−44	8/10
“CH1641-like” natural ovine isolates
TR316211	235+/−26 [26]	8/9	221+/−31	8/11
O104	248+/−50 [26]	10/10	238+/−29	13/13
O100	364+/−61 [26]	12/12	Not available
Other natural isolates
O111	296+/−20 [26]	10/11	399+/−124	8/10
Cattle BSE	421+/−48	10/10	354+/−48	10/10

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].