Table 5.

Endpoint titration in tg338 mice of one 10% brain homogenate sample and WBC samples collected from VRQ/VRQ sheep orally inoculated with PG127 scrapie^a

Inoculum dilution	Results from 10% brain homogenate		Results for WBC from sheep:
			D4		D5		D6
	No. of mice positive for abnormal PrP	Length of incubation (days)	No. of mice positive for abnormal PrP	Length of incubation (days)	No. of mice positive for abnormal PrP	Length of incubation (days)	No. of mice positive for abnormal PrP	Length of incubation (days)
Undiluted	6/6	64 ± 4	6/6	104 ± 6	6/6	102 ± 6	6/6	107 ± 12
10−1	6/6	76 ± 3	4/6	116 ± 10	1/6	131	1/6	118
10−2	6/6	87 ± 2	1/6	132	0/6	>250	0/6	>250
10−3	6/6	97 ± 5	0/6	>250	0/6	>250	0/6	>250
10−4	3/6	110 ± 4	ND		ND		ND
10−5	2/6	117-121	ND		ND		ND
10−6	0/6	>200	ND		ND		ND
10−7	0/6	>200	ND		ND		ND

^aHomogenate (10%, wt/vol) was prepared using posterior brainstem from VRQ/VRQ sheep inoculated with PG127 scrapie at the terminal stage of disease. Groups of 6 mice that overexpress the VRQ ovine PrP (tg338) were intracerebrally (20 μl) inoculated with successive 1/10 dilutions of this homogenate. These data were already used in a previous publication (3). Three susceptible VRQ/VRQ sheep (identified as D4, D5, and D6) were orally challenged with 2 g of brain homogenate (10^6.6 ID₅₀/g IC in tg338 mice) between 6 and 10 months of age. These animals died at 217, 206, and 217 dpi, respectively, and classical scrapie occurrence was confirmed by histopathology (vacuolar changes in the central nervous system) and the detection of abnormal PrP deposits in the central nervous system and lymphoid tissues. Whole-blood (30 ml) samples were collected at 206 dpi, and WBC were prepared. WBC samples equivalent to 10 ml of starting blood were resuspended in 200 μl of 5% glucose solution before homogenization. Successive 1/10 dilutions of homogenate were inoculated intracerebrally into tg338 mice (n = 6; 20 μl per mouse). Mice were observed until the occurrence of clinical signs or were sacrificed after 250 dpi. Mice were considered positive when abnormal PrP deposition was detected in the brain. Incubation periods are presented as means ± SD except for dilutions with which less than 50% of mice were positive. In those cases, incubation times of the positive mice are individually presented. ND, not determined.