Comparison of PrP
levels in BSE and scrapie inocula as determined by IDEXX EIA. Both inocula, 100% BSE (■) and 100% scrapie (●), were analysed by IDEXX Herdchek BSE/scrapie EIA as described in Materials and methods. Colour development, measured in absorbance units (AU), is proportional to the amount of PrPSc in the sample when the growth phase of the curve is linear. Required inocula dilutions to achieve an AU value of 0.6, where the growth phase of the 100% BSE curve was most linear, were 1/912 for 100% scrapie and 1/2.5 for 100% BSE, indicating the quantity of PrPSc in the scrapie inocula was 357 times greater than in the BSE inocula. OD values of ovine BSE and ovine scrapie positive controls were 2.659 and 2.664 AU respectively. OD values of two control negative sheep samples were 0.064 and 0.057 AU.