Scrapie surveillance in Great Britain: results of an abattoir survey, 1997/98

A randomised sample of 2809 apparently healthy sheep, 55 per cent of them l ess than 15 months of age, which were slaughtered for human consumption at abattoirs in Great Britain in 1997/98, was taken to establish the prevalenc e of scrapie infection. The medulla oblongata of each sheep was examined hi stopathologically at the level of the obex, and fresh brain tissue was exam ined for scrapie-associated fibrils (SAF) to establish whether there was ev idence of scrapie. In addition, histological sections of the medulla from 5 00 of the sheep were immunostained with an antiserum to PIP, and the same t echnique was also applied to any animal found positive or inconclusive by t he histological or SAF examinations. Any sheep which was positive by any of these diagnostic methods was also examined by Western immunoblotting, for the detection of the disease-specific protein PrPSc. a total of 2798 sheep (99.6 per cent) were negative by all the methods applied. Ten animals were SAF-positive but negative by all the other methods, and in one animal there was immunohistochemical staining which could not be interpreted unequivoca lly as disease-specific. A mathematical model was used to estimate the prev alence of scrapie infection in the national slaughtered sheep population wh ich would be consistent with these results. By this model, the absence of u nequivocally substantiated cases of scrapie in the sample was consistent wi th a prevalence of infection in the slaughter population of up to 11 per ce nt.