SCRAPIE-ASSOCIATED FIBRIL DETECTION FROM FORMALDEHYDE FIXED BRAIN-TISSUE IN NATURAL CASES OF OVINE SCRAPIE

The medulla oblongata of the brains of 71 scrapie-suspect cases were r outinely fixed in 10 per cent formal saline and assessed for vacuolati on on HE-stained sections. A pool of fresh brain material was also dis sected from each animal and extracts prepared for the routine detectio n of scrapie-associated fibrils by negative stain transmission electro n microscopy. The remaining formaldehyde fixed medulla samples, which were not used for the histological examination, were coded and subject ed to a pretreatment with sodium borohydride and then processed using the routine fibril detection procedure. Of the 71 samples tested 46 we re considered positive by all three test procedures. Sixteen samples w ere negative for all three tests. Four samples were positive by histop athological examination and positive for fibrils using fresh tissue, b ut fibrils could not be detected in the fixed tissue preparations. Con versely, there were five fixed samples in which fibrils could be detec ted which were negative for the other two tests. The fibrils observed in fixed preparations were indistinguishable from those observed in fr esh tissue extracts. The sensitivity of the test for fibril detection using fixed tissue was 92 per cent and the specificity 76 per cent It is concluded that scrapie-associated fibrils can be recovered from for maldehyde fixed tissue, as presented for routine histopathological exa mination, and therefore the method has potential in the retrospective analysis of archived brain tissue where only fixed material was stored .