The Shetland Islands scrapie monitoring and control programme: analysis of the clinical data collected from 772 scrapie suspects 1985-1997.

There were 574 scrapie positive suspects (histopathological scrapie lesions present) and 198 scrapie negative suspects (histopathological scrapie lesions absent). The greatest number of scrapie cases were recorded in sheep of 2, 3 and 4 years of age which represented 17%, 36% and 23% of the scrapie positive suspects, respectively. The sign sensitivities and specificities for the ten recorded signs were, respectively: pruritus (62%, 42%), ataxia (23%, 74%), hyperaesthesia (32%, 74%), wool loss (25%, 73%), fleece discolouration (29%, 85%), bruxism (23%, 69%), nibbling reflex (17%, 58%), head rubbing (47%, 78%), poll rubbing (25%, 83%). These single signs had poor discriminatory values with likelihood ratios close to one (range 0.89-1.21); combinations of the four signs, pruritus, wool loss, ataxia, hyperaesthesia and emaciation were more discriminatory (range 0.30-4.3). This study covered a time period when bovine spongiform encephalopathy (BSE) might have been introduced into the sheep population on the Shetland Islands via contaminated feed. No temporal changes could be detected in the age structure of the affected animals.