MRI changes observed in a case of atypical scrapie in a 7-year-old Herdwick ewe.

Atypical scrapie is a transmissible spongiform encephalopathy that is rarely diagnosed in living animals. In March 2022, a 7-y-old Herdwick ewe was referred to the Scottish Centre for Production Animal Health and Food Safety because of circling behavior and ill thrift. The ewe had a low body condition score, was obtunded, with a wide-based stance of the pelvic limbs, and was circling to the left. Hematologic, biochemical, and CSF analyses were unremarkable, but postmortem magnetic resonance imaging (MRI) findings were consistent with diffuse, bilateral, and symmetrical atrophy of the forebrain and ventriculomegaly. The clinical signs, the involvement of an individual older ewe, and the MRI results led to the clinical diagnosis of scrapie. Immunohistochemistry on the fixed brain, performed by the U.K. Animal and Plant Health Agency, revealed deposits of PrP^Sc, which is a specific disease marker of transmissible spongiform encephalopathies, mainly in the cerebellum and at lower concentrations in the cerebrum and obex, consistent with the diagnosis of atypical scrapie. MRI findings in a sheep with atypical scrapie have not been described previously, to our knowledge. Scrapie should be included in the list of clinical differential diagnoses when veterinarians are presented with sheep with progressive neurologic signs of several weeks' duration.