BSE diagnosis. Recoding electrode placed across a cow's eye in preparation for a test to diagnose BSE (bovine spongiform encephalopathy). BSE is caused by abnormal prions, which are virus-like proteins. The presence of the abnormal prions causes changes to cells in the cow's retina that can be detected by recording the retina's response to a flash of light. This is the first diagnostic test for BSE that can be carried out on a live animal.