Infected cows develop signs of nervousness, incoordination, changes in temperament, aggression, abnormal posture, and decreased milk production.
Symptoms begin to appear two to eight years after the initial infection.
Once the symptoms start, the cow dies within a few weeks to a few months. The original source of Mad Cow disease is unknown. It could have entered cattle through feeding of sheep remains infected with a similar disease, called scrapie. Among cows, researchers say it most likely spread through rendering. After slaughter, unused cattle parts are cooked and the fat removed.